Power-Programmierung/CD2.mdf/doc/helppc/interrup.txt

Text File | 1991-04-14 | 250KB | 8,823 lines

@Interrupt Services DOS∙BIOS∙EMS∙Mouse
:int table:interrupt table:exceptions:IRQ
^Intel Defined CPU Exception Table (see notes)

%    Interrupt      Function

     0    Divide by zero
     1    Single step
     2    Non-maskable  (NMI)
     3    Breakpoint
     4    Overflow trap
     5    BOUND range exceeded (186,286,386)
     6    Invalid opcode (186,286,386)
     7    Coprocessor not available (286,386)
     8    Double fault exception (286,386)
     9    Coprocessor segment overrun (286,386)
     A    Invalid task state segment (286,386)
     B    Segment not present (286,386)
     C    Stack exception (286,386)
     D    General protection exception (286,386)
     E    Page fault (286,386)
     F    Reserved
    10    Coprocessor error (286,386)

^IBM PC Hardware Interrupt Table (in order of priority)

%    IRQ#  Interrupt     Function

    IRQ0     8    ~timer~ (55ms intervals, 18.2 per second)
    IRQ1     9    keyboard service required
    IRQ2     A    slave ~8259~ or EGA/VGA vertical retrace
    IRQ8    70    real time clock  (AT,XT286,PS50+)
    IRQ9    71    software redirected to IRQ2  (AT,XT286,PS50+)
    IRQ10    72    reserved  (AT,XT286,PS50+)
    IRQ11    73    reserved  (AT,XT286,PS50+)
    IRQ12    74    mouse interrupt  (PS50+)
    IRQ13    75    numeric coprocessor error  (AT,XT286,PS50+)
    IRQ14    76    fixed disk controller (AT,XT286,PS50+)
    IRQ15    77    reserved  (AT,XT286,PS50+)
    IRQ3     B    COM2 or COM4 service required, (COM3-COM8 on MCA PS/2)
    IRQ4     C    COM1 or COM3 service required
    IRQ5     D    fixed disk or data request from LPT2
    IRQ6     E    floppy disk service required
    IRQ7     F    data request from LPT1 (unreliable on IBM mono)


^Interrupt Table as Implemented by System BIOS/DOS

%      INT #   Locus        Function

     0    CPU    divide by zero
     1    CPU    single step
     2    CPU    non-maskable
     3    CPU    breakpoint
     4    CPU    overflow trap
     5    BIOS    print screen
     6    CPU    Invalid opcode (186,286,386)
     7    CPU    coprocessor not available (286,386)
     8    IRQ0    ~timer~ (55ms intervals, 18.21590 per second)
     9    IRQ1    keyboard service required (see ~INT 9~)
     A    IRQ2    slave ~8259~ or EGA/VGA vertical retrace
     B    IRQ3    COM2 service required (PS/2 MCA COM3-COM8)
     C    IRQ4    COM1 service required
     D    IRQ5    fixed disk or data request from LPT2
     E    IRQ6    floppy disk service required
     F    IRQ7    data request from LPT1 (unreliable on IBM mono)
    10    BIOS    video (see ~INT 10~)
    11    BIOS    Equipment determination (see ~INT 11~)
    12    BIOS    memory size (see ~INT 12~)
    13    BIOS    disk I/O service  (see ~INT 13~)
    14    BIOS    serial communications (see ~INT 14~)
    15    BIOS    system services, cassette (see ~INT 15~)
    16    BIOS    keyboard services (see ~INT 16~)
    17    BIOS    parallel printer (see ~INT 17~)
    18    BIOS    ROM BASIC loader
    19    BIOS    bootstrap loader (unreliable, see ~INT 19~)
    1A    BIOS    time of day    (see ~INT 1A~)
    1B    BIOS    user defined ctrl-break handler (see ~INT 1B~)
    1C    BIOS    user defined clock tick handler (see ~INT 1C~)
    1D    BIOS    ~6845~ video parameter pointer
    1E    BIOS    diskette parameter pointer (base table)
    1F    BIOS    graphics character table
    20    DOS    general program termination
    21    DOS    function request services (see ~INT 21~)
    22    DOS    terminate address (see ~INT 22~)
    23    DOS    control break termination address (see ~INT 23~)
    24    DOS    critical error handler (see ~INT 24~)
    25    DOS    absolute disk read (see ~INT 25~)
    26    DOS    absolute disk write (see ~INT 26~)
    27    DOS    terminate and stay resident (see ~INT 27~)
    28    DOS    idle loop, issued by DOS when idle (see ~INT 28~)
    29    DOS    fast TTY console I/O (see ~INT 29~)
    2A    DOS    critical section and NETBIOS (see ~INT 2A~)
    2B    DOS    internal, simple ~IRET~ in DOS 2.0-5.0
    2C    DOS    internal, simple IRET in DOS 2.0-5.0
    2D    DOS    internal, simple IRET in DOS 2.0-5.0
    2E    DOS    exec command from base level command
            interpreter    (see ~INT 2E~)
    2F    DOS    multiplexer (see ~INT 2F~)
    30-31    CPM    far jump vector for CPM (not an interrupt)
    31    DPMI    DOS Protected Mode Interface (for DOS extenders)
    32        reserved
    33        mouse support (see ~INT 33~)
    34-3E        Microsoft/Borland floating point emulation
    3F        overlay manager
    40    BIOS    hard disk
    41    BIOS    fixed disk 0 parameters pointer (see ~INT 13,9~)
    42    BIOS    relocated video handler (EGA/VGA/PS)
    43    BIOS    user font table (EGA/VGA/PS)
    44    BIOS    first 128 graphics characters (also Netware)
    45    BIOS    reserved for BIOS
    46    BIOS    fixed disk 1 parameters ptr (see ~INT 13,9~/INT 41)
    47    BIOS    reserved for BIOS
    48    BIOS    PCjr cordless keyboard translation
    49    BIOS    PCjr non-keyboard scancode translation table
    4A    BIOS    user alarm  (AT,CONV,PS/2) (see ~INT 4A~)
    4B-4F    BIOS    reserved
    50    BIOS    periodic alarm from timer (PS/2)
    51-58    BIOS    reserved
    59    BIOS    GSS Computer Graphics Interface
    5A    BIOS    cluster adapter BIOS entry point
    5B    BIOS    cluster adapter boot
    5C    NETBIOS    NETBIOS interface, TOPS interface
    5D-5F    BIOS    reserved for BIOS
    60-67        reserved for user software interrupts
    67    EMS    LIM/EMS specification (see ~INT 67~)
    68        APPC
    69-6B        reserved by IBM
    6C    DOS    DOS 3.2 real time clock update
        BIOS    system resume vector
    6D-6F        reserved
    70    IRQ8    real time clock  (AT,XT286,PS50+, see ~INT 15~)
    71    IRQ9    software redirected to IRQ2    (AT,XT286,PS50+)
    72    IRQ10    reserved  (AT,XT286,PS50+)
    73    IRQ11    reserved  (AT,XT286,PS50+)
    74    IRQ12    mouse interrupt  (PS50+)
    75    IRQ13    numeric coprocessor NMI error  (AT,XT286,PS50+)
    76    IRQ14    fixed disk controller (AT,XT286,PS50+)
    77    IRQ15    reserved  (AT,XT286,PS50+)
    78-79        unused
    80-85        ROM BASIC
    86-F0    DOS    reserved for BASIC interpreter use
    86    NETBIOS    NETBIOS relocated INT 18
    E0    CPM    CP/M 86 function calls
    F1-FF        reserved by IBM
    FE-FF        may be destroyed by return from protected
            mode using VDISK on 286 machines (Apr 86, DDJ)


    - Intel defined 0 through 20h for use for internal CPU;  IBM
      redefined interrupts 0 through 1Fh for its own use, hence
      the duplicate definitions in the tables
    - all interrupts except the internal CPU exceptions push the
      flags and the CS:IP of the next instruction onto the stack.
      CPU exception interrupts are similar but push the CS:IP of the
      causal instruction.    8086/88 divide exceptions are different,
      they return to the instruction following the division
    - interrupts are disabled upon entry into any interrupt routine and
      should be enabled by the user or by an ~IRET~
    - in DOS 3.2+ hardware IRQ interrupts are re-vectored through DOS
      to provide standard stack frames

:int 5
^INT 5 - Print Screen


    no input data


%    related memory:

    50:00    = 00    Print screen has not been called, or upon return
            from a call there were no errors
        = 01    Print screen is already in progress
        = FF    Error encountered during printing


    - invoked from ~INT 9~

:int 8:BIOS timer interrupt
^INT 8 - System timer

    no input data

%    related memory:

    40:6C = Daily timer counter (4 bytes)
    40:70 = 24 hr overflow flag (1 byte)
    40:67 = Day counter on all products after AT
    40:40 = Motor shutoff counter - decremented until 0 then
        shuts off diskette motor


    - ~INT 1C~ is invoked as a user interrupt
    - the byte at 40:70 is a flag that certain DOS functions use
      and adjust the date if necessary.  Since this is a flag and
      not a counter it results in DOS (not the ~RTC~) losing days
      when several midnights pass before a DOS call
    - generated 18.2 times per second by the ~8253~ Programmable Interval
      Timer (PIT)
    - normal INT 8 execution takes approximately 100 microseconds

    - see    ~8253~

:int 9:keyboard interrupt
^INT 9 - Keyboard Interrupt (Hardware Handler)

    no input data

%    related memory:

    40:17 = updates keyboard flag byte 0
    40:18 = updates keyboard flag byte 1
    40:1A = queue head ptr is set to buffer start if Ctrl-Break is hit
    40:1C = updates buffer tail pointer for each keystroke; sets
        queue tail ptr is set to queue start if Ctrl-Break is hit
    40:1E = updates keyboard buffer (32 bytes)
    40:71 = updates bit 7 of the BIOS break flag if Ctrl-Break is hit
    40:72 = updates reset flag with 1234H if Ctrl-Alt-Del pressed
    40:96 = indicates keyboard type (AT,PS/2)
    40:97 = updates keyboard LED flags (AT,PS/2)
    FFFF:0 = reboot code called if Ctrl-Alt-Del pressed

%    related interrupts:

    ~INT 5~     invoked if print screen key pressed
    ~INT 1B~    invoked if Ctrl-Break key sequence pressed
    ~INT 15,85~ invoked on AT if system request key is pressed
    ~INT 15,4F~ invoked on machines after PC/AT with AL = scan code


    - records key press and key release via IRQ1/8259 and
      stores scan codes in the BIOS buffer located at 40:1C
    - keyboard controllers also buffer data when interrupts are
      disabled at the ~8259~ interrupt controller
    - keyboard controller is capable of storing 16 keystrokes
      even when interrupts are disabled at the 8259
    - normal INT 9 execution takes approximately 500 microseconds;
      at least one standard XT BIOS is known to take up to 1.3
      milliseconds to execute

    - see    ~MAKE CODES~   ~KB FLAGS~

:int 10:BIOS video services:video interrupt
^INT 10 - Video BIOS Services

%    For more information, see the following topics:

     ~INT 10,0~ - Set video mode
     ~INT 10,1~ - Set cursor type
     ~INT 10,2~ - Set cursor position
     ~INT 10,3~ - Read cursor position
     ~INT 10,4~ - Read light pen
     ~INT 10,5~ - Select active display page
     ~INT 10,6~ - Scroll active page up
     ~INT 10,7~ - Scroll active page down
     ~INT 10,8~ - Read character and attribute at cursor
     ~INT 10,9~ - Write character and attribute at cursor
     ~INT 10,A~ - Write character at current cursor
     ~INT 10,B~ - Set color palette
     ~INT 10,C~ - Write graphics pixel at coordinate
     ~INT 10,D~ - Read graphics pixel at coordinate
     ~INT 10,E~ - Write text in teletype mode
     ~INT 10,F~ - Get current video state
    ~INT 10,10~ - Set/get palette registers (EGA/VGA)
    ~INT 10,11~ - Character generator routine (EGA/VGA)
    ~INT 10,12~ - Video subsystem configuration (EGA/VGA)
    ~INT 10,13~ - Write string (BIOS after 1/10/86)
    ~INT 10,14~ - Load LCD char font (convertible)
    ~INT 10,15~ - Return physical display parms (convertible)
    ~INT 10,1A~ - Video Display Combination (VGA)
    ~INT 10,1B~ - Video BIOS Functionality/State Information (MCGA/VGA)
    ~INT 10,1C~ - Save/Restore Video State  (VGA only)
    ~INT 10,FE~ - Get DESQView/TopView Virtual Screen Regen Buffer
    ~INT 10,FF~ - Update DESQView/TopView Virtual Screen Regen Buffer


    Warning: Some BIOS implementations have a bug that causes register
    BP to be destroyed.   It is advisable to save BP before a call to
    Video BIOS routines on these systems.

    - registers CS, DS, ES, SS, BX, CX, DX are preserved unless
      explicitly changed
    - see  ~INT 1F~  ~INT 1D~  ~INT 29~  ~INT 21,2~  ~INT 21,6~  ~INT 21,9~

:int 10,0:video modes
^INT 10,0 - Set Video Mode

    AH = 00
    AL = 00  40x25 B/W text (CGA,EGA,MCGA,VGA)
       = 01  40x25 16 color text (CGA,EGA,MCGA,VGA)
       = 02  80x25 16 shades of gray text (CGA,EGA,MCGA,VGA)
       = 03  80x25 16 color text (CGA,EGA,MCGA,VGA)
       = 04  320x200 4 color graphics (CGA,EGA,MCGA,VGA)
       = 05  320x200 4 color graphics (CGA,EGA,MCGA,VGA)
       = 06  640x200 B/W graphics (CGA,EGA,MCGA,VGA)
       = 07  80x25 Monochrome text (MDA,HERC,EGA,VGA)
       = 08  160x200 16 color graphics (PCjr)
       = 09  320x200 16 color graphics (PCjr)
       = 0A  640x200 4 color graphics (PCjr)
       = 0B  Reserved (EGA BIOS function 11)
       = 0C  Reserved (EGA BIOS function 11)
       = 0D  320x200 16 color graphics (EGA,VGA)
       = 0E  640x200 16 color graphics (EGA,VGA)
       = 0F  640x350 Monochrome graphics (EGA,VGA)
       = 10  640x350 16 color graphics (EGA or VGA with 128K)
         640x350 4 color graphics (64K EGA)
       = 11  640x480 B/W graphics (MCGA,VGA)
       = 12  640x480 16 color graphics (VGA)
       = 13  320x200 256 color graphics (MCGA,VGA)
       = 8x  EGA, MCGA or VGA ignore bit 7, see below
       = 9x  EGA, MCGA or VGA ignore bit 7, see below


    - if AL bit 7=1, prevents EGA,MCGA & VGA from clearing display
    - function updates byte at 40:49;  bit 7 of byte 40:87
      (EGA/VGA Display Data Area) is set to the value of AL bit 7

:int 10,1
^INT 10,1 - Set Cursor Type


    AH = 01
    CH = cursor starting scan line (cursor top) (low order 5 bits)
    CL = cursor ending scan line (cursor bottom) (low order 5 bits)


    returns nothing


    - cursor scan lines are zero based
    - cursor size can also be set via the ~6845~ CRT controller
    - cursor size can be determined using the CRTC,  ~INT 10,3~  or the
      ~BIOS Data Area~ bytes 40:60 (ending scan line) and 40:61 (starting
      scan line)
    - the following is a list of the cursor scan lines associated with
      most common adapters;  screen sizes over 40 lines may differ
      depending on adapters.

%        Line     Starting     Ending      Character
%    Video    Count     Scan Line    Scan Line   Point Size

    CGA     25        06         07          08
    MDA     25        0B         0C          0E
    EGA     25        06         07          0E
    EGA     43      04/06      07          08
    VGA     25        0D         0E          10
    VGA     40        08         09          0A
    VGA     50        06         07          08

    - use CX = 2000h to disable cursor

:int 10,2
^INT 10,2 - Set Cursor Position


    AH = 02
    BH = page number (0 for graphics modes)
    DH = row
    DL = column


    returns nothing


    - positions relative to 0,0 origin
    - 80x25 uses coordinates 0,0 to 24,79;    40x25 uses 0,0 to 24,39
    - the ~6845~ can also be used to perform this function
    - setting the data in the BIOS Data Area at location 40:50 does not
      take immediate effect and is not recommended
    - see    ~VIDEO PAGES~   ~6845~   ~BDA~

:int 10,3
^INT 10,3 - Read Cursor Position and Size


    AH = 03
    BH = video page


    on return:
    CH = cursor starting scan line (low order 5 bits)
    CL = cursor ending scan line (low order 5 bits)
    DH = row
    DL = column


    - returns data from ~BIOS DATA AREA~ locations 40:50, 40:60 and 40:61
    - the ~6845~ can also be used to read the cursor position
    - the return data can be circumvented by direct port I/O to the 6845
      CRT Controller since this function returns the data found in the
      BIOS Data Area without actually checking the controller

:int 10,4
^INT 10,4 - Read Light Pen Position


    AH = 04


    on return:
    AH = 0    light pen switch not triggered
       = 1    light pen triggered
    BX = pixel column (0-319 or 0-639, mode dependent)
    CH = raster line (0-199) (CGA and EGA modes 4, 5 and 6)
    CX = raster line (EGA modes except 4, 5 and 6)
    DH = row (0-24)
    DL = column (0-79 or 0-79 mode dependent)


    - data returned as a byte coordinate, leaving horizontal
      accuracy to within 2 pixels (320) or 4 pixels (640)
    - vertical accuracy within 2 lines
    - PS/2's don't support the light pen interface

:int 10,5
^INT 10,5 - Select Active Display Page

    AH = 05
    AL = new page number, see ~VIDEO PAGES~

    for PCjr only:
    AL = 80h to read CRT/CPU page registers
         81h to set CPU page register
         BL = CPU page register
         82h to set CRT page register
         BH = CRT page register
         83h to set CPU and page registers
         BH = CRT page register
         BL = CPU page register


    on return: (PCjr only)
    BH = CRT page register
    BL = CPU page register

:int 10,6
^INT 10,6 - Scroll Window Up


    AH = 06
    AL = number of lines to scroll, previous lines are
         blanked, if 0 or AL > screen size, window is blanked
    BH = attribute to be used on blank line
    CH = row of upper left corner of scroll window
    CL = column of upper left corner of scroll window
    DH = row of lower right corner of scroll window
    DL = column of lower right corner of scroll window


    returns nothing


    - in video mode 4 (300x200 4 color) on the EGA, MCGA and VGA
      this function scrolls page 0 regardless of the current page
    - can be used to scroll graphics screens, using character coords
    - on CGA's this function disables video adapter, causing flitter
:int 10,7
^INT 10,7 - Scroll Window Down


    AH = 07
    AL = number of lines to scroll, previous lines are
         blanked, if 0 or AL > screen size, window is blanked
    BH = attribute to be used on blank line
    CH = row of upper left corner of scroll window
    CL = column of upper left corner of scroll window
    DH = row of lower right corner of scroll window
    DL = column of lower right corner of scroll window


    returns nothing


    - in video mode 4 (300x200 4 color) on the EGA, MCGA and VGA
      this function scrolls page 0 regardless of the current page
    - can be used to scroll graphics screens, using character coords
    - on CGA's this function disables video adapter, causing flitter

:int 10,8
^INT 10,8 - Read Character and Attribute at Cursor Position


    AH = 08
    BH = display page


    on return:
    AH = attribute of character (alpha modes only)
    AL = character at cursor position


    - in video mode 4 (300x200 4 color) on the EGA, MCGA and VGA
      this function works only on page zero

:int 10,9
^INT 10,9 - Write Character and Attribute at Cursor Position


    AH = 09
    AL = ASCII character to write
    BH = display page  (or mode 13h, background pixel value)
    BL = character attribute (text) foreground color (graphics)
    CX = count of characters to write (CX >= 1)


    returns nothing


    - does not move the cursor
    - in graphics mode (except mode 13h), if BL bit 7=1 then
      value of BL is XOR'ed with the background color

:int 10,a
^INT 10,A - Write Character Only at Current Cursor Position


    AH = 0A
    AL = ASCII character to write
    BH = display page  (or mode 13h, background pixel value)
    BL = foreground color (graphics mode only)
    CX = count of characters to write (CX >= 1)


    return nothing


    - similar to ~INT 10,9~ except color ignored in text modes

:int 10,b
^INT 10,B - Set Color Palette

    AH = 0B
    BH = palette color ID
       = 0    to set background and border color
       = 1    to select 4 color palette
    BL = color value (when BH = 0)
       = palette value (when BH = 1)

    Palette    Pixel      Color
       0         0        current background color
             1        green (2)
             2        red (4)
             3        brown (6)
       1         0        current background color
             1        cyan (3)
             2        magenta (5)
             3        white (7)

    - does not work for all EGA and VGA video modes
    - sets border color in text mode (BH = 0)

:int 10,c
^INT 10,C - Write Graphics Pixel at Coordinate


    AH = 0C
    AL = color value (XOR'ED with current pixel if bit 7=1)
    BH = page number, see ~VIDEO PAGES~
    CX = column number (zero based)
    DX = row number (zero based)


    returns nothing


    - if bit 7 is 1, color specified is XOR'ed with current pixel
    - page number in BH ignored for 320x200 4 color graphics mode
    - this function is known to destroy AX and possibly SI and DI on
      on some PS/2 VGA systems

:int 10,d
^INT 10,D - Read Graphics Pixel at Coordinate


    AH = 0D
    BH = page number, see ~VIDEO PAGES~
    CX = column number (zero based)
    DX = row number (zero based)


    on return:
    AL = color of pixel read


    - 64K IBM EGAs with BIOS dated 9/13/84 in 350 line video
      modes,  return invalid data in AL
    - page number in BH ignored for 320x200 4 color graphics mode

:int 10,e
^INT 10,E - Write Text in Teletype Mode


    AH = 0E
    AL = ASCII character to write
    BH = page number (text modes)
    BL = foreground pixel color (graphics modes)


    returns nothing


    - cursor advances after write
    - characters BEL (7), BS (8), LF (A), and CR (D) are
      treated as control codes
    - for some older BIOS (10/19/81), the BH register must point
      to the currently displayed page
    - on CGA adapters this function can disable the video signal while
      performing the output which causes flitter.

:int 10,f
^INT 10,F - Get Video State


    AH = 0F


    on return:
    AH = number of screen columns
    AL = mode currently set (see ~VIDEO MODES~)
    BH = current display page


    - video modes greater than 13h on EGA, MCGA and VGA indicate
      ~INT 10,0~ was called with the high bit of the mode (AL) set
      to 1, meaning the display does not need cleared
    - function returns byte value at 40:49;  On EGA, MCGA and
      VGA bit 7 of register AL is determined by bit 7 of BIOS Data
      Area byte 40:87.   This bit is usually set by INT 10,0
      with bit 7 of the requested mode (in AL) set to 1

:int 10,10
^INT 10,10 - Set/Get Palette Registers (EGA/VGA)

    AH = 10h

%    AL = 00  set individual palette register
       BH = color value
       BL = palette register


%    AL = 01  set border color (overscan register)
       BH = color value


%    AL = 02  set all palette registers and border
       ES:DX = pointer to 17 byte table representing 16 palette
           registers and border color register


%    AL = 03  toggle intensity/blinking (EGA)
       BL = 0  enable intensity
        1  enable blinking


%    AL = 07  read palette register (PS/2)
       BL = palette register to read (0-15)

    on return:
      BH = value of palette register


%    AL = 08  read border color (overscan register, PS/2)

    on return:
      BH = value of border color (overscan register)


%    AL = 09  read palette registers and border (PS/2)
       ES:DX = pointer to 17 byte table representing 16 palette
           registers and border color register

    on return:
    ES:DX = pointer to table provided as input


%    AL = 10  set DAC color register
       BX = color register to set
       CH = green value
       CL = blue value
       DH = red value


%    AL = 12  set block of DAC color registers
       BX = first color register to set
       CX = number of color registers to set
       ES:DX = pointer to table of color values to set


%    AL = 13  set attribute controller color select state
       BL = 0  set Mode Control register bit 7
          BH = value for bit 7
       BL = 1  set color select register
          BH = value for color select register


%    AL = 15  read DAC color register (PS/2)
       BX = color register to read

    on return:
    CH = green value
    CL = blue value
    DH = red value


%    AL = 17  read block of DAC color registers
       BX = first color register to read
       CX = number of color registers to read
       ES:DX = pointer to buffer for color registers

    on return:
    ES:DX = pointer to color table provided as input


%    AL = 18  update video DAC mask register
       BL = new mask


%    AL = 19  read video DAC mask register

    on return:
    BL = value read from video DAC mask register


%    AL = 1A  read color page state
       BL = bit 7 of Mode Control Register
       BH = bits 2 thru 3 of Color select register if BL = 0
          = bits 0 thru 3 of Color select register if BL = 1

    on return:
    BL = current paging mode
    CX = current page



%    AL = 1B  sum color values to shades of gray
       BX = first color register to sum
       CX = number of color registers to sum


    - controls the pixel color mapping bit values
    - BIOS extension to EGA/VGA systems

:int 10,11
^INT 10,11 - Character Generator Routine (EGA/VGA)

    AH = 11h

%    AL = 00  user character load
       BH = number of bytes per character
       BL = table in character generator RAM
       CX = count of characters in table
       DX = ASCII code of first character defined
       ES:BP = pointer to user table


%    AL = 01  ROM BIOS 8x14 monochrome set
       BL = table in character generator RAM


%    AL = 02  ROM BIOS 8x8 double dot
       BL = table in character generator RAM


%    AL = 03  set displayed definition table
       BL = value for character Map Select register (EGA,VGA)
          = character generator RAM table numbers (MCGA)


%    AL = 04  ROM BIOS 8x16 character set
       BL = table in character generator RAM


%    AL = 10  user specified character definition table
       BH = bytes per character (points)
       BL = table in character generator RAM
       CX = number of characters defined in table
       DX = ASCII code of first character defined
       ES:BP = pointer to user table


%    AL = 11  ROM BIOS 8x14 monochrome character set
       BL = table in character generator RAM


%    AL = 12  ROM 8x8 double dot character definitions
       BL = table in character generator RAM


%    AL = 14  ROM 8x16 double dot character definitions
       BL = table in character generator RAM


%    AL = 20  pointer to graphics character table for ~INT 1F~ (8x8)
       ES:BP = pointer to user table


%    AL = 21  user graphics character pointer at INT 43
       BL = row specifier
          = 0 - user specified  (DL = rows)
          = 1 is 14 rows
          = 2 is 25 rows
          = 3 is 43 rows
       CX = bytes per character (points)
       DL = rows (when BL = 0)
       ES:BP = pointer to user table


%    AL = 22  ROM 8x14 character set
       BL = number of rows (see AL=21)
       DL = rows (when BL = 0)


%    AL = 23  ROM 8x8 double dot character set
       BL = row specifier (see AL=21)
       DL = rows (when BL = 0)


%    AL = 24  ROM 8x16 character set
       BL = row specifier (see AL=21)
       DL = rows (when BL = 0)


%    AL = 30  get current character generator information
       BH = information desired:
          = 0  ~INT 1F~ pointer
          = 1  INT 44h pointer
          = 2  ROM 8x14 pointer
          = 3  ROM 8x8 double dot pointer (base)
          = 4  ROM 8x8 double dot pointer (top)
          = 5  ROM 9x14 alpha alternate pointer
          = 6  ROM 8x16 character table pointer
          = 7  ROM 9x16 alternate character table pointer

    on return:
    CX = bytes per character (points)
    DL = rows (less 1)
    ES:BP = pointer to table

:int 10,12
^INT 10,12 - Video Subsystem Configuration (EGA/VGA)

    AH = 12h


%    BL = 10  return video configuration information

    on return:
    BH = 0 if color mode in effect
       = 1 if mono mode in effect
    BL = 0 if 64k EGA memory
       = 1 if 128k EGA memory
       = 2 if 192k EGA memory
       = 3 if 256k EGA memory
    CH = feature bits
    CL = switch settings


%    BL = 20  select alternate print screen routine

%    BL = 30  select scan lines for alphanumeric modes
       AL = 0  200 scan lines
          = 1  350 scan lines
          = 2  400 scan lines

    on return:
    AL = 12


%    BL = 31  select default palette loading
       AL = 0 enable default palette loading
          = 1 disable default palette loading

    on return:
    AL = 12


%    BL = 32  CPU access to video RAM
       AL = 0  enable CPU access to video RAM and I/O ports
          = 1  disable CPU access to video RAM and I/O ports

    on return:
    AL = 12


%    BL = 33  Gray scale summing
       AL = 0  enable gray scale summing
          = 2  disable gray scale summing

    on return:
    AL = 12


%    BL = 34  cursor emulation
       AL = 0  enable cursor emulation
          = 1  disable cursor emulation

    on return:
    AL = 12


%    BL = 35  PS/2 video display switching
       AL = 0 initial adapter video off
          = 1 initial planar video on
          = 2 switch active video off
          = 3 switch inactive video on
       ES:DX pointer to 128 byte save area (when AL = 0, 2 or 3)

    on return:
    AL = 12


%    BL = 36  video refresh control
       AL = 0 enable refresh
          = 1 disable refresh

    on return:
    AL = 12

:int 10,13
^INT 10,13 - Write String (BIOS versions from 1/10/86)

    AH = 13h
    AL = write mode (see bit settings below)
       = 0 string is chars only, attribute in BL, cursor not moved
       = 1 string is chard only, attribute in BL, cursor moved
       = 2 string contains chars and attributes, cursor not moved
       = 3 string contains chars and attributes, cursor moved
    BH = video page number
    BL = attribute if mode 0 or 1 (AL bit 1=0)
    CX = length of string (ignoring attributes)
    DH = row coordinate
    DL = column coordinate
    ES:BP = pointer to string


    Bit settings for write mode (register AL):

    │7│6│5│4│3│2│1│0│  AL
     │ │ │ │ │ │ │ └──── 0=don't move cursor, 1=move cursor
     │ │ │ │ │ │ └───── 0=BL has attributes, 1=string has attributes
     └─┴─┴─┴─┴─┴────── unused


    returns nothing


    - BEL, BS, CR, LF are treated as ASCII control codes
    - wraps data and scrolls if unable to fit data on one line

:int 10,14
^INT 10,14 - Load LCD Character Font (convertible only)

    AH = 14h

%    AL = 0 - load user specified font
       ES:DI = pointer to character font
       CX = number of characters to store
       DX = char offset into ram font area
       BH = number of bytes per character
       BL = 0  load main font (block 0)
          = 1  load alternate font (block 1)

%    AL = 1 - load system ROM default font
       BL = 0  load main font (block 0)
          = 1  load alternate font (block 1)

%    AL = 2 - set mapping of LCD high intensity attribute
       BL = 0  ignore high intensity attribute
          = 1  map high intensity to underscore
          = 2  map high intensity to reverse video
          = 3  map high intensity to select alternate font
:int 10,15
^INT 10,15 - Return Physical Display Parms (convertible)


    AH = 15h

    on return:
    AX = alternate display adapter type
    ES:DI = pointer to parameter table:


%    Offset Size      Description

      01   word   monitor model number
      02   word   vertical pels per meter
      03   word   horizontal pels per meter
      04   word   total number of vertical pels
      05   word   total number of horizontal pels
      06   word   horizontal pel separation in micrometers
      07   word   vertical pel separation in micrometers

:int 10,1a
^INT 10,1A - Video Display Combination (VGA)

    AH = 1A
    AL = 00 get video display combination
       = 01 set video display combination
         BL = active display  (see table below)
         BH = inactive display


    on return:
    AL = 1A, if a valid function was requested in AH
    BL = active display  (AL=00, see table below)
    BH = inactive display  (AL=00)

%    Valid display codes:

     FF  Unrecognized video system
     00  No display
     01  MDA with monochrome display
     02  CGA with color display
     03  Reserved
     04  EGA with color display
     05  EGA with monochrome display
     06  Professional graphics controller
     07  VGA with analog monochrome display
     08  VGA with analog color display
     09  Reserved
     0A  MCGA with digital color display
     0B  MCGA with analog monochrome display
     0C  MCGA with analog color display

    - returns value at byte 40:8A indicating display combination status
    - used to detect video display capabilities

:int 10,1b
^INT 10,1B - Video BIOS Functionality and
^State Information (MCGA/VGA)

    AH = 1B
    BX = implementation type (must be zero)
    ES:DI = pointer to 64 byte buffer


    on return:
    AL = 1B
    ES:DI = pointer to updated buffer  (see below)


    - returns static and dynamic information about the current
      state and capabilities of the current video system
    - bytes 0-3 of the dynamic data table at ES:DI contain a far
      pointer to the video static information table


^Video BIOS Dynamic Functionality State Table (MCGA/VGA)

%    Dynamic Video State Table

    00  dword   address of static functionality table
    04  byte    video mode
    05  word    number of columns
    07  word    length of displayed video buffer (# bytes)
    09  word    start address of upper left corner of video buffer
    0B  16bytes cursor position table for 8 pages (col,row)
    1B  byte    cursor end line
    1C  byte    cursor start line
    1D  byte    active video page
    1E  word    I/O port for CRTC address register
    20  byte    current value of CRTC 3x8 register
    21  byte    current value of CRTC 3x9 register
    22  byte    number of displayed character rows
    23  word    height of character matrix (points)
    25  byte    active display combination code
    26  byte    inactive display combination code
    27  word    number of displayed colors (mono = 0)
    29  byte    number of supported video pages
    2A  byte    raster scan lines 0=200, 1=350, 2=400, 3=480
    2B  byte    text character table used
    2C  byte    text character table used
    2D  byte    other state information:

        │7│6│5│4│3│2│1│0│ State Information byte at offset 2D
         │ │ │ │ │ │ │ └─── 1 = all modes active (MCGA always 0)
         │ │ │ │ │ │ └──── 1 = gray scale summing enabled
         │ │ │ │ │ └───── 1 = monochrome display attached
         │ │ │ │ └────── 1 = default palette loading disabled
         │ │ │ └─────── 1 = cursor emulation enabled
         │ │ └──────── 1 = blinking attribute enabled
         └─┴───────── 1 = reserved

    2E 3bytes   reserved
    31  byte    video RAM available 0=64K, 1=128K, 2=192K, 3=256K
    32  byte    save area status

        │7│6│5│4│3│2│1│0│ Save Area Status
         │ │ │ │ │ │ │ └─── 1 = two text char sets are active
         │ │ │ │ │ │ └──── 1 = dynamic save area is active
         │ │ │ │ │ └───── 1 = text char set override is active
         │ │ │ │ └────── 1 = graphics char set is override active
         │ │ │ └─────── 1 = palette override is active
         │ │ └──────── 1 = display combination code ext. active
         └─┴───────── 1 = reserved

    33  dword   reserved


^Video BIOS Static Functionality Table    (EGA/VGA)

        │7│6│5│4│3│2│1│0│ Video modes supported, byte at offset 00
         │ │ │ │ │ │ │ └─── 1 = mode 0
         │ │ │ │ │ │ └──── 1 = mode 1
         │ │ │ │ │ └───── 1 = mode 2
         │ │ │ │ └────── 1 = mode 3
         │ │ │ └─────── 1 = mode 4
         │ │ └──────── 1 = mode 5
         │ └───────── 1 = mode 6
         └────────── 1 = mode 7

        │7│6│5│4│3│2│1│0│ Video modes supported, byte at offset 01
         │ │ │ │ │ │ │ └─── 1 = mode 8
         │ │ │ │ │ │ └──── 1 = mode 9
         │ │ │ │ │ └───── 1 = mode A
         │ │ │ │ └────── 1 = mode B
         │ │ │ └─────── 1 = mode C
         │ │ └──────── 1 = mode D
         │ └───────── 1 = mode E
         └────────── 1 = mode F

        │7│6│5│4│3│2│1│0│ Video modes supported, byte at offset 02
         │ │ │ │ │ │ │ └─── 1 = mode 10
         │ │ │ │ │ │ └──── 1 = mode 11
         │ │ │ │ │ └───── 1 = mode 12
         │ │ │ │ └────── 1 = mode 13
         └─┴─┴─┴─────── reserved

    03  dword  reserved
    07  byte   scan lines supported in text modes

        │7│6│5│4│3│2│1│0│ Scan lines supported, byte at offset 07
         │ │ │ │ │ │ │ └─── 1 = 200 lines
         │ │ │ │ │ │ └──── 1 = 350 lines
         └─┴─┴─┴─┴─┴───── 1 = 400 lines

    08  byte   max number of displayable text character sets
    09  byte   # of text definition tables in char generator RAM
    0A  byte   other capability flags

        │7│6│5│4│3│2│1│0│ Other flags, byte at offset 0A
         │ │ │ │ │ │ │ └─── 1 = all modes (0 on MCGA)
         │ │ │ │ │ │ └──── 1 = gray scale summing
         │ │ │ │ │ └───── 1 = character set loading
         │ │ │ │ └────── 1 = default palette loading
         │ │ │ └─────── 1 = cursor emulation
         │ │ └──────── 1 = 64 color palette
         │ └───────── 1 = video DAC loading
         └────────── 1 = DAC controlled by ACCS

    0B  byte   other capability flags

        │7│6│5│4│3│2│1│0│ Other flags, byte at offset 0B
         │ │ │ │ │ │ │ └─── 1 = light pen support
         │ │ │ │ │ │ └──── 1 = save/restore video state
         │ │ │ │ │ └───── 1 = blinking/background intensity
         │ │ │ │ └────── 1 = display combination code
         └─┴─┴─┴─────── reserved

    0C  word   reserved
    0E  byte   save area capabilities

        │7│6│5│4│3│2│1│0│  save area capabilities at offset 0E
         │ │ │ │ │ │ │ └──── 1 = multiple text character sets
         │ │ │ │ │ │ └───── 1 = dynamic save area
         │ │ │ │ │ └────── 1 = text character set override
         │ │ │ │ └─────── 1 = graphics character set override
         │ │ │ └──────── 1 = palette override
         │ │ └───────── 1 = display combination code extension
         └─┴────────── reserved

    0F  byte    reserved
:int 10,1c
^INT 10,1C - Save/Restore Video State  (VGA only)

    AH = 1C

%    AL = 0    get save buffer size
       CX = requested states
        bit 0: video hardware state
        bit 1: video BIOS data areas
        bit 2: video DAC state

    on return:
    AL = 1C
    BX = buffer size in 64 byte blocks

%    AL = 1    save requested state
       CX = requested states (see AL = 0)
       ES:BX = pointer to buffer

    returns nothing

%    AL = 2    restore requested states
       CX = requested states (see AL = 0)
       ES:BX = pointer to buffer

    returns nothing

:int 10,fe
^INT 10,FE - Get DESQView/TopView Virtual Screen Regen Buffer


    AH = FE
    ES:DI = set to sentinel value (test for INT 10,FE supported)


    returns:
    ES:DI = address of DESQView/TopView video buffer, DI will always
        be zero


    - on return ES:DI should be tested against the original value;
      the value will change if this function is supported (DESQView
      or TopView loaded), otherwise it will remain unchanged
    - if ES:DI changes this address can be used as the video screen
      regen buffer

:int 10,ff
^INT 10,FF - Update DESQView/TopView Virtual Screen Regen Buffer


    AH = FF
    CX = number of characters changed
    ES:DI = pointer to first character in buffer to change,  ES is
        set to segment returned by ~INT 10,FE~


    returns nothing


    - the physical screen does not get updated until INT 10,FF is
      called in TopView
    - it is not necessary to make this call under DESQView since it
      handles updates automatically
    - calling this function under DESQView will cancel the automatic
      update mode

:int 11:equipment flags
^INT 11 - BIOS Equipment Determination / BIOS Equipment Flags

    no input data

    on return:
    AX contains the following bit flags:

    │F│E│D│C│B│A│9│8│7│6│5│4│3│2│1│0│  AX
     │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ └──── IPL diskette installed
     │ │ │ │ │ │ │ │ │ │ │ │ │ │ └───── math coprocessor
     │ │ │ │ │ │ │ │ │ │ │ │ └─┴────── old PC system board RAM < 256K
     │ │ │ │ │ │ │ │ │ │ │ │ │ └───── pointing device installed (PS/2)
     │ │ │ │ │ │ │ │ │ │ │ │ └────── not used on PS/2
     │ │ │ │ │ │ │ │ │ │ └─┴─────── initial video mode
     │ │ │ │ │ │ │ │ └─┴────────── # of diskette drives, less 1
     │ │ │ │ │ │ │ └───────────── 0 if DMA installed
     │ │ │ │ └─┴─┴────────────── number of serial ports
     │ │ │ └─────────────────── game adapter installed
     │ │ └──────────────────── unused, internal modem (PS/2)
     └─┴───────────────────── number of printer ports


    - bits 3 & 2,  system board RAM if less than 256K motherboard
        00 - 16K             01 - 32K
        10 - 16K             11 - 64K (normal)

    - bits 5 & 4,  initial video mode
        00 - unused          01 - 40x25 color
        10 - 80x25 color         11 - 80x25 monochrome


    - bits 7 & 6,  number of disk drives attached, when bit 0=1
        00 - 1 drive         01 - 2 drives
        10 - 3 drive         11 - 4 drives


    - returns data stored at ~BIOS Data Area~ location 40:10
    - some flags are not guaranteed to be correct on all machines
    - bit 13 is used on the PCjr to indicate serial printer

:int 12
^INT 12 - Memory Size Determination


    no input data


    on return:
    AX  = the number of contiguous 1k memory blocks found at startup

    - contiguous memory does not include video memory or extended RAM

:int 13:BIOS disk services
^INT 13 - Diskette BIOS Services

^For more information see the following topics:

     ~INT 13,0~  Reset disk system
     ~INT 13,1~  Get disk status
     ~INT 13,2~  Read disk sectors
     ~INT 13,3~  Write disk sectors
     ~INT 13,4~  Verify disk sectors
     ~INT 13,5~  Format disk track
     ~INT 13,6~  Format track and set bad sector flag (XT & portable)
     ~INT 13,7~  Format the drive starting at track (XT & portable)
     ~INT 13,8~  Get current drive parameters (XT & newer, see note ╪)
     ~INT 13,9~  Initialize 2 fixed disk base tables (XT & newer, see note ╪)
     ~INT 13,A~  Read long sector (XT & newer, see note ╪)
     ~INT 13,B~  Write long sector (XT & newer, see note ╪)
     ~INT 13,C~  Seek to cylinder (XT & newer, see note ╪)
     ~INT 13,D~  Alternate disk reset (XT & newer, see note ╪)
     ~INT 13,E~  Read sector buffer (XT & portable only)
     ~INT 13,F~  Write sector buffer (XT & portable only)
    ~INT 13,10~  Test for drive ready (XT & newer, see note ╪)
    ~INT 13,11~  Recalibrate drive (XT & newer, see note ╪)
    ~INT 13,12~  Controller ram diagnostic (XT & portable only)
    ~INT 13,13~  Drive diagnostic (XT & portable only)
    ~INT 13,14~  Controller internal diagnostic (XT & newer, see note ╪)
    ~INT 13,15~  Read disk type/DASD type (XT BIOS from 1/10/86 & newer)
    ~INT 13,16~  Disk change line status (XT BIOS from 1/10/86 & newer)
    ~INT 13,17~  Set dasd type for format (XT BIOS from 1/10/86 & newer)
    ~INT 13,18~  Set media type for format (BIOS date specific)
    ~INT 13,19~  Park fixed disk heads (AT & newer)
    ~INT 13,1A~  Format ESDI drive unit (PS/2 50+)


    ╪  The 1983 version of the "IBM XT BIOS Technical Reference"
       shows these functions are available for the XT but many other
       programming references say they aren't available until the AT.
       This is probably due to misunderstanding the design of the disk
       BIOS.   Upon bootup the hard disk BIOS replaces the default
       INT 13h diskette handler with the hard disk INT 13h handler.


%    Most disk BIOS calls use the following parameter scheme:

        AH = function request number
        AL = number of sectors  (1-128 dec.)
        CH = cylinder number  (0-1023 dec.)
        CL = sector number    (1-17 dec.)
        DH = head number  (0-15 dec.)
        DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)
        DL = drive number (0=A:, 1=2nd floppy, 80h=C:, 81h=D:)
         Note that some programming references use (0-3) as the
         drive number which represents diskettes only.
        ES:BX = address of user buffer


%    and return with:
    CF = 0 if successful
       = 1 if error
    AH = status of operation  (see INT 13,STATUS)


    - INT 13 diskette read functions should be retried at least 3
      times to assure the disk motor has time to spin up to speed
    - physical sector numbers can be converted to and from DOS sector
      numbers with the following formulas:

      dos_sector = (sector - 1) + (head * sectors_per_track) +
               (track * sectors_per_track * num_heads)

      physical_sector = 1 + (dos_sector  MOD  sectors_per_track)
      physical_head = (dos_sector / sectors_per_track)  MOD  num_heads
      physical_track = dos_sector / (sectors_per_track * num_heads)

    - registers DS, BX, CX and DX are preserved
    - see  ~INT 13,STATUS~

:int 13,0
^INT 13,0 - Reset Disk System


    AH = 00
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)


    on return:
    AH = disk operation status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error


    - clears reset flag in controller and pulls heads to track 0
    - setting the controller reset flag causes the disk to recalibrate
      on the next disk operation
    - if bit 7 is set, the diskette drive indicated by the lower 7 bits
      will reset then the hard disk will follow; return code in AH is
      for the drive requested

:int 13,1:int 13,status
^INT 13,1 - Disk Status

    AH = 01

    on return:
    AL = status:

%    Status in AL

       00  no error
       01  bad command passed to driver
       02  address mark not found or bad sector
       03  diskette write protect error
       04  sector not found
       05  fixed disk reset failed
       06  diskette changed or removed
       07  bad fixed disk parameter table
       08  DMA overrun
       09  DMA access across 64k boundary
       0A  bad fixed disk sector flag
       0B  bad fixed disk cylinder
       0C  unsupported track/invalid media
       0D  invalid number of sectors on fixed disk format
       0E  fixed disk controlled data address mark detected
       0F  fixed disk DMA arbitration level out of range
       10  ECC/CRC error on disk read
       11  recoverable fixed disk data error, data fixed by ECC
       20  controller error (NEC for floppies)
       40  seek failure
       80  time out, drive not ready
       AA  fixed disk drive not ready
       BB  fixed disk undefined error
       CC  fixed disk write fault on selected drive
       E0  fixed disk status error/Error reg = 0
       FF  sense operation failed


    - codes represent controller status after last disk operation
    - returns the status byte located at 40:41 in the ~BIOS Data Area~

:int 13,2
^INT 13,2 - Read Disk Sectors

    AH = 02
    AL = number of sectors to read    (1-128 dec.)
    CH = track/cylinder number  (0-1023 dec., see below)
    CL = sector number  (1-17 dec.)
    DH = head number  (0-15 dec.)
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)
    ES:BX = pointer to buffer


    on return:
    AH = status  (see ~INT 13,STATUS~)
    AL = number of sectors read
    CF = 0 if successful
       = 1 if error


    - BIOS disk reads should be retried at least three times and the
      controller should be reset upon error detection
    - be sure ES:BX does not cross a 64K segment boundary or a
      DMA boundary error will occur
    - many programming references list only floppy disk register values
    - only the disk number is checked for validity
    - the parameters in CX change depending on the number of cylinders;
      the track/cylinder number is a 10 bit value taken from the 2 high
      order bits of CL and the 8 bits in CH (low order 8 bits of track):

      │F│E│D│C│B│A│9│8│7│6│5-0│  CX
       │ │ │ │ │ │ │ │ │ │    └─────    sector number
       │ │ │ │ │ │ │ │ └─┴───────  high order 2 bits of track/cylinder
       └─┴─┴─┴─┴─┴─┴─┴──────────  low order 8 bits of track/cyl number

    - see    ~INT 13,A~

:int 13,3
^INT 13,3 - Write Disk Sectors

    AH = 03
    AL = number of sectors to write  (1-128 dec.)
    CH = track/cylinder number  (0-1023 dec.)
    CL = sector number  (1-17 dec., see below)
    DH = head number  (0-15 dec.)
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)
    ES:BX = pointer to buffer


    on return:
    AH = 0 if CF=0; otherwise disk status  (see ~INT 13,STATUS~)
    AL = number of sectors written
    CF = 0 if successful
       = 1 if error


    - BIOS disk write attempts should reset the controller on error
    - be sure ES:BX does not cross a 64K segment boundary or a
      DMA boundary error will occur
    - IBM PC XT 286 does not require a value in AL, though it is
      recommended that one be supplied for portability
    - many programming references list only floppy disk register values
    - only the disk number is checked for validity
    - the parameters in CX change depending on the number of cylinders;
      the track/cylinder number is a 10 bit value taken from the 2 high
      order bits of CL and the 8 bits in CH (low order 8 bits of track):

      │F│E│D│C│B│A│9│8│7│6│5-0│  CX
       │ │ │ │ │ │ │ │ │ │    └─────    sector number
       │ │ │ │ │ │ │ │ └─┴───────  high order 2 bits of track/cylinder
       └─┴─┴─┴─┴─┴─┴─┴──────────  low order 8 bits of track/cyl number

    - see    ~INT 13,B~

:int 13,4
^INT 13,4 - Verify Disk Sectors

    AH = 04
    AL = number of sectors to verify  (1-128 dec.)
    CH = track/cylinder number  (0-1023 dec., see below)
    CL = sector number  (1-17 dec.)
    DH = head number  (0-15 dec.)
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)
    ES:BX = pointer to buffer

    on return:
    AH = status  (see ~INT 13,STATUS~)
    AL = number of sectors verified
    CF = 0 if successful
       = 1 if error


    - BIOS disk reads should be retried at least three times and the
      controller should be reset upon error detection
    - causes controller to calculate the CRC of the disk data and
      compare it against the CRC stored in the sector header
    - BIOS before 11/15/85 required ES:BX point to a valid buffer
      that doesn't cross DMA boundaries.   More recent BIOS versions
      actually ignore the buffer and the DMA boundary requirement
    - use this function to check for valid formatted diskette in a
      the specified drive and for drive ready for read
    - only the disk number is checked for validity
    - the parameters in CX change depending on the number of cylinders;
      the track/cylinder number is a 10 bit value taken from the 2 high
      order bits of CL and the 8 bits in CH (low order 8 bits of track):

      │F│E│D│C│B│A│9│8│7│6│5-0│  CX
       │ │ │ │ │ │ │ │ │ │    └─────    sector number
       │ │ │ │ │ │ │ │ └─┴───────  high order 2 bits of track/cylinder
       └─┴─┴─┴─┴─┴─┴─┴──────────  low order 8 bits of track/cyl number

    - see    ~DETECTING~

:int 13,5
^INT 13,5 - Format Disk Track

    AH = 05
    AL = interleave value (XT only)
    CX = track/cylinder number (see below for format)
    DH = head number  (0-15 dec.)
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)
    ES:BX = pointer to block of "track address fields" containing
        four byte fields for each sector to be formatted of the form:

           1 byte  track number
           1 byte  head number         Size      #
           1 byte  sector number         Codes   Bytes
           1 byte  sector size code        0      128
                           1      256
                           2      512
                           3     1024
    on return:
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error


    - BIOS disk write attempts should reset the controller on error
    - ~INT 13,17~ should be called to set the DASD type
    - this function is capable of doing great damage if the parameters
      are incorrectly specified; only the drive number is checked
    - initializes disk address fields and data sectors
    - interleave is specified by ordering of track address fields
    - after INT 13 disk format, if the disk is to be used with DOS the
      DOS data structure must be written
    - only the disk number is checked for validity
    - the parameters in CX change depending on the number of cylinders;
      the track/cylinder number is a 10 bit value taken from the 2 high
      order bits of CL and the 8 bits in CH (low order 8 bits of track):

    │F│E│D│C│B│A│9│8│7│6│5-0│  CX (cylinder value 0-1023 dec.)
     │ │ │ │ │ │ │ │ │ │  └─────  unused
     │ │ │ │ │ │ │ │ └─┴───────    high order 2 bits of track/cylinder
     └─┴─┴─┴─┴─┴─┴─┴──────────  low order 8 bits of track/cyl number

:int 13,6
^INT 13,6 - Format Track and Set Bad Sector Flags (XT & portable)

    AH = 06
    AL = Interleave value (XT only)
    BX = format buffer, size = 512 bytes;  the first
         2*(sectors/track) bytes contain F,N for each sector
         F = 00h for good sector,
         F = 80h for bad sector
         N = sector number


    on return:
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error


    - BIOS disk write attempts should reset the controller on error
    - only the disk number is checked for validity
:int 13,7
^INT 13,7 - Format Drive Starting at Specified Track (XT & portable)

    AH = 07
    AL = interleave value (XT only)
    BX = format buffer, size = 512 bytes;  the first 2*(sectors/track)
         bytes contain F, N for each sector where:

        F = 00h for good sector
        F = 80h for bad sector
        N = sector number

    on return:
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error


    - BIOS disk write attempts should reset the controller on error
    - only the disk number is checked for validity
:int 13,8
^INT 13,8 - Get Current Drive Parameters (XT & newer)

    AH = 08
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)


    on return:
    AH = status  (see ~INT 13,STATUS~)
    BL = CMOS drive type
         01 - 5¼  360K         03 - 3½  720K
         02 - 5¼  1.2Mb         04 - 3½ 1.44Mb
    CH = cylinders (0-1023 dec. see below)
    CL = sectors per track    (see below)
    DH = number of sides (0 based)
    DL = number of drives attached
    ES:DI = pointer to 11 byte ~Disk Base Table~ (DBT)
    CF = 0 if successful
       = 1 if error


%    Cylinder and Sectors Per Track Format

    │F│E│D│C│B│A│9│8│7│6│5│4│3│2│1│0│  CX
     │ │ │ │ │ │ │ │ │ │ └─┴─┴─┴─┴─┴──  sectors per track
     │ │ │ │ │ │ │ │ └─┴──────────    high order 2 bits of cylinder count
     └─┴─┴─┴─┴─┴─┴─┴──────────  low order 8 bits of cylinder count

    - the track/cylinder number is a 10 bit value taken from the 2 high
      order bits of CL and the 8 bits in CH (low order 8 bits of track)
    - many good programming references indicate this function is only
      available on the AT, PS/2 and later systems, but all hard disk
      systems since the XT have this function available
    - only the disk number is checked for validity
:int 13,9
^INT 13,9 - Initialize Fixed Disk Table (XT & newer)

    AH = 09
    DL = fixed disk number    (80h=drive 0, 81h=drive 1)


    on return:
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error


    - sets specified fixed disk table to the default values from ROM
    - many good programming references indicate this function is only
      available on the AT, PS/2 and later systems, but all hard disk
      systems since the XT have this function available
    - INT 41h vector is pointer to table for drive 0
    - INT 46h vector is pointer to table for drive 1


^Table    definitions located by interrupt vectors  41H & 46H

%    Offset Size        Description

      00   word  maximum number of cylinders
      02   byte  maximum number of heads
      03   word  starting reduced write current cylinder
      05   word  starting write pre-comp cylinder
      07   byte  maximum ECC data burst length
      08   byte  control byte:


        │7│6│5│4│3│2│1│0│  Control byte
         │ │ │ │ │ └─┴─┴──── drive option
         │ │ └─┴─┴───────── always zero
         │ └────────────── disable ECC retries
         └─────────────── disable access retires
:int 13,a
^INT 13,A - Read Long Sector (XT & newer)

    AH = 0A
    AL = number of sectors    (1-121 dec.)
    CH = track number  (0-1023 dec., see below)
    CL = sector number  (1-17 dec., see below)
    DH = head number  (0-15 dec.)
    DL = fixed drive number  (80h=drive 0, 81h=drive 1)
    ES:BX = address of buffer


    on return:
    AH = status  (see ~INT 13,STATUS~)
    AL = number of sectors actually transferred
    CF = 0 if successful
       = 1 if error


    - BIOS disk reads should be retried at least three times and the
      controller should be reset upon error detection
    - many good programming references indicate this function is only
      available on the AT, PS/2 and later systems, but all hard disk
      systems since the XT have this function available
    - reads regular data sectors (128-1024 bytes) with an additional
      4 byte ECC code included
    - a DMA boundary error will occur if the buffer at ES:BX crosses
      a 64K segment boundary
    - only the disk number is checked for validity
    - the parameters in CX change depending on the number of cylinders;
      the track/cylinder number is a 10 bit value taken from the 2 high
      order bits of CL and the 8 bits in CH (low order 8 bits of track):

      │F│E│D│C│B│A│9│8│7│6│5│4│3│2│1│0│  CX
       │ │ │ │ │ │ │ │ │ │ └─┴─┴─┴─┴─┴────    sector number
       │ │ │ │ │ │ │ │ └─┴────────────  high order 2 bits of track number
       └─┴─┴─┴─┴─┴─┴─┴────────────    low order 8 bits of track number

    - see    ~INT 13,2~

:int 13,b
^INT 13,B - Write Long Sectors (XT & newer)

    AH = 0B
    AL = number of sectors    (1-121 dec.)
    CH = track number  (0-1023 dec., see below)
    CL = sector number  (1-17 dec., see below)
    DH = head number  (0-15 dec.)
    DL = fixed drive number  (80h=drive 0, 81h=drive 1)
    ES:BX = address of buffer


    on return:
    AL = number of sectors actually transferred
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error


    - BIOS disk write attempts should reset the controller on error
    - many good programming references indicate this function is only
      available on the AT, PS/2 and later systems, but all hard disk
      systems since the XT have this function available
    - writes regular sectors (128-1024 bytes) with additional 4 byte
      ECC code included in data
    - a DMA boundary error will occur if the buffer at ES:BX crosses
      a 64K segment boundary
    - only the disk number is checked for validity
    - the parameters in CX change depending on the number of cylinders;
      the track/cylinder number is a 10 bit value taken from the 2 high
      order bits of CL and the 8 bits in CH (low order 8 bits of track):

      │F│E│D│C│B│A│9│8│7│6│5│4│3│2│1│0│  CX
       │ │ │ │ │ │ │ │ │ │ └─┴─┴─┴─┴─┴────    sector number
       │ │ │ │ │ │ │ │ └─┴────────────  high order 2 bits of track number
       └─┴─┴─┴─┴─┴─┴─┴────────────    low order 8 bits of track number

    - see    ~INT 13,3~

:int 13,c
^INT 13,C - Seek to Cylinder (XT & newer)

    AH = 0C
    CH = low order byte of cylinder number (see below)
    CL = high order byte of cylinder number (see below)
    DH = head number (0-15)
    DL = fixed drive number (80h=drive 0, 81h=drive 1)


    on return:
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error


    - many good programming references indicate this function is only
      available on the AT, PS/2 and later systems, but all hard disk
      systems since the XT have this function available
    - notice that CX is in reverse byte format similar to the way data
      is stored in memory and reverse of normal register storage
    - only the disk number is checked for validity
    - the parameters in CX change depending on the number of cylinders;
      the track/cylinder number is a 10 bit value taken from the 2 high
      order bits of CL and the 8 bits in CH (low order 8 bits of track):

      │F│E│D│C│B│A│9│8│7│6│5-0│  CX
       │ │ │ │ │ │ │ │ │ │    └─────    unused
       │ │ │ │ │ │ │ │ └─┴───────  high order 2 bits of track number
       └─┴─┴─┴─┴─┴─┴─┴──────────  low order 8 bits of track number
:int 13,d
^INT 13,D - Alternate Disk Reset  (XT & newer)

    AH = 0D
    DL = fixed drive number  (80h=drive 0, 81h=drive 1)


    on return:
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error


    - many good programming references indicate this function is only
      available on the AT, PS/2 and later systems, but all hard disk
      systems since the XT have this function available
    - used to force drive recalibration similar to ~INT 13,0~
    - drive heads are positioned at track zero
:int 13,e
^INT 13,E - Read Sector Buffer (XT & portable only)


    AH = 0E


    on return:
    AL = number of sectors actually transferred
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error
:int 13,f
^INT 13,F - Write Sector Buffer (XT & portable only)


    AH = 0F


    on return:
    AL = number of sectors actually transferred
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error
:int 13,10
^INT 13,10 - Test for Drive Ready (XT  & newer)


    AH = 10h
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)


    on return:
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error


    - many good programming references indicate this function is only
      available on the AT, PS/2 and later systems, but all hard disk
      systems since the XT have this function available
:int 13,11
^INT 13,11 - Recalibrate Drive (XT & newer)


    AH = 11h
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)


    on return:
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error


    - many good programming references indicate this function is only
      available on the AT, PS/2 and later systems, but all hard disk
      systems since the XT have this function available
:int 13,12
^INT 13,12 - Controller RAM Diagnostic (XT & portable only)


    AH = 12h


    on return:
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error
:int 13,13
^INT 13,13 - Drive Diagnostic (XT & portable only)


    AH = 13h


    on return:
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error
:int 13,14
^INT 13,14 - Controller Internal Diagnostic (XT & newer)


    AH = 14h


    on return:
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error


    - many good programming references indicate this function is only
      available on the AT, PS/2 and later systems, but all hard disk
      systems since the XT have this function available
    - not valid on PS/2 model 30
:int 13,15
^INT 13,15 - Read DASD Type (XT BIOS from 1/10/86 & newer)

    AH = 15h
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)


    on return:
    AH = 00 drive not present
       = 01 diskette, no change detection present
       = 02 diskette, change detection present
       = 03 fixed disk present
    CX:DX = number of fixed disk sectors; if 3 is returned in AH
    CF = 0 if successful
       = 1 if error


    - XT's must have a BIOS date 1/10/86 or newer
    - used to determine if ~INT 13,16~ can detect disk change

    - see    ~INT 13,STATUS~
:int 13,16
^INT 13,16 - Change of Disk Status (XT BIOS from 1/10/86 & newer)


    AH = 16h
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)


    on return:
    AH = 00 no disk change
       = 01 disk changed
    CF = set if disk has been removed or an error occurred


    - used to detect if a disk change has occurred
    - see    ~INT 13,STATUS~    ~INT 13,15~

:int 13,17
^INT 13,17 - Set DASD Type for Format (XT BIOS from 1/10/86 & newer)


    AH = 17h
    AL = 00 no disk
       = 01  320k/360k diskette in 320k/360k drive
       = 02  320k/360k diskette in 1.2Mb drive
       = 03  1.2Mb diskette in 1.2Mb drive
       = 04  720k diskette in 720k drive  (BIOS 6/10/85 & newer)
         720K diskette in 1.44Mb drive (PS/2)
         1.44Mb diskette in 1.44Mb drive (PS/2)
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)


    returns nothing


    - only the disk number is checked for validity
    - tells BIOS format routine about the disk type

:int 13,18
^INT 13,18 - Set Media Type for Format (BIOS date specific)


    AH = 18h
    CH = lower 8 bits of number of tracks  (0-1023 dec., see below)
    CL = sectors per track (1-17 dec., see below)
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)


    on return:
    ES:DI = pointer to 11-byte ~Disk Base Table~ (DBT)
    AH = 00h if requested combination supported
       = 01h if function not available
       = 0Ch if not supported or drive type unknown
       = 80h if there is no media in the drive
    CF = 0 if successful
       = 1 if error


    - valid only for XT BIOS dated after 1/10/86, AT after 11/15/86,
      XT 286 and the PS/2 line
    - only disk number is checked for validity
    - track number is a 10 bit value taken from the 2 high order
      bits of CL and the 8 bits in CH (low order 8 bits of track):

      │F│E│D│C│B│A│9│8│7│6│5│4│3│2│1│0│  CX
       │ │ │ │ │ │ │ │ │ │ └─┴─┴─┴─┴─┴────    sectors per track count
       │ │ │ │ │ │ │ │ └─┴────────────  high order 2 bits track/cyl count
       └─┴─┴─┴─┴─┴─┴─┴────────────    low order 8 bits of track/cyl count

    - see    ~INT 13,STATUS~

:int 13,19
^INT 13,19 - Park Fixed Disk Heads (AT & newer)


    AH = 19h
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)


    on return:
    AH = status  (see ~INT 13,STATUS~)
    CF = 0 if successful
       = 1 if error


    - available only on AT, XT 283 and PS/2 machines
:int 13,1a
^INT 13,1A - Format Unit (PS/2 model 50+)

    AH = 1Ah
    AL = defect table count
    DL = drive number (0=A:, 1=2nd floppy, 80h=drive 0, 81h=drive 1)
    ES:BX = far pointer to defect table
    CL = modifier bits

        │7│6│5│4│3│2│1│0│  Format Unit Modifier Bits
         │ │ │ │ │ │ │ └──── 1=ignore primary defect map, 0=use map
         │ │ │ │ │ │ └───── 1=ignore secondary defect map, 0=use map
         │ │ │ │ │ └────── 1=update secondary defect map, 0=don't
         │ │ │ │ └─────── 1=do extended surface analysis, 0=don't
         │ │ │ └──────── 1=periodic interrupt status on, 0=off
         └─┴─┴───────── reserved

    returns nothing

    - only the disk number is checked for validity
    - see    ~INT 13,STATUS~
:int 14:BIOS serial services:BIOS communications:int 14,status
^INT 14 - BIOS Asynchronous Communications Services

%    For more information, see the following topics:

    ~INT 14,0~  Initialize serial port parameters
    ~INT 14,1~  Send character in AL
    ~INT 14,2~  Receive character in AL
    ~INT 14,3~  Get Serial port status
    ~INT 14,4~  Extended initialize  (PS/2)
    ~INT 14,5~  Extended communication port control (PS/2)


    - all functions have:
      AH = function number
      AL = character to send or receive
      DX = zero based RS232 card number
    - all registers are preserved except AX
    - these functions use hardware flow control
    - used by DOS MODE to redirect LPTx output to a serial device
    - see individual functions for more data


^BIOS Asynchronous Communications Services (Status)

        │7│6│5│4│3│2│1│0│  AL  modem status
         │ │ │ │ │ │ │ └──── clear to send status changed
         │ │ │ │ │ │ └───── data set ready status changed
         │ │ │ │ │ └────── trailing edge ring indicator
         │ │ │ │ └─────── receive line signal changed
         │ │ │ └──────── clear to send
         │ │ └───────── data set ready
         │ └────────── ring indicator
         └─────────── receive line signal detected

        │7│6│5│4│3│2│1│0│  AH  port status
         │ │ │ │ │ │ │ └──── data ready
         │ │ │ │ │ │ └───── overrun error
         │ │ │ │ │ └────── parity error
         │ │ │ │ └─────── framing error
         │ │ │ └──────── break detect
         │ │ └───────── transmit holding register empty
         │ └────────── transmit shift register empty
         └─────────── time out (N/A for functions 1 and 2)

:int 14,0
^INT 14,0 - Initialize Communications Port Parameters

    AH = 00
    AL = parms for initialization (see tables below)
    DX = zero based serial port number (0-1) (0-3 for AT)

    │7│6│5│4│3│2│1│0│  AL               Parity (bits 4 & 3)
     │ │ │ │ │ │ └─┴──── word length bits           00 = none
     │ │ │ │ │ └─────── stop bits flag           01 = odd
     │ │ │ └─┴──────── parity bits               10 = none
     └─┴─┴─────────── baud rate bits           11 = even

%    Word length (bits 1 & 0)        Stop bit count (bit 2)

       10 = 7 bits                  0 = 1 stop bit
       11 = 8 bits                  1 = 2 stop bits

%    Baud rate (bits 7, 6 & 5)

    000 = 110 baud        100 = 1200 baud
    001 = 150 baud        101 = 2400 baud
    010 = 300 baud        110 = 4800 baud
    011 = 600 baud        111 = 9600 baud (4800 on PCjr)


    on return:
    AH = port status
    AL = modem status


    - for status bits see ~INT 14,STATUS~

:int 14,1
^INT 14,1 - Send Character to Communications Port


    AH = 01
    AL = character to send
    DX = zero based serial port number (0-1) (0-3 for AT)


    on return:
    AH = port status  (see ~INT 14,STATUS~)
         bit 7=0 indicates success
         bit 7=1 indicates error, bits 0-6 indicate cause


    - ~INT 14,3~ should be used to determine the actual cause
      of the error since the time-out bit of the status register
      is always set during an error on this call
    - uses hardware flow control
:int 14,2
^INT 14,2 - Receive Character from Communications Port

    AH = 02
    DX = zero based serial port number (0-1) (0-3 for AT)


    on return:
    AH = port status  (see ~INT 14,STATUS~)
         bit 7 = 0 if successful
         bit 7 = 1 if call failed
    AL = character received if call was success


    - ~INT 14,3~ should be used to determine the actual cause of
      the error since the time-out bit of the status register is
      always set during an error on this call
    - uses hardware flow control
:int 14,3
^INT 14,3 - Get Serial Port Status


    AH = 03
    DX = zero based serial port number (0-1) (0-3 for AT)


    on return:
    AH = port status
    AL = modem status

    - for status bits see ~INT 14,STATUS~
    - the status check performs a poll of the port and does not
      perform character I/O
:int 14,4
^INT 14,4 - Serial Port Extended Initialization (PS/2 & later systems)

    AH = 04
    AL = break setting
    BH = parity setting
    BL = stop bit setting
    CH = word length setting
    CL = baud rate setting
    DX = zero based serial port number (0-3)

%             --- Break Setting ---
                  1 = break
                  0 = no break

%    ----- Parity Setting -----      --- Stop Bit Setting ---

       0 = no parity           0 = one stop bit
       1 = odd parity           1 = 2 stop bits
       2 = even parity               1½ if register CH=0
       3 = stick parity odd            (5 bit word length)
       4 = stick parity even

%    -- Word Length Setting --    ---- Baud rate Setting ----

           0 = 5 bits          0 = 110    5 = 2400
           1 = 6 bits          1 = 150    6 = 4800
           2 = 7 bits          2 = 300    7 = 9600
           3 = 8 bits          3 = 600    8 = 19200
                      4 = 1200

    on return:
    AH = port status
    AL = modem status


    - for status bits see ~INT 14,STATUS~

:int 14,5
^INT 14,5 - Extended Communication Port Control (PS/2 & later systems)

    AH = 05
    AL = read or write modem control register
       = 00 read modem control register
       = 01 read modem control register
    DX = zero based serial port number (0-1) (0-3 for AT)

    BL = modem control register

        │7│6│5│4│3│2│1│0│    BL
         │ │ │ │ │ │ │ └──── 1 = activate data terminal ready
         │ │ │ │ │ │ └───── 1 = activate request to send
         │ │ │ │ │ └────── OUT1
         │ │ │ │ └─────── OUT2
         │ │ │ └──────── 0 = normal, 1 = loopback test
         └─┴─┴───────── reserved, forced to zero

    on return:
    AH = port status
    AL = modem status
    BL = ~UART~ modem control register

    - for status bits see ~INT 14,STATUS~

:int 15:BIOS system services:system interrupts
^INT 15 - System BIOS Services

^For more information see the following topics:

     ~INT 15,0~  Turn cassette motor on (PC,PCjr only)
     ~INT 15,1~  Turn cassette motor off (PC,PCjr only)
     ~INT 15,2~  Read blocks from cassette (PC,PCjr only)
     ~INT 15,3~  Write blocks to cassette (PC,PCjr only)
     ~INT 15,F~  PS/2 format periodic interrupt (PS/2 only)
    ~INT 15,20~  PRINT.COM Critical region flag (undocumented DOS 3.x+)
    ~INT 15,21~  PS/2 power on self test (PS/2 except 30)
    ~INT 15,40~  Read / modify profiles (convertible only)
    ~INT 15,41~  Wait on external event (convertible only)
    ~INT 15,42~  Request system power off (convertible only)
    ~INT 15,43~  Read system status (convertible only)
    ~INT 15,44~  (De)activate internal modem power (convertible)
    ~INT 15,4F~  Keyboard intercept  (BIOS date specific)
    ~INT 15,80~  Device open
    ~INT 15,81~  Device close
    ~INT 15,82~  Program termination
    ~INT 15,83~  Event wait
    ~INT 15,84~  Joy-Stick support
    ~INT 15,85~  System request key pressed
    ~INT 15,86~  Elapsed time wait (AT and after)
    ~INT 15,87~  Move block to/from extended memory
    ~INT 15,88~  Extended memory size determination
    ~INT 15,89~  Switch processor to protected mode
    ~INT 15,90~  Device busy
    ~INT 15,91~  Interrupt complete
    ~INT 15,C0~  Return system configuration parameters (PS/2 only)
    ~INT 15,C1~  Return extended BIOS data address (PS/2 only)
    ~INT 15,C2~  Pointing device BIOS interface (PS/2 only)
    ~INT 15,C3~  Enable/Disable watchdog timer
    ~INT 15,C4~  Programmable option select


    - CF is set and AX is left unchanged if a function requested in AH
      isn't supported
    - some Tandy BIOS's fail to restore interrupts on return from INT 15

:int 15,0
^INT 15,0 - Turn Cassette Motor On (PC,PCjr only)


    AH = 00


    - all programs using the cassette must actually turn on
      the device before use
    - there is a noticeable delay between turn-on and device ready
    - a noticeable clicking noise can be heard by switching the
      cassette on and off

    - see ~INT 15,1~

:int 15,1
^INT 15,1 - Turn Cassette Motor Off (PC,PCjr only)


    AH = 01


    - all programs using the cassette should turn off
      the device after use
    - a noticeable clicking noise can be heard by switching the
      cassette on and off

    - see  ~INT 15,0~

:int 15,2
^INT 15,2 - Read Blocks from Cassette (PC,PCjr only)

    AH = 02
    CX = count of bytes to read
    ES:BX = pointer to data buffer


    on return:

    AH = error code if CF=1
       = 1    if CRC error
       = 2    data transition lost, (bit signal scrambled)
       = 3    no data found on tape

    DX = count of bytes actually read
    ES:BX = pointer to byte following last byte read


    - data is transferred in 256 byte blocks, though only the
      number of bytes requested are actually stored

:int 15,3
^INT 15,3 - Write Blocks to Cassette (PC,PCjr only)


    AH = 03
    CX = count of bytes to write
    ES:BX = pointer to data buffer


    on return:

    CX = zero
    ES:BX = pointer to byte following last byte written


    - data is actually written in 256 byte blocks
    - if CX is less than 256, then the block is padded
    - no error detection/correction is available for writing

:int 15,f
^INT 15,F - Format Periodic Interrupt (PS/2 only)

    AH = 0F
    AL = phase code
       = 00  reserved
       = 01  surface analysis
       = 02  formatting

    on return:
    CF = 0 continue formatting/scanning
       = 1 end of format/scanning

    - after formatting/scanning each cylinder the formatting
      routine calls this interrupt
    - may be used by a program to gain control after formatting
      a cylinder
    - if invoked from a non-PS/2 CF=1 and AH contains the following
      AH = 80h  for PC and PCjr
         = 86h  on most other machines
:int 15,20
^INT 15,20 - PRINT.COM Critical Region Flag (undocumented DOS 3.x+)

    AH = 20h
    AL = 00  disable user critical region flag
       = 01  enable user critical region flag
    ES:BX = pointer to user critical region flag byte (AH = 1)


    returns nothing


    - provides PRINT.COM with the address of a byte to increment on
      entry to DOS functions
    - see ~bibliography~ reference for "Undocumented DOS"
:int 15,21
^INT 15,21 - Power On Self Test (POST) Error Log  (PS/2 except 30)

    AH = 21h
    AL = 00 read POST error log
       = 01 write error code to POST error log
    BH = Device code  (if write)
    BL = Device error  (if write)

    on return
    CF = 0 if successful
       = 1 if in error (AH contains code)

%    if reading POST error log:
       AH = 00h if successful
          = 80h (PCjr and PC)     = 86h for all other machines
       BX = number of POST error codes stored
       ES:DI pointer to POST error log

%    if writing POST error log:
       AH = 00h if successful     = 01 POST error log full
          = 80h (PCjr and PC)     = 86h for all other machines
:int 15,40
^INT 15,40 - Read / Modify Profiles (convertible only)


    AH = 40h

    AL = 00 - return system profile in CX,BX
         01 - modify system profile
         CX,BX = profile info


    AL = 02 - return internal modem profile in BX


    AL = 03 - modify internal modem profile
         BX = profile info

:int 15,41
^INT 15,41 - Wait on External Event (convertible only)

    AH = 41h
    AL = condition type:

        │7│6│5│4│3│2│1│0│  AL
         │ │ │ │ │ └─┴─┴──── conditions (codes) to wait for
         │ │ │ │ └───────── reserved
         │ │ │ └────────── 1=port address/0=user byte
         └─┴─┴─────────── reserved

%    Condition codes:

         0 = any external event
         1 = compare and return if equal
         2 = compare and return if not equal
         3 = test and return if not zero
         4 = test and return if zero

    BH = condition compare or mask value
    BL = time-out value times 55 ms, 0 if no time limit
    DX = I/O port address (if AL bit 4 = 1)
    ES:DI = pointer to user byte (if AL bit 4 = 0)

:int 15,42
^INT 15,42 - Request System Power Off (convertible only)


    AH = 42h

    AL = 00 to use system profile
       = 01 to force suspend regardless of profile

:int 15,43
^INT 15,43 - Read System Status (convertible only)


    AH = 43h


    on return
    AL = STATUS:

        │7│6│5│4│3│2│1│0│  AL
         │ │ │ │ │ │ │ └──── LCD detached
         │ │ │ │ │ │ └───── reserved
         │ │ │ │ │ └────── RS232/parallel powered on
         │ │ │ │ └─────── internal modem powered on
         │ │ │ └──────── power activated by alarm
         │ │ └───────── standby power lost
         │ └────────── external power in use
         └─────────── battery low

:int 15,44
^INT 15,44 - (De)activate Internal Modem Power (convertible)


    AH = 44h

    AL = 00 to power off
       = 01 to power on

:int 15,4f
^INT 15,4F - Keyboard Intercept  (BIOS date specific)

    AH = 4F
    AL = scan code
    CF = set to 1 (via STC instruction)

    on return

    AH = 80h, CF set  (PC, PCjr)
       = 86h, CF set  (XT BIOS 11/8/82, AT BIOS 1/10/84)
    AL = CF set, new scan code
       = CF clear, original scancode


    - available with XT BIOS after 11/8/82, AT BIOS after 1/10/84
    - called by ~INT 9~, makes allowance for keyboard translation
    - normally returns the scan code in AL, with CF set
    - if function returns with CF clear, INT 9 ignores keystroke
    - do not rely on this function being called for each INT 9 since
      any user INT 9 handler can exit prematurely and circumvent
      this function
:int 15,80
^INT 15,80 - Device Open


    AH = 80h
    BX = device id
    CX = process id


    on return:
    CF = 0 if successful
       = 1 if error
    AH = 80h for PC & PCjr
       = 86h for XT (BIOS after 11/8/82)


    - available on the AT, XT 286, PS/2 and XT with BIOS after 11/8/82
    - designed for use in simple multitasking

:int 15,81
^INT 15,81 - Device Close


    AH = 81h
    BX = device id
    CX = process id


    on return:
    CF = 0 if successful
       = 1 if error
    AH = 80h for PC & PCjr
       = 86h for XT (BIOS after 11/8/82)


    - available on the AT, XT 286, PS/2 and XT with BIOS after 11/8/82
    - designed for use in simple multitasking

:int 15,82
^INT 15,82 - Program Termination


    AH = 82h
    BX = device id


    on return:
    CF = 0 if successful
       = 1 if error
    AH = 80h for PC & PCjr
       = 86h for XT (BIOS after 11/8/82)


    - available on the AT, XT 286, PS/2 and XT with BIOS after 11/8/82
    - designed for use in simple multitasking

:int 15,83
^INT 15,83 - Event Wait


    AH = 83h
    AL = 0    set interval
       = 1    cancel interval (PS/2)
    CX,DX = number of microseconds to wait (976 µs resolution)
    ES:BX = pointer to memory flag (bit 7 is set
        when interval expires)
            (granularity is 976 microseconds)


    on return:
    CF = 1 if function already busy
    AH = 80h for PC
       = 86h for XT & AT


    - available on the AT (BIOS after 1/10/84) and PS/2's except 30
    - designed for use in simple multitasking

:int 15,84
^INT 15,84 - Joy-Stick Support


    AH = 84h
    DX = 0 to read the current switch settings
       = 1 to read the ~joystick~ position (resistive inputs)


    on return (DX=0, read switch setting):
    CF = 0 if successful
       = 1 if error
    AH = 80h error code if CF set on PC
       = 86h error code if CF set on XT before 11/8/82
    AL = switch settings in bits 7-4 (if read switch function)

    on return (DX=1, read joystick position):
    AX = A(X)
    BX = A(Y)
    CX = B(X)
    DX = B(Y)

:int 15,85
^INT 15,85 - System Request Key Pressed


    AH = 85h
    AL = 00 key pressed
       = 01 key released


    on return:
    CF = 0 if successful
       = 1 if error
    AH = 80h for PC or PCjr
       = 86h for XT (BIOS after 11/8/82)


    - called by BIOS when the System Request key is pressed/released
    - available on machines with newer BIOS and keyboards

:int 15,86
^INT 15,86 - Elapsed Time Wait (AT and PS/2)


    AH = 86h
    CX,DX = number of microseconds to wait (976 µs resolution)


    on return:
    CF = set if error (PC,PCjr,XT)
       = set if wait in progress
       = clear if successful wait
    AH = 80h for PC and PCjr
       = 86h for XT


    - AT and PS/2 only for system timing
    - not designed for user application usage

:int 15,87
^INT 15,87 - Move Block to/from Extended Memory

    AH = 87h
    CX = word count of block to be moved
    ES:SI = pointer to Global Descriptor Table (~GDT~)


    on return:
    CF = 0    if successful (AH contains return code)
       = 1    if error detected
    AH = 0    operation successful (ZF also set)
       = 1    RAM parity error  (if CF set)
       = 2    other exception (if CF set)
       = 3    gate address on line 20h failed (if CF set)
       = 80h on PC and PCjr (if CF set)
       = 86h on XT and newer 808x machines (if CF set)

    - transfers data blocks to and from extended memory on 80286
      and 80386 machines by switching from real to protected mode
      for the duration of the transfer
    - all real mode interrupts are disabled
    - processor shuts down during the switch from protected mode
      to real mode on 286 processors
:int 15,88
^INT 15,88 - Extended Memory Size Determination


    AH = 88h


    on return:
    CF = 80h for PC, PCjr
       = 86h for XT and Model 30
       = other machines, set for error, clear for success
    AX = number of contiguous 1k blocks of memory starting
         at address 1024k (100000h)


    - works only on 80286 and 80386 machines
    - retrieves bytes 30 and 31 from the ~CMOS~ RAM (this data is set
      by the boot memory scan
:int 15,89
^INT 15,89 - Switch Processor to Protected Mode

    AH = 89h
    BH = IRQ8 interrupt vector offset
    BL = IRQ0 interrupt vector offset
    CX = offset into protected mode CS to jump
    ES:SI = pointer to Global Descriptor Table (~GDT~)


    on return:
    CF = 0 if successful
       = 1 if error

    - switches 286 and 386 machines into protected mode to take
      advantage of advanced feature and extended memory
    - Global descriptor table must be setup before interrupt
    - normal BIOS functions are not available after the switch
:int 15,90
^INT 15,90 - Device Busy

    AH = 90h
    AL = type code:
       = 00  disk
       = 01  diskette
       = 02  keyboard
       = 03  pointing device
       = 80  network (ES:BX = NCB)
       = FC  fixed disk reset (PS/2)
       = FD  diskette motor start
       = FE  printer
    ES:BX = pointer to network control block if waiting for network

    on return:
    CF = 0 if wait not satisfied
       = 1 if wait time satisfied
    AH = when CF set, 80h for PC & PCjr, 86h for XT (BIOS 11/8/82)

    - tells the OS a a program is about to wait for a device
    - used for multitasking OS development
:int 15,91
^INT 15,91 - Interrupt Complete

    AH = 91h

    on return:
    CF = 0 if successful
       = 1 if error
    AH = when CF set, 80h for PC & PCjr, 86h for XT (BIOS 11/8/82)
    AL = type code:
       = 00  disk
       = 01  diskette
       = 02  keyboard
       = 03  pointing device
       = 80  network (ES:BX = NCB)
       = FC  fixed disk reset (PS/2)
       = FD  diskette motor start
       = FE  printer

    - should not be used by applications software
    - used by the BIOS to indicate device interrupt is complete
    - used for multitasking OS development
:int 15,c0
^INT 15,C0 - Return System Configuration Parameters (PS/2 only)

    AH = C0

    on return:
    CF = 0 if successful
       = 1 if error
    AH = when CF set, 80h for PC & PCjr, 86h for XT
         (BIOS after 11/8/82) and AT (BIOS after 1/10/84)

    ES:BX = pointer to system descriptor table in ROM of the format:

%    Offset Size         Description

      00   word   length of descriptor (8 minimum)
      02   byte   model byte (same as F000:FFFE, not reliable)
      03   byte   secondary model byte
      04   byte   BIOS revision level (zero based)
      05   byte   feature information, see below
      06   dword  reserved


%    Feature Information

    │7│6│5│4│3│2│1│0│  Feature Byte
     │ │ │ │ │ │ │ └──── reserved
     │ │ │ │ │ │ └───── 0=PC bus, 1=Micro Channel
     │ │ │ │ │ └────── Extended BIOS Data Area (~EBDA~)allocated
     │ │ │ │ └─────── wait for external event supported
     │ │ │ └──────── ~INT 15,4F~ used (kbd intercept)
     │ │ └───────── ~RTC~ present
     │ └────────── 2nd ~8259~ present
     └─────────── DMA channel 3 used by fixed disk BIOS

    - see    ~MACHINE ID~

:int 15,c1
^INT 15,C1 - Return Extended BIOS Data Area Segment (PS/2 only)


    AH = C1


    on return:
    CF = 0 if successful
       = 1 if error
    AH = when CF set, 80h for PC & PCjr, 86h for XT (BIOS 11/8/82)
    ES = ~EBDA~ segment address if PS/2


    - the EBDA is used internally by the PS/2 BIOS routines
    - the EBDA is allocated at the top of user memory by the ~POST~
      routine

:int 15,c2
^INT 15,C2 - Pointing Device BIOS Interface  (PS/2 only)

    AH = C2
    AL = 0    enable/disable pointing device (BH contains flag)
       = 1    reset pointing device
       = 2    set sample rate
       = 3    set resolution
       = 4    read device type
       = 5    pointing device initialization
       = 6    extended commands
       = 7    pointing device far call initialization
    BH = 0 indicates enable for AL = 0
       = 1 indicates disable for AL = 0


    on return:
    CF = 0 if successful
       = 1 if error
    AH = when CF set, 80h for PC & PCjr, 86h for XT, AT
       = 01  invalid function call
       = 02  invalid input
       = 03  interface error
       = 04  resend
       = 05  no far call installed

:int 15,c3
^INT 15,C3 - Enable/Disable Watchdog Timer  (PS/2)

    AH = C3
    AL = 0    disable watchdog time-out
       = 1    enable watchdog time-out
    BX = watchdog timer count (1-255)

    on return:
    CF = 0 if successful
       = 1 if error
    AH = when CF set, 80h for PC & PCjr, 86h for XT, AT & Model 30


    - the watchdog timer is available only on PS/2 with 80286 and
      80386 processors.
    - the ~timer~ uses channel timer channel 3 and IRQ0.  Activated
      when IRQ0 is active for more than one channel 0 timer cycle,
      which causes the timer to be decremented.   When the watch-
      dog timer reaches 0, and NMI is generated.
:int 15,c4
^INT 15,C4 - Programmable Option Select  (PS/2)

    AH = C4
    AL = 0    get Programmable Option Select adapter register address
       = 1    enable slot for setup
       = 2    adapter enable


    on return:
    CF = 0 if successful
       = 1 if error
    AH = when CF set, 80h for PC & PCjr, 86h for XT, AT & 30
    BL = slot number (function AL=1)
    DL = POS adapter register address (function AL=0)


    - allows access to PS/2 Programmable Option Select registers
    - available on 80286 and 80386 PS/2's

:int 16:BIOS keyboard services
^INT 16 - Keyboard BIOS Services


^For more information, see the following topics:

    ~INT 16,0~   Wait for keystroke and read
    ~INT 16,1~   Get keystroke status
    ~INT 16,2~   Get shift status
    ~INT 16,3~   Set keyboard typematic rate (AT+)
    ~INT 16,4~   Keyboard click adjustment (AT+)
    ~INT 16,5~   Keyboard buffer write  (AT,PS/2 enhanced keyboards)
    ~INT 16,10~  Wait for keystroke and read  (AT,PS/2 enhanced keyboards)
    ~INT 16,11~  Get keystroke status  (AT,PS/2 enhanced keyboards)
    ~INT 16,12~  Get shift status  (AT,PS/2 enhanced keyboards)


    - with IBM BIOS's, INT 16 functions do not restore the flags to
      the pre-interrupt state to allow returning of information via
      the flags register
    - functions 3 through 12h are not available on all AT machines
      unless the extended keyboard BIOS is present
    - all registers are preserved except AX and FLAGS
    - see    ~SCAN CODES~
:int 16,0
^INT 16,0 - Wait for Keypress and Read Character


    AH = 00


    on return:
    AH = keyboard scan code
    AL = ASCII character or zero if special function key


    - halts program until key with a scancode is pressed
    - see  ~SCAN CODES~

:int 16,1
^INT 16,1 - Get Keyboard Status


    AH = 01


    on return:
    ZF = 0 if a key pressed (even Ctrl-Break)
    AX = 0 if no scan code is available
    AH = ~scan code~
    AL = ASCII character or zero if special function key


    - data code is not removed from buffer
    - ~Ctrl-Break~ places a zero word in the keyboard buffer but does
      register a keypress.
:int 16,2
^INT 16,2 - Read Keyboard Flags

    AH = 02


    on return:
    AL = BIOS keyboard flags (located in ~BIOS Data Area~ 40:17)

        │7│6│5│4│3│2│1│0│  AL or BIOS Data Area 40:17
         │ │ │ │ │ │ │ └──── right shift key depressed
         │ │ │ │ │ │ └───── left shift key depressed
         │ │ │ │ │ └────── CTRL key depressed
         │ │ │ │ └─────── ALT key depressed
         │ │ │ └──────── scroll-lock is active
         │ │ └───────── num-lock is active
         │ └────────── caps-lock is active
         └─────────── insert is active
:int 16,3
^INT 16,3 - Set Keyboard Typematic Rate  (AT+)

    AH = 03
    AL = 00  set typematic rate to default
         01  increase initial delay
         02  slow typematic rate by 1/2
         04  turn off typematic chars
         05  set typematic rate/delay

    BH = repeat delay (AL=5)
         0 = 250ms       2 = 750ms
         1 = 500ms       3 = 1000ms
    BL = typematic rate, one of the following  (AL=5)

         00 - 30.0        01 - 26.7       02 - 24.0      03 - 21.8
         04 - 20.0        05 - 18.5       06 - 17.1      07 - 16.0
         08 - 15.0        09 - 13.3       0A - 12.0      0B - 10.9
         0C - 10.0        0D - 9.2       0E - 8.6      0F - 8.0
         10 - 7.5        11 - 6.7       12 - 6.0      13 - 5.5
         14 - 5.0        15 - 4.6       16 - 4.3      17 - 4.0
         18 - 3.7        19 - 3.3       1A - 3.0      1B - 2.7
         1C - 2.5        1D - 2.3       1E - 2.1      1F - 2.0

    returns nothing

    - if the typematic rate is not within range,no action is taken
    - available on AT and PS/2 machines with extended keyboard support
    - see    ~KEYBOARD COMMANDS~
:int 16,4
^INT 16,4 - Keyboard Click Adjustment (AT+)


    AH = 04
    AL = 1 for click on
       = 0 for click off


    - available only on AT and later machines that support the extended
      keyboard BIOS
:int 16,5
^INT 16,5 - Keyboard Buffer Write  (AT+)


    AH = 05
    CH = ~scan code~
    CL = ASCII character


    on return:
    AL = 00 if success
         01 if buffer full


    - available on AT and PS/2 machines with extended keyboard support
    - stores normal keystroke into keyboard buffer
    - will not store attribute keys like Shift, Alt, Ctrl, etc...

:int 16,10
^INT 16,10 - Extended Wait for Keypress and Read Character  (AT+)


    AH = 10h


    on return:
    AH = ~scan code~
    AL = ASCII character or zero if special function key


    - available on AT and PS/2 machines with extended keyboard support
    - similar to ~INT 16,0~
:int 16,11
^INT 16,11 - Extended Get Keyboard Status  (AT+)


    AH = 11h


    on return:
    ZF = 0 if key pressed (data waiting)
    AX = 0 if no scan code is available
    AH = ~scan code~
    AL = ASCII character or zero if special function key


    - available on AT and PS/2 machines with extended keyboard support
    - data is not removed from buffer
    - similar to ~INT 16,1~

:int 16,12
^INT 16,12 - Extended Get Keyboard Status  (AT+)

    AH = 12h

    on return:
    AH = BIOS keyboard flags (~BIOS Data Area~ location 40:18)

        │7│6│5│4│3│2│1│0│  AH
         │ │ │ │ │ │ │ └──── left CTRL key depressed
         │ │ │ │ │ │ └───── left ALT key depressed
         │ │ │ │ │ └────── right CTRL key pressed
         │ │ │ │ └─────── right ALT key depressed
         │ │ │ └──────── scroll-lock depressed
         │ │ └───────── num-lock key depressed
         │ └────────── caps-lock key depressed
         └─────────── system request key depressed

    AL = BIOS keyboard flags (BIOS Data Area location 40:17)

        │7│6│5│4│3│2│1│0│  AL
         │ │ │ │ │ │ │ └──── right shift key depressed
         │ │ │ │ │ │ └───── left shift key depressed
         │ │ │ │ │ └────── CTRL key depressed
         │ │ │ │ └─────── ALT key depressed
         │ │ │ └──────── scroll-lock is active
         │ │ └───────── num-lock is active
         │ └────────── caps-lock is active
         └─────────── insert is active


    - available on AT and PS/2 machines with extended keyboard support
    - similar to ~INT 16,2~

    - see    ~KB FLAGS~

:int 17:BIOS printer services:int 17,status:printer status
^INT 17 - Printer BIOS Services

%    For more information, see the following topics:

    ~INT 17,0~   Print character
    ~INT 17,1~   Initialize printer port
    ~INT 17,2~   Read printer port status

    Status flags returned in register AH

    │7│6│5│4│3│2│1│0│  AH (status)
     │ │ │ │ │ │ │ └────  time-out
     │ │ │ │ │ └─┴─────  unused
     │ │ │ │ └────────  1 = I/O error  (~parallel~ pin 15)
     │ │ │ └─────────  1 = printer selected/on-line  (parallel pin 13)
     │ │ └──────────  1 = out of paper  (parallel pin 12)
     │ └───────────  1 = printer acknowledgment  (parallel pin 10)
     └────────────    1 = printer not busy  (parallel pin 11)

    - on the AT, XT 286, and PS/2 when the BIOS determines the printer
      is busy,  ~INT 15,90~  is executed
    - all printers do not return reliable status information;  bit 3 and
      bit 7 are usually reliable.
    - all registers are preserved except AH
:int 17,0
^INT 17,0 - Print Character


    AH = 00
    AL = character to print
    DX = printer to be used (0-2)


    on return:
    AH = printer status, see ~INT 17,STATUS~


    - writes character and returns status
:int 17,1
^INT 17,1 - Initialize Printer Port


    AH = 01
    DX = printer port to initialize (0-2)


    on return:
    AH = status, see ~INT 17,STATUS~


    - initializes printer port and returns status
    - outputs characters 0x08 and 0x0C to printer port which
      reset ~Epson~ and IBM printers, but may produce undesirable
      effects on other printers

:int 17,2
^INT 17,2 - Read Printer Port Status

    AH = 02
    DX = printer port to be used (0-2)


    on return:
    AH = status:

        │7│6│5│4│3│2│1│0│  Printer status bits
         │ │ │ │ │ │ │ └──── time out
         │ │ │ │ │ └─┴───── unused
         │ │ │ │ └──────── I/O error
         │ │ │ └───────── selected
         │ │ └────────── out of paper
         │ └─────────── acknowledge
         └──────────── not busy


    - returns status of specified printer port
:int 19
^INT 19 - Bootstrap Loader


    DL = physical drive where boot sector is located


    no output


    - track 0, sector 1 is loaded into address 0:7C00
      and control is transferred there
    - not a preferred method for rebooting by applications.  A
      better method is to set the word at location 40:72 to 1234h
      and jump to location FFFF:0 in ROM
    - memory is not cleared when rebooted through this interrupt

    - see    ~WARM BOOT~

:int 1a:BIOS clock services
^INT 1A - System and Real Time Clock BIOS Services


%    For more informations, see the following topics:

    ~INT 1A,0~   Read system clock counter
    ~INT 1A,1~   Set system clock counter
    ~INT 1A,2~   Read real time clock time (AT,PS/2)
    ~INT 1A,3~   Set real time clock time (AT,PS/2)
    ~INT 1A,4~   Read real time clock date (AT,PS/2)
    ~INT 1A,5~   Set real time clock date (AT,PS/2)
    ~INT 1A,6~   Set real time clock alarm (AT,PS/2)
    ~INT 1A,7~   Reset real time clock alarm (PS/2)
    ~INT 1A,8~   Set RTC activated power on mode (convertible,PS/2)
    ~INT 1A,9~   Read RTC alarm time and status (convertible,PS/2)
    ~INT 1A,A~   Read system day counter (PS/2)
    ~INT 1A,B~   Set system day counter (PS/2)
    ~INT 1A,80~  Set up sound multiplexer (PCjr only)

    - function is specified in register AH
    - see    ~RTC~
:int 1a,0
^INT 1A,0 - Read System Clock Counter


    AH = 00


    on return:
    AL = midnight flag, 1 if 24 hours passed since reset
    CX = high order word of tick count
    DX = low order word of tick count


    - incremented approximately 18.206 times per second
    - at midnight CX:DX is zero
    - this function can be called in a program to assure the date is
      updated after midnight;  this will avoid the passing two midnights
      date problem
:int 1a,1
^INT 1A,1 - Set System Clock Counter


    AH = 01
    CX = high order word of tick count
    DX = low order word of tick count


    returns nothing


    - CX:DX should be set to the number of seconds past
      midnight multiplied by approximately 18.206

:int 1a,2
^INT 1A,2 - Read Time From Real Time Clock  (XT 286,AT,PS/2)


    AH = 02


    on return:
    CF = 0 if successful
       = 1 if error, ~RTC~ not operating
    CH = hours in BCD
    CL = minutes in BCD
    DH = seconds in BCD
    DL = 1 if daylight savings time option


    - on AT with BIOS before 6/10/85, DL is not returned
:int 1a,3
^INT 1A,3 - Set Time on Real Time Clock  (XT 286,AT,PS/2)


    AH = 03
    CH = hours in BCD
    CL = minutes in BCD
    DH = seconds in BCD
    DL = 1 if daylight savings time option
       = 0 if standard time


    returns nothing


    - clock values must be in BCD
    - see    ~RTC~
:int 1a,4
^INT 1A,4 - Read Real Time Clock Date  (XT 286,AT,PS/2)


    AH = 04


    on return:
    CH = century in BCD (decimal 19 or 20)
    CL = year in BCD
    DH = month in BCD
    DL = day in BCD
    CF = 0 if successful
       = 1 if error or clock not operating

    - calling this interrupt will update the DOS maintained date and
      reset the ~BIOS Data Area~ date rollover flag at 40:70
    - see    ~RTC~
:int 1a,5
^INT 1A,5 - Set Real Time Clock Date  (XT 286,AT,PS/2)


    AH = 05
    CH = century in BCD (decimal 19 or 20)
    CL = year in BCD
    DH = month in BCD
    DL = day in BCD


    returns nothing


    - all values must be in BCD
    - see    ~RTC~
:int 1a,6
^INT 1A,6 - Set Real Time Clock Alarm  (XT 286,AT,PS/2)


    AH = 06
    CH = hours in BCD
    CL = minutes in BCD
    DH = seconds in BCD


    on return:
    CF = 1 if alarm already set or clock inoperable


    - alarm setting is not relative like some sources claim, but the
      actually clock time the interrupt should occur
    - settings values must be in BCD
    - when the alarm time is reached ~INT 4A~ is executed
    - INT 4A vector should be replaced with address of the alarm
      handling interrupt routine before setting the alarm
    - ~INT 1A,7~ should be called to disable the ~RTC~ alarm
:int 1a,7
^INT 1A,7 - Disable Real Time Clock Alarm  (XT,AT,PS/2)


    AH = 07


    returns nothing


    - should be called previous to setting or resetting the
      ~RTC~ alarm with ~INT 1A,6~
:int 1a,8
^INT 1A,8 - Set RTC Activated Power On Mode (convertible)


    AH = 08
    CH = hours in BCD
    CL = minutes in BCD
    DH = seconds in BCD


    - see    ~RTC~
:int 1a,9
^INT 1A,9 - Read RTC Alarm Time and Status (convertible,PS/2)


    AH = 09


    on return:
    CH = hours in BCD
    CL = minutes in BCD
    DH = seconds in BCD
    DL = alarm status:
       = 00 if alarm not enabled (AIE=0)
       = 01 if alarm enabled but will not power up system (AIE=1)
       = 02 if alarm will power up system (AIE=1)


    - see    ~RTC~
:int 1a,a
^INT 1A,A - Read System Day Counter (PS/2)


    AH = 0Ah


    on return:
    CX = count of days since 1-1-1980

:int 1a,b
^INT 1A,B - Set System Day Counter (PS/2)


    AH = 0Bh
    CX = count of days since 1-1-1980

:int 1a,80
^INT 1A,80 - Setup Sound Multiplexer  (PCjr only)


    AL = 00 - source is ~8253~ channel 2
       = 01 - source is cassette input
       = 02 - source is I/O channel "AUDIO IN"
       = 03 - source is sound generator chip

:int 1b:Ctrl-Break:BIOS break interrupt
^INT 1B - BIOS Ctrl-Break Handler Address


    - holds address of the BIOS Ctrl-Break interrupt handler
    - called by ~INT 9~ after Ctrl-Break key is pressed; INT 9 sets the
      BIOS keyboard head and tail pointers to the buffer start and
      places a zero word in the buffer
    - the BIOS initially sets this value to point to a simple ~IRET~ but
      DOS re-vectors this to its own code, usually 2 lines of code that
      sets an internal flag (to 3) and then returns via IRET.
    - DOS checks this flag on entry to many of its subfunctions.  If the
      flag is set, it invokes ~INT 23~.
    - pointing this address to a null function with and IRET disables
      Ctrl-Break aborts
    - if the INT 1B code chooses to retain control of the system, it
      must issue an EOI for any interrupt pending on the ~8259~ and reset
      all I/O devices
    - should be terminated via an IRET
    - should not be called directly by user application
:int 1c:user timer routine
^INT 1C - System Timer Tick (User Routine)


    - called by ~INT 8~, approximately 18.206 times per second
      (unless the program modifies the ~8253~ PIT
    - this is a user routine which the BIOS defaults to a simple ~IRET~
    - this interrupt vector can be used for TSR popup utilities,
      animated graphics updates and event polling
    - since many TSR's don't maintain the integrity of the interrupt
      call chain, relying on this for TSR popup isn't recommended
:int 1d
^INT 1D - Video Initialization Parameter Table Vector


    - not a true interrupt, but a far pointer to a video
      initialization parameter table for video controllers

:int 1e
^INT 1E - Disk Initialization Parameter Table Vector

    - not an interrupt, but a far pointer to the diskette base table
    - this table contains initialization parameters for the disk
      controller used during formatting, reading, and writing to a disk

%    Disk Initialization Parameter Table Vector Definition:

%    Offset       Description

      00  │7│6│5│4│3│2│1│0│  Disk Controller Mode byte 1
           │ │ │ │ └─┴─┴─┴────  head step rate in milliseconds (0-F)
           └─┴─┴─┴───────────  head unload time in milliseconds (0-F)

      01  │7│6│5│4│3│2│1│0│  Disk Controller Mode byte 2
           │ │ │ │ │ │ │ └──── DMA flag (0 = use DMA)
           └─┴─┴─┴─┴─┴─┴───── (head load time/2)-1 in milliseconds

      02  clock ticks until motor off
      03  FM or MFM Mode:  Log2 (Bytes per Sector/128)
          FM=Frequency Modulation used on single-density disks
          MFM=Modified Frequency Modulation used on double-sided disks
      04  last sector on track
      05  gap length in bytes:

             Bytes Per     Sectors     Write    Format
          MFM     Sector    Per Track     Gap    Gap
           1    256        18          0Ah    0Ch
           1    256        16          20h    32h
           2    512         8          2Ah    50h
           2    512         9          1Bh    6Ch
           3       1024         4          80h    F0h
           4       2048         2          C8h    FFh
           5       4096         1          C8h    FFh

      06  disk data length
          80h for 128 bytes/sector
          FFh otherwise
      07  gap length when formatting
      08  data pattern used during formatting
      09  floppy head bounce delay (in milliseconds)
      0A  floppy motor start delay (in 0.125 second intervals)
      0B  floppy motor stop delay (in 0.25 second intervals)

    - see ~INT 13,18~

:int 1f
^INT 1F - Graphics Display Character Bit Map Table


    - not a true interrupt, but a far pointer to a table of
      character bit maps for the graphics mode representation of
      ASCII characters 128 through 255
    - can be used to change character fonts in CGA graphics mode

:DOS interrupts
^DOS Interrupt Summary


%    See the following topics for more information:

    ~INT 20~  Program terminate
    ~INT 21~  DOS Function Dispatcher
    ~INT 22~  Program Terminate
    ~INT 23~  Ctl-break exit address
    ~INT 24~  Critical error handler address
    ~INT 25~  Absolute disk read
    ~INT 26~  Absolute disk write
    ~INT 27~  Terminate but stay resident
    ~INT 28~  DOS idle loop/scheduler (undocumented)
    ~INT 29~  Fast character output (undocumented)
    ~INT 2E~  Execute command using base level COMMAND.COM (undoc.)
    ~INT 2F~  Multiplex interrupt (DOS 3.x+)

:int 20
^INT 20 - Program Terminate


    CS = address of ~PSP~


    returns nothing


    - INT 20 restores the terminate, ~Ctrl-Break~, and critical error
      exit addresses from values in the PSP, flushes all buffers, frees
      memory and returns to DOS via the termination handler address
    - does not close ~FCB~s
    - this function is no longer recommended, but can be used by
      version of DOS before 2.0, see  ~INT 21,4C~  and  ~INT 21,0~

:int 21:DOS services:DOS functions
^INT 21 - DOS Function Dispatcher

^For more information, see the following topics:

    ~INT 21,0~  Program terminate
    ~INT 21,1~  Keyboard input with echo
    ~INT 21,2~  Display output
    ~INT 21,3~  Wait for auxiliary device input
    ~INT 21,4~  Auxiliary output
    ~INT 21,5~  Printer output
    ~INT 21,6~  Direct console I/O
    ~INT 21,7~  Wait for direct console input without echo
    ~INT 21,8~  Wait for console input without echo
    ~INT 21,9~  Print string
    ~INT 21,A~  Buffered keyboard input
    ~INT 21,B~  Check standard input status
    ~INT 21,C~  Clear keyboard buffer, invoke keyboard function
    ~INT 21,D~  Disk reset
    ~INT 21,E~  Select disk
    ~INT 21,F~  Open file using ~FCB~
    ~INT 21,10~  Close file using FCB
    ~INT 21,11~  Search for first entry using FCB
    ~INT 21,12~  Search for next entry using FCB
    ~INT 21,13~  Delete file using FCB
    ~INT 21,14~  Sequential read using FCB
    ~INT 21,15~  Sequential write using FCB
    ~INT 21,16~  Create a file using FCB
    ~INT 21,17~  Rename file using FCB
    ~INT 21,18~  DOS dummy function (CP/M) (not used/listed)
    ~INT 21,19~  Get current default drive
    ~INT 21,1A~  Set disk transfer address
    ~INT 21,1B~  Get allocation table information
    ~INT 21,1C~  Get allocation table info for specific device
    ~INT 21,1D~  DOS dummy function (CP/M) (not used/listed)
    ~INT 21,1E~  DOS dummy function (CP/M) (not used/listed)
    ~INT 21,1F~  Get pointer to default drive parameter table (undocumented)
    ~INT 21,20~  DOS dummy function (CP/M) (not used/listed)
    ~INT 21,21~  Random read using ~FCB~
    ~INT 21,22~  Random write using FCB
    ~INT 21,23~  Get file size using FCB
    ~INT 21,24~  Set relative record field for FCB
    ~INT 21,25~  Set interrupt vector
    ~INT 21,26~  Create new program segment
    ~INT 21,27~  Random block read using FCB
    ~INT 21,28~  Random block write using FCB
    ~INT 21,29~  Parse filename for FCB
    ~INT 21,2A~  Get date
    ~INT 21,2B~  Set date
    ~INT 21,2C~  Get time
    ~INT 21,2D~  Set time
    ~INT 21,2E~  Set/reset verify switch
    ~INT 21,2F~  Get disk transfer address
    ~INT 21,30~  Get DOS version number
    ~INT 21,31~  Terminate process and remain resident
    ~INT 21,32~  Get pointer to drive parameter table (undocumented)
    ~INT 21,33~  Get/set ~Ctrl-Break~ check state & get boot drive
    ~INT 21,34~  Get address to DOS critical flag (undocumented)
    ~INT 21,35~  Get vector
    ~INT 21,36~  Get disk free space
    ~INT 21,37~  Get/set switch character (undocumented)
    ~INT 21,38~  Get/set country dependent information
    ~INT 21,39~  Create subdirectory (mkdir)
    ~INT 21,3A~  Remove subdirectory (rmdir)
    ~INT 21,3B~  Change current subdirectory (chdir)
    ~INT 21,3C~  Create file using handle
    ~INT 21,3D~  Open file using handle
    ~INT 21,3E~  Close file using handle
    ~INT 21,3F~  Read file or device using handle
    ~INT 21,40~  Write file or device using handle
    ~INT 21,41~  Delete file
    ~INT 21,42~  Move file pointer using handle
    ~INT 21,43~  Change file mode
    ~INT 21,44~  I/O control for devices (~IOCTL~)
    ~INT 21,45~  Duplicate file handle
    ~INT 21,46~  Force duplicate file handle
    ~INT 21,47~  Get current directory
    ~INT 21,48~  Allocate memory blocks
    ~INT 21,49~  Free allocated memory blocks
    ~INT 21,4A~  Modify allocated memory blocks
    ~INT 21,4B~  EXEC load and execute program (func 1 undocumented)
    ~INT 21,4C~  Terminate process with return code
    ~INT 21,4D~  Get return code of a sub-process
    ~INT 21,4E~  Find first matching file
    ~INT 21,4F~  Find next matching file
    ~INT 21,50~  Set current process id (undocumented)
    ~INT 21,51~  Get current process id (undocumented)
    ~INT 21,52~  Get pointer to DOS "INVARS" (undocumented)
    ~INT 21,53~  Generate drive parameter table (undocumented)
    ~INT 21,54~  Get verify setting
    ~INT 21,55~  Create ~PSP~ (undocumented)
    ~INT 21,56~  Rename file
    ~INT 21,57~  Get/set file date and time using handle
    ~INT 21,58~  Get/set memory allocation strategy (3.x+, undocumented)
    ~INT 21,59~  Get extended error information (3.x+)
    ~INT 21,5A~  Create temporary file (3.x+)
    ~INT 21,5B~  Create new file (3.x+)
    ~INT 21,5C~  Lock/unlock file access (3.x+)
    ~INT 21,5D~  Critical error information (undocumented 3.x+)
    ~INT 21,5E~  Network services (3.1+)
    ~INT 21,5F~  Network redirection (3.1+)
    ~INT 21,60~  Get fully qualified file name (undocumented 3.x+)
    ~INT 21,62~  Get address of program segment prefix (3.x+)
    ~INT 21,63~  Get system lead byte table (MSDOS 2.25 only)
    ~INT 21,64~  Set device driver look ahead  (undocumented 3.3+)
    ~INT 21,65~  Get extended country information (3.3+)
    ~INT 21,66~  Get/set global code page (3.3+)
    ~INT 21,67~  Set handle count (3.3+)
    ~INT 21,68~  Flush buffer (3.3+)
    ~INT 21,69~  Get/set disk serial number (undocumented DOS 4.0+)
    ~INT 21,6A~  DOS reserved (DOS 4.0+)
    ~INT 21,6B~  DOS reserved
    ~INT 21,6C~  Extended open/create (4.x+)
    ~INT 21,F8~  Set OEM INT 21 handler (functions F9-FF) (undocumented)


    - int 21 functions are called with the function number in AH
    - register AX may be altered, its contents are not guaranteed
    - if an error occurs, CF is set to 1 and AX contains a simple
      error code;    ~INT 21,59~ can be used to determine cause.
    - most INT 21 functions do not restore the flags to pre-interrupt
      state to allow returning of information via the flags register

:int 21,0
^INT 21,0 - Program Terminate


    AH = 00
    CS = ~PSP~ segment address


    returns nothing


    - restores the terminate, ~Ctrl-Break~, and critical error
      exit addresses, flushes all buffers, frees memory
      and returns to DOS via the termination handler address
    - does not close ~FCB~s
    - this function is no longer recommended, but can be used by
      version of DOS before 2.0, see  ~INT 21,4C~  and  ~INT 20~

:int 21,1
^INT 21,1 - Keyboard Input with Echo


    AH = 01


    on return:
    AL = character from standard input device


    - waits for keyboard input from STDIN and echoes to STDOUT
    - returns 0 for extended keystroke, then function must be
      called again to return scan code
    - if ~Ctrl-Break~ is detected, ~INT 23~ is executed

:int 21,2
^INT 21,2 - Display Output


    AH = 02
    DL = character to output


    returns nothing


    - outputs character to STDOUT
    - backspace is treated as non-destructive cursor left
    - if ~Ctrl-Break~ is detected, ~INT 23~ is executed

:int 21,3
^INT 21,3 - Wait for Auxiliary Device Input


    AH = 03


    on return:
    AL = character from the auxiliary device


    - does not supply error returns
    - waits for character and reads from STDAUX
    - default DOS AUX parameters are 2400,N,8,1

:int 21,4
^INT 21,4 - Auxiliary Output


    AH = 04
    DL = character to output


    returns nothing


    - sends character in DL to STDAUX
    - does not supply error returns
    - waits until STDAUX is available
    - default DOS AUX parameters are 2400,N,8,1

:int 21,5
^INT 21,5 - Printer Output


    AH = 05
    DL = character to output


    returns nothing


    - sends character in DL to STDPRN
    - waits until STDPRN device is ready before output

:int 21,6
^INT 21,6 - Direct Console I/O

    AH = 06
    DL = (0-FE) character to output
       = FF if console input request

    on return:
    AL = input character if console input request (DL=FF)
    ZF = 0    if console request character available (in AL)
       = 1    if no character is ready, and function request
         was console input


    - reads from or writes to the console device depending on
      the value of DL
    - cannot output character FF  (DL=FF indicates read function)
    - for console read, no echo is produced
    - returns 0 for extended keystroke, then function must be
      called again to return scan code
    - ignores Ctrl-Break and Ctrl-PrtSc

:int 21,7
^INT 21,7 - Direct Console Input Without Echo


    AH = 07


    on return:
    AL = character from STDIN


    - waits for keyboard input until keystroke is ready
    - character is not echoed to STDOUT
    - returns 0 for extended keystroke, then function must be
      called again to return scan code
    - ignores Ctrl-Break and Ctrl-PrtSc
    - see    ~INT 21,1~

:int 21,8
^INT 21,8 - Console Input Without Echo


    AH = 08


    on return:
    AL = character from STDIN


    - returns 0 for extended keystroke, then function must be
      called again to return scan code
    - waits for character from STDIN and returns data in AL
    - if ~Ctrl-Break~ is detected, ~INT 23~ is executed

:int 21,9
^INT 21,9 - Print String


    AH = 09
    DS:DX = pointer to string ending in "$"


    returns nothing


    - outputs character string to STDOUT up to "$"
    - backspace is treated as non-destructive
    - if ~Ctrl-Break~ is detected, ~INT 23~ is executed

:int 21,a
^INT 21,A - Buffered Keyboard Input

    AH = 0A
    DS:DX = pointer to input buffer of the format:

    │ max │ count │  BUFFER (N bytes)
       │      │         └────── input buffer
       │      └──────────── number of characters returned (byte)
       └────────────── maximum number of characters to read (byte)


    returns nothing

    - since strings can be pre-loaded, it is recommended that the
      default string be terminated with a CR
    - N bytes of data are read from STDIN into buffer+2
    - max buffer size is 255, minimum buffer size is 1 byte
    - chars up to and including a CR are placed into the buffer
      beginning at byte 2;    Byte 1 returns the number of chars
      placed into the buffer  (extended codes take 2 characters)
    - DOS editing keys are active during this call
    - ~INT 23~ is called if Ctrl-Break or Ctrl-C detected

:int 21,b
^INT 21,B - Check Standard Input Status


    AH = 0B


    on return:
    AL = 00 if no character available
       = FF if character available


    - checks STDIN for available characters
    - character is not returned
    - if Ctrl-Break is detected ~INT 23~ is executed

:int 21,c
^INT 21,C - Clear Keyboard Buffer and Invoke Keyboard Function


    AH = 0C
    AL = 01, 06, 07, 08 or 0A   (INT 21 input functions)


    on return:
    see return values from INT 21,AL where AL is 1, 6, 7, 8 or A


    - main function is to clear the input buffer and call INT 21h with
      the specified function (in AL)
    - see ~INT 21,1~,  ~INT 21,6~,  ~INT 21,7~,  ~INT 21,8~ & ~INT 21,A~

:int 21,d
^INT 21,D - Disk Reset


    AH = 0D


    returns nothing


    - all file buffers are flushed to disk
    - does NOT update directory entry

:int 21,e
^INT 21,E - Select Disk


    AH = 0E
    DL = zero based, drive number (0-25, A: - Z:)


    on return:
    AL = one based, total number of logical drives including
         hardfiles (1-26)


    - for DOS 3.x+, this function returns the number of logical
      drives or the value of LASTDRIVE (default of 5) in the
      CONFIG.SYS file

:int 21,f
^INT 21,F - Open a File Using FCB

    AH = 0F
    DS:DX = pointer to unopened ~FCB~


    on return:
    AL = 00 if file opened
       = FF if unable to open


    - opens an existing file using a previously setup FCB
    - the FCB fields drive identifier, filename and extension
      must be filled in before call
    - sets default FCB fields; current block number is set to 0;
      record size is set to 80h; file size, date and time are set
      to the values from the directory
    - does not create file, see  ~INT 21,16~
    - DOS 2.x allows opening of subdirectories, DOS 3.x does not
:int 21,10
^INT 21,10 - Close a File Using FCB


    AH = 10h
    DS:DX = pointer to opened ~FCB~


    on return:
    AL = 00  if file closed
       = FF  if file not closed


    - closes a previously opened file opened with an FCB
    - FCB must be setup with drive id, filename, and extension
      before call

:int 21,11
^INT 21,11 - Search for First Entry Using FCB

    AH = 11h
    DS:DX = pointer to unopened ~FCB~

    on return:
    AL = 00 if matching file found
       = FF if file not found

    - searches for first occurrence of filename specified in FCB
    - FCB must have drive id, filename, and extension before call
    - extended FCB can be used to specify a search criteria based
      on attributes; hidden, system, label, and directory attributes
      can be used to narrow the search  (see ~FILE ATTRIBUTES~)
    - after successful call DTA holds an unopened ~FCB~/~XFCB~ for
      the requested file.    Using any of the other FCB functions
      destroys this ~DTA~ copy of the FCB/XFCB
    - searching can be continued with the FCB find-next function
    - "?" wildcard supported after DOS 2.1, "*" supported in DOS 3.x
    - DOS 2.x can't find . and .. entries, DOS 3.x can (unless in root)
    - see    ~INT 21,12~

:int 21,12
^INT 21,12 - Search for Next Entry Using FCB

    AH = 12h
    DS:DX = pointer to unopened ~FCB~ returned from
        ~INT 21,11~  or  ~INT 21,12~


    on return:
    AL = 00 if file found
       = FF if file not found


    - finds next matching file after calls to ~INT 21,11~ and
      ~INT 21,12~
    - FCB should be the same across calls to INT 21,11 and 12
    - after successful call ~DTA~ holds an unopened ~FCB~/~XFCB~ for
      the requested file.    Using any of the other FCB functions
      destroys this DTA copy of the FCB/XFCB
:int 21,13
^INT 21,13 - Delete File Using FCB


    AH = 13h
    DS:DX = pointer to an unopened ~FCB~


    on return:
    AL = 00 if file deleted
       = FF if file not found


    - deletes unopened file with normal attributes
    - FCB must contain drive id, filename, and extension before call
    - "?" wildcard supported after DOS 2.1, "*" supported by DOS 3.x+
    - DOS 2.x allowed deletion of a subdirectory if ~XFCB~ was provided,
      even if files existed, causing lost clusters.  DOS 3.x does not
:int 21,14
^INT 21,14 - Sequential Read Using FCB

    AH = 14h
    DS:DX = pointer to an opened ~FCB~


    on return:
    AL = 00 if successful read
       = 01 if end of file (no data read)
       = 02 if ~DTA~ is too small
       = 03 if end of file or partial record read


    - reads a record from file pointed to by FCB at the location
      specified in current block and current record number
    - data record is read into the DTA
    - FCB record number is updated
:int 21,15
^INT 21,15 - Sequential Write Using FCB


    AH = 15h
    DS:DX = pointer to an opened ~FCB~


    on return:
    AL = 00 if write was successful
       = 01 if diskette is full or read only
       = 02 if ~DTA~ is too small


    - writes a record from the DTA to the current record position
      in file specified by the opened FCB
    - record size and output location are maintained in the FCB
:int 21,16
^INT 21,16 - Create a File Using FCB


    AH = 16h
    DS:DX = pointer to an unopened ~FCB~


    on return:
    AL = 00 if file created
       = FF if file creation failed


    - creates file using FCB and leaves open for later output
    - FCB must be setup with drive id, filename, and extension
      before call
    - an extended FCB can be used to also set ~file attributes~
:int 21,17
^INT 21,17 - Rename a File Using FCB

    AH = 17h
    DS:DX = pointer to a modified ~FCB~ of the format:

      Offset     Description
        00       drive designator
        01       original file name
        09       original file extension
        11       new file name
        19       new extension


    on return:
    AL = 00 if file renamed
       = FF if file not renamed


    - allows renaming of files with normal attributes
    - "?" wildcard supported after DOS 2.1, "*" supported by
      DOS 3.x+
:int 21,19
^INT 21,19 - Get Current Default Drive


    AH = 19h


    on return:
    AL = current default drive (0=A,1=B,etc)


    - determines the current default drive

:int 21,1a
^INT 21,1A - Set Disk Transfer Address (DTA)


    AH = 1A
    DS:DX = pointer to disk transfer address (~DTA~)


    returns nothing


    - specifies the disk transfer address to DOS
    - DTA cannot overlap 64K segment boundary
    - offset 80h in the ~PSP~ is a 128 byte default DTA supplied
      by DOS upon program load
    - use of the DTA provided by DOS will result in the loss
      of the program command tail which also occupies the 128
      bytes starting at offset 80h of the PSP
    - see    ~INT 21,2F~
:int 21,1b
^INT 21,1B - Get Allocation Table Information


    AH = 1B


    on return:
    AL = sectors per cluster
    CX = bytes per sector
    DX = clusters on disk
    DS:BX = pointer to ~Media Descriptor Byte~ found in ~FAT~


    - retrieves information on capacity and format of default drive
    - DS:BX can be used to determine if drive is RAMDISK or removable
    - see  ~INT 21,1C~

:int 21,1c
^INT 21,1C - Get Allocation Table Info for Specified Drive


    AH = 1C
    DL = drive number (0 for default, 1 = A:, Z: = 26)


    on return:
    AL = sectors per cluster
    CX = bytes per sector
    DX = clusters on disk
    DS:BX = pointer to ~Media Descriptor Byte~ found in ~FAT~


    - retrieves information on capacity and format of specified drive
    - DS:BX can be used to determine if drive is RAMDISK or removable
    - see  ~INT 21,1B~

:int 21,1f
^INT 21,1F - Get Pointer to Current Drive Parameter Table
^(Undocumented)


    AH = 1F
    DL = drive number (0=default, 1=A, ...)


    on return:
    AL = 00  DS:BX is pointer to drive parameter table  (~DPT~)
         FF  drive does not exist
    DS:BX = pointer to drive parameter table (DPT) if AL=0


    - the format of the DPT varies between DOS versions
    - calls ~INT 21,32~ with DL=00 for DOS version 2.x+
:int 21,21
^INT 21,21 - Random Read Using FCB


    AH = 21h
    DS:DX = pointer to an opened ~FCB~


    on return:
    AL = 00 if read successful
       = 01 if EOF (no data read)
       = 02 if ~DTA~ is too small
       = 03 if EOF (partial record read)


    - reads random records from a file opened with an FCB
      to the DTA
    - FCB must be setup with drive id, filename, extension,
      record position and record length before call
    - current record position field in FCB is not updated

:int 21,22
^INT 21,22 - Random Write Using FCB


    AH = 22h
    DS:DX = far pointer to an opened ~FCB~


    on return:
    AL = 00 if write successful
       = 01 if diskette full or read only
       = 02 if ~DTA~ is too small


    - write records to random location in file opened with FCB
    - FCB must be setup with drive id, filename, extension,
      record position and record length before call
    - current record position field in FCB is not updated

:int 21,23
^INT 21,23 - Get File Size Using FCB


    AH = 23h
    DS:DX = pointer to an unopened ~FCB~


    on return:
    AL = 00 if successful
       = FF if file not found


    - determines the number of records in a file
    - FCB must be setup with drive id, complete filename and
      extension plus record length before call
    - updates random record position in FCB located at DS:DX
      with file record count

:int 21,24
^INT 21,24 - Set Relative Record Field in FCB


    AH = 24h
    DS:DX = pointer to an opened ~FCB~


    returns nothing


    - modifies opened FCB for random operation
    - sets FCB random record field to current sequential block
      and record fields

:int 21,25
^INT 21,25 - Set Interrupt Vector


    AH = 25h
    AL = interrupt number
    DS:DX = pointer to interrupt handler


    returns nothing


    - provides a safe method for changing interrupt vectors


    - see    ~INT 21,35~

:int 21,26
^INT 21,26 - Create New Program Segment Prefix


    AH = 26h
    DX = segment address of new ~PSP~


    returns nothing


    - allocates memory for a PSP and copies current PSP there
    - used before DOS 2.x to spawn a child process
    - the application is responsible for allocating any memory
      necessary for the child process
    - ~INT 21,4B~ (EXEC) is now the recommended method for starting
      a child process, so this function should be avoided


    - see also ~INT 21,55~

:int 21,27
^INT 21,27 - Random Block Read Using FCB


    AH = 27h
    CX = number of records to read
    DS:DX = pointer to an opened ~FCB~


    on return:
    AL = 00 if read was successful
       = 01 if EOF (no data read)
       = 02 if ~DTA~ is too small
       = 03 if EOF (partial record read)
    CX = actual number of records read


    - allows random access and sequential reading of a group
      of records from a file opened with an FCB into the DTA
    - FCB must be setup with drive id, filename, extension
      record length and random record number before call

:int 21,28
^INT 21,28 - Random Block Write Using FCB


    AH = 28h
    CX = number of records to write
    DS:DX = pointer to an opened ~FCB~


    on return:
    AL = 00 if write successful
       = 01 if diskette full or read only
       = 02 if ~DTA~ is too small
    CX = number of records written


    - allows random access and sequential writing of a group
      of records from a file opened with an FCB into the DTA
    - FCB must be setup with random record number and record size
    - updates random record number, current block and current
      record fields

:int 21,29
^INT 21,29 - Parse a Filename for FCB

    AH = 29h
    AL = bit pattern to control parsing (see bit meanings below)
    DS:SI = pointer to a filespec to parse
    ES:DI = pointer to a buffer for unopened ~FCB~


    Bit patterns for parsing control found in AL:

    │7│6│5│4│3│2│1│0│  AL
     │ │ │ │ │ │ │ └──── 1 = ignore leading separators
     │ │ │ │ │ │ │         0 = don't ignore leading separators
     │ │ │ │ │ │ └───── 1 = modify drive ID if specified
     │ │ │ │ │ │        0 = modify drive ID regardless
     │ │ │ │ │ └────── 1 = modify filename if specified
     │ │ │ │ │       0 = modify filename regardless
     │ │ │ │ └─────── 1 = modify extension if specified
     │ │ │ │      0 = modify extension regardless
     └─┴─┴─┴──────── unused

    on return:
    AL = 00 if no wildcard characters present
       = 01 if wildcards present in string
       = FF if drive specifier is invalid
    DS:SI = pointer to the first character after parsed filename
    ES:DI = pointer to the updated unopened FCB


    - retrieves filename from the command line string and places
      the filename components into an unopened FCB for later use
    - if no filename is found a pointer is returned in ES:DI that
      has a blank at ES:DI+1
    - this function can be used to detect the existence of logical DOS
      drives by creating a dummy filespec with a drive letter and colon
      prepended.  If the drive is invalid, this function will return FF
      in AL
:int 21,2a
^INT 21,2A - Get Date


    AH = 2A


    on return:
    AL = day of the week (0=Sunday)
    CX = year (1980-2099)
    DH = month (1-12)
    DL = day (1-31)


    - retrieves system date based on the DOS maintained clock
    - updates ~BIOS Data Area~ current date and date rollover flag
      at location 40:70

:int 21,2b
^INT 21,2B - Set Date


    AH = 2B
    CX = year (1980-2099)
    DH = month (1-12)
    DL = day (1-31)


    on return:
    AL = 00 if date change successful
       = FF if invalid date


    - sets DOS maintained clock
    - DOS versions 3.3+ also update the ~CMOS~ date where applicable

:int 21,2c
^INT 21,2C - Get Time


    AH = 2C


    on return:
    CH = hour (0-23)
    CL = minutes (0-59)
    DH = seconds (0-59)
    DL = hundredths (0-99)


    - retrieves DOS maintained clock time
:int 21,2d
^INT 21,2D - Set Time


    AH = 2D
    CH = hour (0-23)
    CL = minutes (0-59)
    DH = seconds (0-59)
    DL = hundredths (0-99)


    on return:
    AL = 00 if time change successful
       = FF if time invalid


    - changes DOS maintained clock
    - DOS version 3.3+ also update CMOS clock where applicable

:int 21,2e
^INT 21,2E - Set/Reset Verify Switch


    AH = 2E
    AL = 00 to set off
       = 01 to set verify on
    DH = 00 for DOS versions before 3.0


    returns nothing


    - with the verify setting on, disk I/O is more secure but
      takes longer to complete


    - see    ~INT 21,54~

:int 21,2f
^INT 21,2F - Get Disk Transfer Address (DTA)


    AH = 2F


    on return:
    ES:BX = pointer to current ~DTA~


    - returns the DTA address
    - the default DTA is a 128 byte block at address 80h in the
      Program Segment Prefix (~PSP~).  This area also contains the
      command tail at program startup it must be saved or the DTA
      must be relocated before use to preserve the command tail

    - see    ~INT 21,1A~

:int 21,30
^INT 21,30 - Get DOS Version Number


    AH = 30h


    on return:
    AL = major version number (2-5)
    AH = minor version number (in hundredths decimal)
    BH = FF  indicates MS-DOS, only if OEM vendor chooses to identify
       = 00  indicates PC-DOS
    BL:CX = 24 bit OEM serial number if BH is FF


    - for an example DOS version 2.1 returns AL=2 and AH=10
    - DOS versions prior to DOS 2.0 return zero in AH and AL
    - DOS version 4.0 and 4.1 usually return the same value of 4.00
    - the OEM serial number is a rarity, though some older OEM DOS
      versions implemented this feature
    - the OS/2 compatibility box returns 10.10 for OS/2 1.1, 10.20
      for OS/2 1.2, etc...
    - when testing for version, a specific test can often cause your
      code to not work in following versions of DOS.  It is often better
      to test for a version number greater or equal to the minimum rather
      than a specific version number where possible
    - see ~DOS Versions~
:int 21,31
^INT 21,31 - Terminate Process and Remain Resident


    AH = 31h
    AL = exit code (returned to batch files)
    DX = memory size in paragraphs to reserve


    returns nothing


    - preferred method for Terminate and Stay Resident programs
    - terminates process without releasing allocated memory and
      without closing open files
    - attempts allocation of memory specified in DX from memory
      allocated by DOS at startup.     ~INT 21,48~ memory allocation
      is not affected

    - see    ~INT 27~

:int 21,32
^INT 21,32 - Get Pointer to Drive Parameter Table (Undocumented)

    AH = 32h
    DL = drive (0=default, 1=A:, 2=B:, 3=C:, ...)


    on return:
    AL =  FF if the drive number in DL was invalid
    DS:BX = address of drive parameter table (~DPT~)


    - available since DOS 2.0
    - used by DOS commands CHKDSK and RECOVER
    - forces a media check, which clears byte DS:[BX+17h] or DS:[BX+18h]
      in the Drive Parameter Table
    - actually accesses the disk, causing a critical error if a disk
      error occurs
    - can be used to determine if a drive is SUBST'ed by comparing
      DS:[BX+1] and DS:[BX] for a match.  If not equal, then the
      drive is possibly SUBST'ed (though not guaranteed, this may also
      indicate a Bernoulli box)
    - can be used to determine if a drive is a RAM disk; if the disk is
      NOT removable and (DS:[BX+1] == 0), then the disk is a RAM disk
      (see ~IOCTL,0~ bit number 0Bh to determine if the disk is removable)
:int 21,33
^INT 21,33 - Get/Set System Values (Ctl-Break/Boot Drive)


    AH = 33h
    AL = 00 to get Ctrl-Break checking flag
       = 01 to set Ctrl-Break checking flag
       = 02 to set extended Ctrl-Break checking
       = 05 get boot drive (DOS 4.x)
    DL = 00 to set Ctrl-Break checking off
       = 01 to set Ctrl-Break checking on
       = boot drive for subfunction 5;  (1=A:, 2=B:, ...)


    on return:
    DL = 00 Ctrl-Break checking OFF (AL=0 or AL=2)
       = 01 Ctrl-Break checking ON    (AL=0 or AL=2)
       = boot drive number (1-26, A: - Z:) (function 05)


    - retrieves DOS Ctrl-Break or extended Ctrl-Break setting which
      determines if DOS will check for Ctrl-Break during INT 21 calls
:int 21,34:INDOS
^INT 21,34 - Get Address to DOS Critical Flag
^INDOS    (Undocumented DOS 2.0+)

    AH = 34h

    on return:
    ES:BX = address of a byte indicating whether a DOS call is
        in progress.  No DOS calls should be made if set.


    - AKA the INDOS flag, this critical section flag may be checked from
      within an interrupt handler before requesting a DOS service. It is
      a semaphore that is non-zero when DOS is busy, and zero otherwise.
    - though this flag indicates whether a DOS interrupt is active, it
      should not be used alone to determine DOS is safe for re-entry;
      Here's the standard rule for safe DOS entry: if ~INT 28~ is active
      or this flag and the critical error flag are clear then it is safe
      to call DOS
    - this interrupt should be used only during TSR initialization;
      the returned pointer should be used thereafter
    - this flag is cleared after a critical error (~INT 24~)
    - ES:BX-1 points to the critical error flag for DOS 3.x+
      ES:BX+1 points to the critical error flag for DOS 2.x
      ES:BX-1AA points to the critical error flag for COMPAQ DOS 3.0
      ~INT 21,5D~ in DOS 3.x+ can be used to locate the critical
      error flag
:int 21,35
^INT 21,35 - Get Interrupt Vector


    AH = 35h
    AL = interrupt vector number


    on return:
    ES:BX = pointer to interrupt handler


    - standard method for retrieving interrupt vectors


    - see    ~INT 21,25~

:int 21,36
^INT 21,36 - Get Disk Free Space


    AH = 36h
    DL = drive number (0=default, 1=A:)


    on return:
    AX = sectors per cluster
       = FFFF if drive is invalid
    BX = number of available clusters
    CX = number of bytes per sector
    DX = number of clusters per drive


    - used to determine available space on specified disk
    - see    ~INT 21,1B~   ~INT 21,1C~

:int 21,37
^INT 21,37 - Get/Set Switch Character (Undocumented, DOS 2.0+)


    AH = 37h
    AL = 0    get switch character into DL;  some systems return "-"
       = 1    set switch character to value in DL
       = 2    read device prefix flag into DL;  returns DL = 0 indicating
        devices must be accessed using /DEV/device. A non-zero value
        indicates devices may be accessed without prefix
       = 3    set device prefix flag, device names must begin with \DEV\.
    DL = new switch character (AL=1)
       = 00  \DEV\ must preceed device names (AL=3)
       = 01  \DEV\ is not neccesary in device names (AL=3)

    on return:
    AL = FF  illegal subfunction code specified
    DL = current switch character (AL=0)
       = device availability (AL=2, always FF with DOS 4.x+)


    - subfunctions 0 and 1 were formerly available as a CONFIG.SYS
      command in DOS versions before 3.x;    also supportedin the OS/2
      compatibility box
    - subfunctions 2 and 3 were available in DOS 2.x only;    they have
      no effect in DOS 4.x+
    - /DEV/ prefix is valid in DOS 2.x by default, setting the flag makes
      it mandatory.  DOS internal commands like DIR, TYPE and DEL do not
      recognize filenames the are identical to device names regardless of
      the setting of the AVAILDEV flag.
:int 21,38
^INT 21,38 - Get/Set Country Dependent Information

    AH = 38h
    AL = 00 to get current country information
       = 00-FE country codes  (DOS 3.x+)
       = FF for country codes >= FF, country codes are in BX
    BX = country code if AL = FF (country code > 255)
    DX = FFFF to set country information
    DS:DX = pointer to buffer to contain country data (if get data)

    on return:
    AX = error code if CF set
       = 02 invalid country
    BX = country code (DOS 3.x+)
    DS:DX = pointer to returned country data (see ~COUNTRY CODES~)


    - returns a pointer to country specific data, for DOS 3.x+ this
      function can be used to also set this information
:int 21,39
^INT 21,39 - Create Subdirectory (mkdir)


    AH = 39h
    DS:DX = pointer to ASCIIZ path name


    on return:
    CF = 0 if successful
       = 1 if error
    AX = error code  (see ~DOS ERROR CODES~)


    - creates specified subdirectory
    - returns error if directory already exists, element of the path
      is not found, directory full or write protected disk

:int 21,3a
^INT 21,3A - Remove Subdirectory (rmdir)


    AH = 3A
    DS:DX = pointer to ASCIIZ path name


    on return:
    CF = 0 if successful
       = 1 if error
    AX = error code  (see ~DOS ERROR CODES~)


    - allows deletion of a subdirectory as long as it exists, is empty
      and not the current directory

:int 21,3b
^INT 21,3B - Change Current Directory (chdir)


    AH = 3B
    DS:DX = pointer to ASCIIZ path name


    on return:
    CF = 0 if successful
       = 1 if error
    AX = error code if CF set  (see ~DOS ERROR CODES~)


    - changes the current directory to the directory specified
      by pointer DS:DX

:int 21,3c
^INT 21,3C - Create File Using Handle


    AH = 3C
    CX = file attribute  (see ~FILE ATTRIBUTES~)
    DS:DX = pointer to ASCIIZ path name


    on return:
    CF = 0 if successful
       = 1 if error
    AX = files handle if successful
       = error code if failure  (see ~DOS ERROR CODES~)


    - if file already exists, it is truncated to zero bytes on opening

:int 21,3d
^INT 21,3D - Open File Using Handle

    AH = 3D
    AL = open access mode
         00  read only
         01  write only
         02  read/write
    DS:DX = pointer to an ASCIIZ file name

    on return:
    AX = file handle if CF not set
       = error code if CF set  (see ~DOS ERROR CODES~)


%    Access modes in AL:

    │7│6│5│4│3│2│1│0│  AL
     │ │ │ │ │ └─┴─┴──── read/write/update access mode
     │ │ │ │ └───────── reserved, always 0
     │ └─┴─┴────────── sharing mode (see below) (DOS 3.1+)
     └─────────────── 1 = private, 0 = inheritable (DOS 3.1+)


%    Sharing mode bits (DOS 3.1+):           Access mode bits:
%    654                       210
    000  compatibility mode (exclusive)    000  read access
    001  deny others read/write access     001  write access
    010  deny others write access           010  read/write access
    011  deny others read access
    100  full access permitted to all


    - will open normal, hidden and system files
    - file pointer is placed at beginning of file

:int 21,3e
^INT 21,3E - Close File Using Handle


    AH = 3E
    BX = file handle to close


    on return:
    AX = error code if CF set  (see ~DOS ERROR CODES~)


    - if file is opened for update, file time and date stamp
      as well as file size are updated in the directory
    - handle is freed

:int 21,3f
^INT 21,3F - Read From File or Device Using Handle


    AH = 3F
    BX = file handle
    CX = number of bytes to read
    DS:DX = pointer to read buffer


    on return:
    AX = number of bytes read is CF not set
       = error code if CF set  (see ~DOS ERROR CODES~)


    - read specified number of bytes from file into buffer DS:DX
    - when AX is not equal to CX then a partial read occurred due
      to end of file
    - if AX is zero, no data was read, and EOF occurred before read

:int 21,40
^INT 21,40 - Write To File or Device Using Handle


    AH = 40h
    BX = file handle
    CX = number of bytes to write, a zero value truncates/extends
         the file to the current file position
    DS:DX = pointer to write buffer


    on return:
    AX = number of bytes written if CF not set
       = error code if CF set  (see ~DOS ERROR CODES~)


    - if AX is not equal to CX on return, a partial write occurred
    - this function can be used to truncate a file to the current
      file position by writing zero bytes
:int 21,41
^INT 21,41 - Delete File


    AH = 41h
    DS:DX = pointer to an ASCIIZ filename


    on return:
    AX = error code if CF set  (see DOS ERROR CODES)


    - marks first byte of file directory entry with E5 to indicate
      the file has been deleted.  The rest of the directory entry
      stays intact until reused.   ~FAT~ pointers are returned to DOS
    - documented as not accepting wildcards in filename but actually
      does in several DOS versions
:int 21,42
^INT 21,42 - Move File Pointer Using Handle


    AH = 42h
    AL = origin of move:
         00 = beginning of file plus offset  (SEEK_SET)
         01 = current location plus offset    (SEEK_CUR)
         02 = end of file plus offset  (SEEK_END)
    BX = file handle
    CX = high order word of number of bytes to move
    DX = low order word of number of bytes to move


    on return:
    AX = error code if CF set  (see ~DOS ERROR CODES~)
    DX:AX = new pointer location if CF not set


    - seeks to specified location in file
:int 21,43
^INT 21,43 - Get/Set File Attributes

    AH = 43h
    AL = 00 to get attribute
       = 01 to set attribute
    DS:DX = pointer to an ASCIIZ path name
    CX = attribute to set

    │5│4│3│2│1│0│  CX  valid file attributes
     │ │ │ │ │ └──── 1 = read only
     │ │ │ │ └───── 1 = hidden
     │ │ │ └────── 1 = system
     │ └─┴─────── not used for this call
     └────────── 1 = archive


    on return:
    AX = error code if CF set  (see ~DOS ERROR CODES~)
    CX = the attribute if AL was 00

    - see    ~DIRECTORY~

:int 21,44
^INT 21,44 - I/O Control for Devices  (IOCTL)

%    Standard Call Format

    AH = 44h
    AL = function value
    BX = file handle
    BL = logical device number (0=default, 1=A:, 2=B:, 3=C:, ...)
    CX = number of bytes to read or write
    DS:DX = data or buffer


    on return:
    AX = error code if CF set
    AX = # of bytes transferred if CF not set


%    For more information, see the following topics:

    ~IOCTL,0~   Get Device Information
    ~IOCTL,1~   Set Device Information
    ~IOCTL,2~   Read From Character Device
    ~IOCTL,3~   Write to Character Device
    ~IOCTL,4~   Read From Block Device
    ~IOCTL,5~   Write to Block Device
    ~IOCTL,6~   Get Input Status
    ~IOCTL,7~   Get Output Status
    ~IOCTL,8~   Device Removable Query
    ~IOCTL,9~   Device Local or Remote Query
    ~IOCTL,A~   Handle Local or Remote Query
    ~IOCTL,B~   Set Sharing Retry Count
    ~IOCTL,C~   Generic I/O for Handles
    ~IOCTL,D~   Generic I/O for Block Devices (3.2+)
    ~IOCTL,E~   Get Logical Drive (3.2+)
    ~IOCTL,F~   Set Logical Drive (3.2+)


    - see:    ~DEVICE COMMAND CODES~
        ~DEVICE REQUEST HEADER~
        ~DEVICE STATUS~
        ~DEVICE HEADER~
        ~DEVICE ATTRIBUTES~

:int 21,44,0:IOCTL,0
^INT 21,44,0 / IOCTL,0 - Get Device Information

    AH = 44h
    AL = 00
    BX = handle (must be an opened device)


    on return
    AX = error code if CF set  (see ~DOS ERROR CODES~)
    DX = device information  (see tables below)


    │F│E│D│C│B│A-8│7│6│5-0│  DX  Block Device Information
     │ │ │ │ │  │  │ │  └───── drive number (0=A:,1=B:)
     │ │ │ │ │  │  │ └─────── 0 = file has been written
     │ │ │ │ │  │  └──────── 0 = disk file; 1 = character device
     │ │ │ │ │  └────────── reserved, must be zero
     │ │ │ │ └──────────── 1 = media not removable
     │ │ │ └───────────── 1 = network device (DOS 3.x+)
     │ │ └────────────── 1 = reserved
     │ └─────────────── 1 = don't update file time or date (DOS 4.x+)
     └──────────────── 1 = file is remote (DOS 3.x+)

    │F│E│D│C│B│A-8│7│6│5│4│3│2│1│0│  DX  Character Device Information
     │ │ │ │ │  │  │ │ │ │ │ │ │ └──── 1 = standard input device
     │ │ │ │ │  │  │ │ │ │ │ │ └──── 1 = standard output device
     │ │ │ │ │  │  │ │ │ │ │ └──── 1 = NUL device
     │ │ │ │ │  │  │ │ │ │ └──── 1 = clock device
     │ │ │ │ │  │  │ │ │ └──── uses DOS ~INT 29~ for fast character output
     │ │ │ │ │  │  │ │ └──── 1 = binary mode, 0 = translated
     │ │ │ │ │  │  │ └──── 0 = end of file on input
     │ │ │ │ │  │  └──── 1 = character device, 0 if disk file
     │ │ │ │ │  └───── reserved
     │ │ │ │ └────── 1 = media not removable
     │ │ │ └────── 1 = network device (DOS 3.x+)
     │ │ └────── reserved
     │ └────── 1 = supports IOCTL, via functions 2 & 3
     └────── reserved


    - BIT 7 of register DX can be used to detect if STDIN/STDOUT is
      redirected to/from disk; if a call to this function has DX BIT 7
      set it's not redirected from/to disk; if it's clear then it is
      redirected to/from disk
    - BIT B of register DX can be used to determine if a drive is
      removable.

:int 21,44,1:IOCTL,1
^INT 21,44,1 / IOCTL,1 - Set Device Information

    AH = 44h
    AL = 01
    BX = handle
    DH = must be zero
    DL = device data low order byte  (see below)


    on return
    AX = error code if CF set
    DX = device information  (see below)


    - applicable to character devices only
    - allows setting of device data word for character devices
    - usually used to change from binary to translated I/O
    - handle in BX must be an opened file or device


^Device Data Word

    │F│E│D│C│B│A│9│8│7│6│5│4│3│2│1│0│  Device Data Word
     │ │ │ │ │ │ │ │ │ │ │ │ │ │ │ └──── 1 = standard input device
     │ │ │ │ │ │ │ │ │ │ │ │ │ │ └──── 1 = standard output device
     │ │ │ │ │ │ │ │ │ │ │ │ │ └──── 1 = NUL device
     │ │ │ │ │ │ │ │ │ │ │ │ └──── 1 = clock device
     │ │ │ │ │ │ │ │ │ │ │ └──── reserved
     │ │ │ │ │ │ │ │ │ │ └──── 1 = binary mode, 0 = translated
     │ │ │ │ │ │ │ │ │ └──── 0 = end of file on input
     │ │ │ │ │ │ │ │ └──── 1 = character device
     └─┴─┴─┴─┴─┴─┴─┴──── reserved, must be zero


:int 21,44,2:IOCTL,2
^INT 21,44,2 / IOCTL,2 - Read From Character Device


    AH = 44h
    AL = 02
    BX = handle
    CX = number of bytes to read
    DS:DX = pointer to data buffer


    on return
    AX = number of bytes read if CF clear
       = error code if CF set   (see ~DOS ERROR CODES~)


    - see bit 14 if IOCTL function 00h to determine if
      driver can support IOCTL control strings

:int 21,44,3:IOCTL,3
^INT 21,44,3 / IOCTL,3 - Write to Character Device


    AH = 44h
    AL = 03
    BX = handle
    CX = number of bytes to send
    DS:DX = pointer to data buffer


    on return
    AX = number of bytes written if CF clear
       = error code if CF set   (see ~DOS ERROR CODES~)


    - see bit 14 if IOCTL function 00h to determine if
      driver can support IOCTL control strings

:int 21,44,4:IOCTL,4
^INT 21,44,4 / IOCTL,4 - Read from Block Device


    AH = 44h
    AL = 04
    BL = drive number (0=default, 1=A:, 2=B:, 3=C:, ...)
    CX = number of bytes to read
    DS:DX = pointer to data buffer


    on return
    AX = number of bytes read if CF clear
       = error code if CF set  (see ~DOS ERROR CODES~)


    - block drivers are not required to support this function
    - returns AX = 01 (invalid function code) if not supported

:int 21,44,5:IOCTL,5
^INT 21,44,5 / IOCTL,5 - Write to Block Device


    AH = 44h
    AL = 05
    BL = drive number (0=default, 1=A:, 2=B:, 3=C:, ...)
    CX = number of bytes to send
    DS:DX = pointer to data buffer


    on return
    AX = number of bytes written if CF clear
       = error code if CF set  (see ~DOS ERROR CODES~)


    - block devices are not required to support this function
    - returns AX = 01 (invalid function code) if not supported

:int 21,44,6:IOCTL,6
^INT 21,44,6 / IOCTL,6 - Get Input Status


    AH = 44h
    AL = 06
    BX = handle


    on return
    CF = 0 if successful
       = 1 if error
    AX = error code if CF set  (see ~DOS ERROR CODES~)
    AL = 00  if EOF (files), or not ready (char devices)
       = FF  if not EOF (files), or ready (char devices)


    - used to determine if a file or device is ready for input
    - can be used to determine EOF unless EOF caused by ~INT 21,42~

:int 21,44,7:IOCTL,7
^INT 21,44,7 / IOCTL,7 - Get Output Status


    AH = 44h
    AL = 07
    BX = handle


    on return
    CF = 0 if successful
       = 1 if error
    AX = error code if CF set  (see ~DOS ERROR CODES~)
    AL = 00  if ready (files), or not ready (char devices)
       = FF  ready (files or char device)


    - indicates if a device or file is ready for output
    - files always return ready, character devices don't

:int 21,44,8:IOCTL,8
^INT 21,44,8 / IOCTL,8 - Device Removable Query

    AH = 44h
    AL = 08
    BL = drive number (0=default, 1=A:, 2=B:, 3=C:, ...)


    on return
    CF = 0 if successful
       = 1 if error
    AX = 00  removable media
       = 01  non-removable media
       = error code if CF set  (see ~DOS ERROR CODES~)


    - used to determine if a device supports removable media
    - RAM disks are not considered removable media
    - device drivers compatible with DOS 2.0 do not always respond
      correctly to this query
    - implemented from DOS 3.0;  for earlier DOS versions drive A: and
      B: are removable media since DRIVER.SYS/SUBST are DOS 3.0+ only
:int 21,44,9:IOCTL,9
^INT 21,44,9 / IOCTL,9 - Device Local or Remote Query


    AH = 44h
    AL = 09
    BL = drive number (0=default, 1=A:, 2=B:, 3=C:, ...)


    on return
    AX = error code if CF set  (see ~DOS ERROR CODES~)
    DX = device attribute word
         bit 12 = 1 if drive is remote
         bit 12 = 0 if drive is local


    - used to determine if block device is local or remote
    - returns invalid function if networking not started
    - implemented from DOS 3.1

:int 21,44,a:IOCTL,a
^INT 21,44,A / IOCTL,A - Handle Local or Remote Query


    AH = 44h
    AL = 0A
    BX = handle


    on return
    AX = error code if CF set  (see ~DOS ERROR CODES~)
    DX = device attribute word
         bit 15 = 1 if drive is remote
         bit 15 = 0 if drive is local


    - used to determine if block device is local or remote
    - returns invalid function if networking not started
    - implemented from DOS 3.1

:int 21,44,b:IOCTL,b
^INT 21,44,B / IOCTL,B - Set Sharing Retry Count


    AH = 44h
    AL = 0B
    CX = pause between retries  (default 1)
    DX = number of retries    (default 3)


    on return
    AX = error code if CF set  (see ~DOS ERROR CODES~)


    - retry parameters are associated with file locking
    - differences in CPU and clock speeds affect length of pauses
    - requires SHARE be loaded or invalid function is returned
    - implemented from DOS 3.0

:int 21,44,c:IOCTL,c
^INT 21,44,C / IOCTL,C - Generic I/O for Handles

    AH = 44h
    AL = 0C
    BX = handle
    CH = device type
       = 00  unknown device type  (DOS 3.3+)
       = 01  COMx  (DOS 3.3+)
       = 03  CON  (DOS 3.3+)
       = 05  LPTx  (DOS 3.3+),  printer  (DOS 3.2)
    CL = minor function code (when CH = 3 or CH = 5)
       = 45  set iteration count  (DOS 3.2 only)

       = 4A  select code page (DOS 3.3+); parameter format:
           00  word  length of data
           02  word  code page ID
           04 nwords character set data array (see offset 00)

       = 4C  code page prepare start  (DOS 3.3+); parameter format:
           00  word  flags
           02  word  length of remainder of parameter block
           04  word  number of code pages following
           06 nwords code page 1,...,N

       = 4D  code page prepare end    (DOS 3.3+); parameter format:
           00  word  length of data
           02  word  code page ID

       = 5F  set display info  (DOS 4.x, when CH=3); parameter format:
           00  byte  level  (0 for DOS 4.0)
           01  byte  reserved
           02  word  length of following data
           04  word  control flags
                 bit 0 set for blink, clear for intensity
                 bits 1 to 15 reserved
           06  byte  mode type (1=text, 2=graphics)
           07  byte  reserved
           08  word  colors;  0=monochrome, n=bits per pixel
           0A  word  pixel columns
           0C  word  pixel rows
           0E  word  character columns
           10  word  character rows

       = 65  get iteration count  (DOS 3.2 only)

       = 6A  query selected code page  (DOS 3.3+); parameter format:
           00  word  length of data
           02  word  code page ID
           04 nwords character set data array  (see offset 00)

       = 6B  query prepare list  (DOS 3.3+); Parameter format:
           00  word  length of following data
           02  word  number of hardware code pages
           04 nwords hardware code page array
           xx  word  number of prepared code pages
           xx nwords prepared code page array

       = 7F  get display info  (DOS 4.x, CH = 3)
           00  byte  level  (0 for DOS 4.0)
           01  byte  reserved
           02  word  length of following data
           04  word  control flags
                   bit 0 set for blink, clear for intensity
                   bits 1 to 15 reserved
           06  byte  mode type (1=text, 2=graphics)
           07  byte  reserved
           08  word  colors;  0=monochrome, n=bits per pixel
           0A  word  pixel columns
           0C  word  pixel rows
           0E  word  character columns
           10  word  character rows

    DS:DX = pointer to iteration count word  (DOS 3.2)
          = pointer to parameter block  (DOS 3.3)


    on return
    AX = error code if CF set  (see ~DOS ERROR CODES~)


    - iteration count word specifies the number of times to retry
      an operation before aborting
    - DOS 3.3 changed this function to handle code page switching
    - implemented from DOS 3.2
    - DOS 4.x adds support for double byte characters
    - see IBM DOS Technical Reference Manual for more details

:int 21,44,d:IOCTL,d
^INT 21,44,D / IOCTL,D - Generic I/O for Block Devices

    AH = 44h
    AL = 0D
    BL = drive number (0=default, 1=A:, 2=B:, 3=C:, ...)
    CH = device type
       = 08 for disk drive    (block device)
    CL = minor function code
       = 40  set device parameters
       = 41  write track on logical device
       = 42  format/verify track on logical drive
       = 47  set access flag  (DOS 4.x)
       = 60  get device parameters
       = 61  read track on logical device
       = 62  verify track on logical drive
       = 67  get access flag  (DOS 4.x)
    DS:DX = pointer to parameter block


    on return
    AX = error code if CF set  (see ~DOS ERROR CODES~)


    - provides device independent primitive control operations
    - implemented from DOS 3.2
    - see IBM DOS Technical Reference Manual for more details

:int 21,44,e:IOCTL,e
^INT 21,44,E / IOCTL,E - Get Logical Drive


    AH = 44h
    AL = 0E
    BL = physical drive number (0=default, 1=A:, 2=B:, 3=C:, ...)


    on return
    AX = error code if CF set  (see ~DOS ERROR CODES~)
    AL = logical drive number assigned to physical drive
       = 0 if drive accessed by only one drive specifier
       = 1 if drive A:, 2 if drive B:, etc


    - used to determine the last drive designator used to access
      a drive if more than one logical drive designation applies
      to a device
    - implemented from DOS 3.2

:int 21,44,f:IOCTL,f
^INT 21,44,F / IOCTL,F - Set Logical Drive


    AH = 44h
    AL = 0F
    BL = new drive number (0=default, 1=A:, 2=B:, 3=C:, ...)


    on return
    AX = error code if CF set  (see ~DOS ERROR CODES~)
    AL = logical drive number (should be equal to BL input)
       = 0 if drive accessed by only one drive specifier
       = 1 if drive A:, 2 if drive B:, etc



    - changes the logical drive designator of the physical drive
      to be accessed next
    - implemented from DOS 3.2

:int 21,45
^INT 21,45 - Duplicate File Handle


    AH = 45h
    BX = file handle


    on return:
    AX = new file handle if CF not set
       = error code if CF set  (see ~DOS ERROR CODES~)


    - gets another file handle for the same file
    - both file handles move in unison
    - often used to flush file data and update a file directory
      entry without closing the initial file

:int 21,46
^INT 21,46 - Force Duplicate File Handle


    AH = 46h
    BX = existing file handle
    CX = second file handle


    on return:
    AX = error code if CF set  (see ~DOS ERROR CODES~)


    - if file handle in CX is currently open, current file
      identified by CX is closed and the handle in BX is
      dupped and placed in CX
    - after duping file handles move together through file
    - similar to ~INT 21,45~

:int 21,47
^INT 21,47 - Get Current Directory


    AH = 47h
    DL = drive number (0 = default, 1 = A:)
    DS:SI = pointer to a 64 byte user buffer


    on return:
    DS:SI = pointer ASCIIZ directory path string
    AX = error code if CF set  (see ~DOS ERROR CODES~)


    - returns the current directory relative to the root directory
    - the leading slash "\" and drive designator are omitted

:int 21,48
^INT 21,48 - Allocate Memory

    AH = 48h
    BX = number of memory paragraphs requested


    on return:
    AX = segment address of allocated memory block (MCB + 1para)
       = error code if CF set  (see ~DOS ERROR CODES~)
    BX = size in paras of the largest block of memory available
         if CF set, and AX = 08 (Not Enough Mem)
    CF = 0 if successful
       = 1 if error


    - returns segment address of allocated memory block AX:0000
    - each allocation requires a 16 byte overhead for the ~MCB~
    - returns maximum block size available if error

    - see  ~INT 21,49~,  ~INT 21,4A~
:int 21,49
^INT 21,49 - Free Allocated Memory


    AH = 49h
    ES = segment of the block to be returned (~MCB~ + 1para)


    on return:
    AX = error code if CF set  (see ~DOS ERROR CODES~)


    - releases memory and MCB allocated by ~INT 21,48~
    - may cause unpredictable results is memory wasn't allocated using
      INT 21,48 or if memory wasn't allocated by the current process
    - checks for valid MCB id, but does NOT check for process ownership
    - care must be taken when freeing the memory of another process, to
      assure the segment isn't in use by a TSR or ISR
    - this function is unreliable in a TSR once resident, since
      COMMAND.COM and many other .COM files take all available memory
      when they load
    - see  INT 21,4A
:int 21,4a
^INT 21,4A - Modify Allocated Memory Block  (SETBLOCK)


    AH = 4A
    BX = new requested block size in paragraphs
    ES = segment of the block (~MCB~ + 1 para)


    on return:
    AX = error code if CF set  (see ~DOS ERROR CODES~)
    BX = maximum block size possible, if CF set and AX = 8


    - modifies memory blocks allocated by  ~INT 21,48~
    - can be used by programs to shrink or increase the size
      of allocated memory
    - PC-DOS version 2.1 and DOS 3.x will actually allocate the largest
      available block if CF is set.  BX will equal the size allocated.
    - see also  ~INT 21,49~
:int 21,4b:exec function
^INT 21,4B - EXEC/Load and Execute Program

    AH = 4B
    AL = 00  to load and execute program
       = 01  (Undocumented)  create program segment prefix and load
         program, but don't execute.  The CS:IP and SS:SP of the
         program is placed in parameter block.    Used by debuggers
       = 03  load program only
       = 04  called by MSC spawn() when P_NOWAIT is specified
    DS:DX = pointer to an ASCIIZ filename
    ES:BX = pointer to a parameter block


    on return:
    AX = error code if CF set  (see ~DOS ERROR CODES~)
    ES:BX = when AL=1, pointer to parameter block similar to:

%    Offset Size           Description

      00   word   when AL=1, segment of env. or zero if using parents env.
           word   when AL=3, segment of load point for overlay
      02   dword  when AL=1, pointer to cmd line at PSP 80h
           word   when AL=3, relocation factor for EXE overlay
      06   dword  pointer to default ~FCB~ passed at ~PSP~ 5Ch
      0A   dword  pointer to default FCB passes at PSP 6Ch
      0E   dword  value of program SS:SP
      12   dword  value of program CS:IP


    - allows execution of an external program as well as overlay
      management from within an application
    - all registers except CS and IP are destroyed
    - SS and SP should be preserved in code segment before call
      since a bug in DOS version 2.x destroys these
    - return code can be retrieved if child process exits via ~INT 21,4C~
    - calling process must assure presence of enough unallocated memory
    - subfunction 4 returns with an error under DOS 4.x+
    - calls ~INT 21,55~

    - see also  ~INT 21,26~

:int 21,4c
^INT 21,4C - Terminate Process With Return Code


    AH = 4C
    AL = return code (for batch files)


    returns nothing


    - approved method of program termination
    - restores the terminate, ~Ctrl-Break~, and critical error exit
      addresses, flushes all buffers, frees memory and returns to
      DOS via the termination handler address
    - does not close FCBs
    - this function is not supported in versions of DOS before 2.x,
      so use ~INT 21,0~  or     ~INT 20~  to exit.


    - see also  ~INT 27~   ~INT 21,31~

:int 21,4d
^INT 21,4D - Get Return Code of Sub-process


    AH = 4D


    on return:
    AH = system exit code  (indicates normal termination)
       = 00 for normal termination
       = 01 if terminated by ctl-break
       = 02 if critical device error
       = 03 if terminated by ~INT 21,31~
    AL = child exit code


    - retrieve child process and system exit codes
    - this function can only be used to retrieve the exit code once
      multiple attempts to read exit codes will cause errors

:int 21,4e
^INT 21,4E - Find First Matching File

    AH = 4E
    CX = attribute used during search  (see ~FILE ATTRIBUTES~)
    DS:DX = pointer to ASCIIZ filespec, including wildcards


    on return:
    AX = error code if CF set  (see ~DOS ERROR CODE~)
    ~DTA~ = data returned from call in the format:

%    Offset Size     Description

      00   byte    attribute of search (undocumented)
           byte    drive letter used in search (DOS 3.1-4.x, undocumented)
      01   byte    drive letter used in search (undocumented)
         11bytes   search name used (DOS 3.1-4.x, undocumented)
      02 11bytes   search name used (undocumented)
      0C   byte    attribute of search (DOS 3.1-4.x, undocumented)
      0D   word    directory entry number (0 based, undocumented)
      0F   word    starting cluster number of current directory; zero
               for root directory  (DOS 3.2+, undocumented)
           dword   pointer to DTA (DOS 2.x-3.1, undocumented)
      11   word    reserved
      13   word    starting cluster number of current directory; zero
               for root directory  (DOS 2.x+, undocumented)
      15   byte    attribute of matching file
      16   word    file time  (see ~FILE ATTRIBUTES~)
      18   word    file date  (see FILE ATTRIBUTES)
      1A   word    file size
      1E 13bytes   ASCIIZ filename and extension in the form NAME.EXT
               with blanks stripped

    - returns information on first file matching specifications
    - use ~INT 21,4F~ to retrieve following file matches
    - DOS 2.x cannot find . and .. entries, while DOS 3.x can unless
      they represent the root directory
    - character devices return a zero for size, time and date in DOS 2.x,
      while DOS 3.0 returns a 40h attribute and current time and date.
    - multiple calls to this function with a character device will
      result in unpredictable results
    - normal files are always included along with files that match the
      requested attributes except when the LABEL attribute is requested.
      DOS 2.x returns all normal files when label is specified but 3.x
      doesn't.  It's up to the programmer to determine which actually
      match the requested attributes.
    - bit 8 of CX (file attributes) indicates Novell Netware shareable
    - see    INT 21,1A

:int 21,4f
^INT 21,4F - Find Next Matching File


    AH = 4F
    DS:DX = unchanged from previous function 4E


    on return:
    AX = error code if CF set  (see ~DOS ERROR CODES~)


    - ~INT 21,4E~ should be called to find the first file and this
      function should be called to find all other matching files
    - normal files are always included along with files that match
      the requested attributes except when the LABEL attribute is
      requested.  It's up to the programmer to determine which
      actually match the requested attributes.
    - see  ~INT 21,1A~  ~DTA~  ~FILE ATTRIBUTES~

:int 21,50
^INT 21,50 - Set Current Process ID  (Undocumented DOS 2.x)


    AH = 50h
    BX = process ID number (process ~PSP~ segment address)


    returns nothing


    - the process ID number is actually the segment address of a
      program's PSP.  This is useful for TSR's to access their own
      file handle table inside their PSP.
    - this function cannot be called while in an ~INT 28~ handler in
      DOS 2.x unless the critical error flag is set or stack corruption
      will occur
    - safe in DOS 3.x INT 28 handlers since they use a different stack
      by default
    - available in OS/2 compatibility box
    - see  ~INT 21,51~   ~INT 21,62~

:int 21,51
^INT 21,51 - Get Current Process ID  (Undocumented DOS 2.x)

    AH = 51h

    on return:
    BX = process ID


    - The process ID  number is actually the segment address of program's
      PSP.     This in conjunction with ~INT 21,50~ is useful for TSR's to
      access their own file handle table in their respective ~PSP~.
    - this function cannot be called while in an ~INT 28~ handler in
      DOS 2.x unless the critical error flag is set or stack corruption
      will occur
    - ~INT 21,62~ is highly recommended for DOS 3.x due to a possible bug
      when activated from a TSR.  DOS may switch to the wrong internal
      stack which may cause a problems with TSR's if called during an
      INT 28.
    - see INT 21,62 (Get PSP segment) for DOS 3.x applications
:int 21,52:INVARS
^INT 21,52 - Get Pointer to DOS "INVARS"  (Undocumented)

    AH = 52h

    on return:
    ES:BX = pointer to DOS "invars", a table of pointers used by DOS.
        Known "invars" fields follow (varies with DOS version):

%    Offset Size         Description

     -12   word   sharing retry count (DOS 3.1-3.3)
     -10   word   sharing retry delay  (DOS 3.1-3.3)
      -8   dword  pointer to current disk buffer (DOS 3.x)
      -4   word   pointer in DOS code segment of unread CON input;
              0 indicates no unread input (DOS 3.x)
      -2   word   segment of first Memory Control Block (~MCB~)
      00   dword  pointer to first ~DRIVE PARAMETER TABLE~ (A:) in chain
      04   dword  pointer to DOS ~System File Table~ (SFT)
      08   dword  pointer to $CLOCK device driver
      0C   dword  pointer to CON device driver
      10   byte   number of logical drives in system
      11   word   maximum bytes/block of any block device
      13   dword  pointer to DOS cache buffer header
      17 18bytes  NUL device header, first 4 bytes of device header
              point to the next device in device chain


^DOS 3.0 Specific Information

%    Offset Size        Description

      10   byte   number of block devices
      11   word   max sector size used by DOS prior to loading block
              device drivers;  limits allowable device sector size
      13   dword  pointer to DOS cache buffer header
      17   dword  pointer to drive information table
      1B   byte   value of LASTDRIVE command in CONFIG.SYS
      1C   dword  pointer to STRING= workspace area
      20   word   size of STRING area (CONFIG.SYS value of STRING=x)
      22   dword  ~FCB~ file table pointer
      26   word   number of protected FCBs
      28  18bytes NUL device header, first 4 bytes of device header
              point to the next device in device chain


^DOS 3.1-3.3  Specific Information

%    Offset Size         Description

      10   word   max sector size used by DOS prior to loading block
              device drivers;  limits allowable device sector size
      12   dword  pointer to DOS cache buffer header
      16   dword  pointer to drive information table
      1A   dword  ~FCB~ file table pointer
      1E   word   number of protected FCBs
      20   byte   number of block devices
      21   byte   value of LASTDRIVE command in CONFIG.SYS
      22  18bytes NUL device header, first 4 bytes of device header
              point to the next device in device chain
      34   byte   number of JOIN'ed drives


^DOS 4.x  Specific Information

%    Offset Size        Description

      10   word   max sector size used by DOS prior to loading block
              device drivers;  limits allowable device sector size
      12   dword  pointer to DOS cache buffer header
      16   dword  pointer to drive information table
      1A   dword  FCB file table pointer
      1E   word   number of protected FCBs
      20   byte   number of block devices
      21   byte   value of LASTDRIVE command in CONFIG.SYS (default 5)
      22  18bytes NUL device header, first 4 bytes of device header
              point to the next device in device chain
      34   byte   number of joined drives
      35   word   pointer within IBMDOS code segment to list of special
              program names
      37   dword  pointer to resident IFS utility function
      3B   dword  pointer to chain of installable file system drivers
      3F   word   the x in BUFFERS x,y (rounded up to multiple of 30
              if EMS is used)
      41   word   the y in BUFFERS x,y
      43   byte   boot drive (1=A:)
      44   byte   ???
      45   byte   extended memory size in K bytes


    - see ~Bibliography~ references for Bernd Schemmer & "Undocumented DOS"
    - see   ~INDOS~  ~INT 21,5D~
:int 21,53
^INT 21,53 - Generate Drive Parameter Table  (Undocumented)


    AH = 53h
    DS:SI = address of BIOS Parameter Block (~BPB~)
    ES:BP = pointer to buffer to hold first Drive Parameter Table (~DPT~)


    returns:
    ES:BP = pointer to buffer containing the first Drive Parameter Table
        in chain


    - available since DOS 2.0+
    - translates BPB  into a DOS Disk Parameter Table
    - see  ~INT 21,32~  ~INT 21,52~
:int 21,54
^INT 21,54 - Get Verify Setting


    AH = 54h


    on return:
    AL = 00  verify off
       = 01  verify on


    - returns value of disk read after write verification flag

    - see    ~INT 21,2E~

:int 21,55
^INT 21,55 - Create New PSP  (Undocumented)

    AH = 55h
    DX = New PSP segment address
    SI = memory size value to place in ~PSP~ offset 02h (DOS 3+)

    returns nothing

    - the following should be observed when using this function

      o  Allocate memory for the PSP and program code  (~INT 21,48~)
      o  Duplicate the PSP     (~INT 21,55~)
      o  Load program code into allocated segment above the new PSP
      o  Save Current PSP
      o  Set PSP to the PSP just created  (~INT 21,50~)
      o  Jump to start of code if .COM otherwise handle relocation,
         setup stack and registers

    - similar to ~INT 21,26~ except the PSP is setup by DOS
    - increments reference count in ~SFT~ for all inherited files
    - files flagged as not inheritable are marked as closed in the
      new PSP
    - invoked by ~INT 21,4B~

    - see    ~UNDOC~   ~INT 21,4B~   ~INT 21,26~
:int 21,56
^INT 21,56 - Rename File


    AH = 56h
    DS:DX = pointer to old ASCIIZ path/filename
    ES:DI = pointer to new ASCIIZ path/filename


    on return:
    AX = error code if CF set  (see ~DOS ERROR CODES~)


    - supports full pathnames and allows renaming files across
      directories and in DOS 3.x allows renaming subdirectories
    - does not support use of wildcards unless invoked from via
      ~INT 21,5D~ in which case error code 12h is returned
    - unpredictable result may occur if an opened file is renamed
    - see ~Bibliography~ reference to "Undocumented DOS"
:int 21,57
^INT 21,57 - Get/Set File Date and Time Using Handle

    AH = 57h
    AL = 00  get date and time
       = 01  set date and time
       = 02  ??? (DOS 4.0+ undocumented)
       = 03  ??? (DOS 4.0+ undocumented)
       = 04  ??? (DOS 4.0+ undocumented)
    BX = file handle
    CX = time to set (if setting)
    DX = date to set (if setting)
    ES:DI = pointer to buffer to contain results


    on return:
    AX = error code if CF set  (see  ~DOS ERROR CODES~)
    CX = file time (if reading, see below)
    DX = file date (if reading, see below)

%    Time encoding:

    │F│E│D│C│B│A│9│8│7│6│5│4│3│2│1│0│  Time in CX
     │ │ │ │ │ │ │ │ │ │ │ └─┴─┴─┴─┴────  two second incr (0-29)
     │ │ │ │ │ └─┴─┴─┴─┴─┴─────────────  minutes 0-59)
     └─┴─┴─┴─┴────────────────────────  hours (0-29)

%    Date Encoding

    │F│E│D│C│B│A│9│8│7│6│5│4│3│2│1│0│  Date in DX
     │ │ │ │ │ │ │ │ │ │ │ └─┴─┴─┴─┴────  day (1-31)
     │ │ │ │ │ │ │ └─┴─┴─┴─────────────  month (1-12)
     └─┴─┴─┴─┴─┴─┴────────────────────  year - 1980
:int 21,58
^INT 21,58 - Get/Set Memory Allocation Strategy
^(Undocumented, DOS 3.x)

    AH = 58h
    AL = 00  get strategy code
       = 01  set strategy code
    BX = strategy code (when AL = 01)
       = 00  first fit  (default)
       = 01  best fit
       = 02  last fit

    on return:
    AX = strategy code if CF clear
       = error if CF set, see  ~DOS ERROR CODES~


    - defaults to first fit, first block large enough is chosen
    - in first fit, DOS searches the ~MCB~ chain from low addresses to
      high for the first block large enough to fill the request.  For
      best fit, DOS searches all memory blocks for the closest fit.
      In last fit, DOS starts at high addresses and works downward
      and uses the high part of the chosen block
    - any strategy value greater than 2 defaults to 2, but the last value
      set, even if incorrect, is the value returned by the get strategy
:int 21,59
^INT 21,59 - Get Extended Error Information (DOS 3.0+)

    AH = 59h
    BX = 00 for versions  3.0, 3.1, 3.2


    on return:
    AX = extended error code (see ~DOS ERROR CODES~)
       = 0 if no error
    BH = error class
    BL = suggested action
    CH = locus


    - may be called after any  ~INT 21~  function or from
      ~INT 24~ when an error is returned
    - must be called immediately after the error occurs
    - registers CX, DX, DI, SI, BP, DS and ES are destroyed
:int 21,5a
^INT 21,5A - Create Temporary File  (DOS 3.0+)


    AH = 5A
    CX = attribute
    DS:DX = pointer to ASCIIZ path ending in '\'


    on return:
    AX = handle if call CF clear
       = error code if CF set  (see ~DOS ERROR CODES~)
    DS:DX = pointer to updated ASCIIZ filespec


    - creates files with random names with any combination of
      the following attributes: normal, system and hidden
    - ending backslash is required

:int 21,5b
^INT 21,5B - Create File  (DOS 3.0+)


    AH = 5B
    CX = attribute
    DS:DX = pointer to ASCIIZ path/filename


    on return:
    AX = handle if CF not set
       = error code if CF set  (see ~DOS ERROR CODES~)


    - standard method of opening files
    - returns a file handle of a file opened with specified
      attributes (combinations of normal, system and hidden)

:int 21,5c
^INT 21,5C - Lock/Unlock File Access  (DOS 3.0+)

    AH = 5C
    AL = 00  lock file
       = 01  unlock file
    BX = file handle
    CX = most significant word of region offset
    DX = least significant word of region offset
    SI = most significant word of region length
    DI = least significant word of region length


    on return:
    AX = error code if CF set  (see ~DOS ERROR CODES~)


    - used for networking and multi-tasking system to preserve
      data integrity
    - duplicated handles inherit access to locked regions
    - EXEC'd programs do not inherit access to locked regions
:int 21,5d
^INT 21,5D - Critical Error Information  (Undocumented, DOS 3.x+)

%    See second reference below for more information about this interrupt
%    information presented here is incomplete

    AH = 5D
    AL = 00  server function call  (DOS 3.1+)
       = 01  commit all files  (DOS 3.1+)
       = 02  SHARE: close file by name  (DOS 3.1+)
       = 03  SHARE: close all files for given computer  (DOS 3.1+)
       = 04  SHARE: close all files for given process  (DOS 3.1+)
       = 05  SHARE: get open file list entry  (DOS 3.1+)
       = 06  get address of DOS swappable area into DS:SI  (DOS 3.0+)
       = 07  get network redirected printer mode  (DOS 3.1+)
       = 08  set network redirected printer mode  (DOS 3.1+)
       = 09  flush network redirected printer output  (DOS 3.1+)
       = 0A  set extended error information  (DOS 3.1+)
       = 0B  get DOS swappable data areas  (DOS 4.x+)
    DS:DX = pointer to 18 byte DOS Parameter List (DPL, if AL=00)
          = pointer to 9 byte data block of the form (AL=0A):

%          Offset Size    Description

        00   word   extended error code to set
        02   dword  pointer to driver address to set
        06   byte   action code to set
        07   byte   class code to set
        08   byte   locus code to set

    on return:
    DS:SI = (if AL was 6) address of critical flag of the form:

%          Offset Size      Description

        00   word   extended error code
        02   byte   action code
        03   byte   class code
        04   byte   pointer to driver address

    - function 0 copies 18 bytes from DS:SI to the DOS internal
      register-save area;  this data will be placed in the registers
      when DOS returns to the caller thereby circumventing the DOS
      register save logic
    - may be used by a TSR to prevent accidental changing of an error
      code and causing problems in the foreground process
    - see ~bibliography~ references for "Undocumented DOS" and "DOS
      Programmers Reference"

    - see    ~INT 21,59~
:int 21,5e
^INT 21,5E  AL=0  Get Machine Name  (DOS 3.1+)

    AH = 5E
    AL = 00 get machine name
    DS:DX = far pointer to buffer receiving name string


    on return:
    AX = error code if CF set, (invalid function)
    CH = 0    if name not defined
       > 0    if name defined
    CL = NETBIOS name number, if CH not 0
    DS:DX = far pointer to buffer containing string if CH not 0


    - returns pointer to an ASCIIZ string identifying the
      computer on a Microsoft network
    - output string is a 15 bytes long, padded with blanks and
      null terminated
    - unpredictable if called without file sharing loaded


^INT 21,5E  AL=01  Set Machine Name  (DOS 3.1+)

    AH = 5E
    AL = 01 set machine name
    CH = 00 undefine name
       = any other value means to define name
    CL = name number
    DS:DX = pointer to 15 byte, blank padded ASCIIZ name string


    on return:
    AX = error code if CF set, (invalid function)


    - specifies the network machine name on a Microsoft network
    - unpredictable if called without file sharing loaded


^INT 21,5E  AL=02  Set Printer Setup  (DOS 3.1+)


    AH = 5E
    AL = 02 set printer setup
    BX = redirection list index (see ~INT 21,5F~ AL=2)
    CX = length of setup string
    DS:SI = far pointer to printer setup string


    on return:
    AX = error code if CF set, (invalid function)


    - see INT 21,5F subfunction 2 for information on obtaining the
      redirection list index
    - fails if file sharing is not loaded


^INT 21,5E  AL=03  Get Printer Setup  (DOS 3.1+)


    AH = 5E
    AL = 03 get printer setup
    BX = redirection list index (see ~INT 21,5F~ AL=2)
    ES:DI = far pointer to buffer to receive setup string


    on return:
    AX = error code if CF set


    - see INT 21,5F subfunction 2 for information on obtaining the
      redirection list index
    - fails if file sharing is not loaded


^INT 21,5E  AL=04  Set Printer Mode  (DOS 3.1+)


    AH = 5E
    AL = 04  set printer setup
    BX = redirection list index (see INT 21,5F AL=2)
    DX = printer mode to set (bit zero is set for binary mode and clear
         for translated/text mode)


    on return:
    AX = error code if CF set


    - see INT 21,5F subfunction 2 for information on obtaining the
      redirection list index
    - fails if file sharing is not loaded
    - issues ~INT 2F,11~  and  ~INT 2F,1F~


^INT 21,5E  AL=05  Get Printer Mode  (DOS 3.1+)


    AH = 5E
    AL = 05  set printer setup
    BX = redirection list index (see INT 21,5F AL=2)


    on return:
    AX = error code if CF set
    DX = printer mode where bit zero is set for binary mode and
         clear for translated mode



    - see ~INT 21,5F~ subfunction 2 for information on obtaining the
      redirection list index
    - fails if file sharing is not loaded
    - issues  ~INT 2F,11~  and  ~INT 2F,1F~
:int 21,5f
^INT 21,5F  AL=00  Get Redirection Mode  (DOS 3.1+)


    AH = 5F
    AL = 00
    BL = redirection type
       = 03  printer
       = 04  disk drive


    on return:
    AX = error code if CF is set
    BH = device redirection state
       = 00  redirection off
       = 01  redirection on


    - determines if disk or printer redirection is enabled
    - fails if file sharing is not loaded


^INT 21,5F  AL=01  Set Redirection Mode  (DOS 3.1+)


    AH = 5F
    AL = 01
    BH = device redirection state
       = 00  turn redirection off
       = 01  turn redirection on
    BL = redirection type
       = 03  printer
       = 04  disk drive


    on return:
    AX = error code if CF is set


    - enables or disables disk or printer redirection
    - local devices are used when redirection is off
    - fails if file sharing is not loaded


^INT 21,5F  AL=02  Get Redirection List Entry  (DOS 3.1+)

    AH = 5F
    AL = 02
    BX = redirection list index
    DS:SI = far pointer to a 16 byte buffer to hold device name
    ES:DI = far pointer to a 128 byte buffer to hold network name

    on return:
    AX = error code if CF is set
       = 1    invalid function code
       = 12 no more files
    BH = device status flag,
       bit 0 = 0  device valid
       bit 0 = 1  device invalid
    BL = device type
       = 3    printer
       = 4    drive
    CX = stored parameter value
    DS:SI = far pointer to ASCIIZ local device name
    ES:DI = far pointer to ASCIIZ network name

    - value returned in CX is the value previously passed to
      INT 21,5F with subfunction 3 in register CX
    - registers DX and BP are destroyed
    - fails if file sharing is not loaded


^INT 21,5F  AL=03  Redirect Device  (DOS 3.1+)

    AH = 5F
    AL = 03
    BL = device type
       = 3 if printer
       = 4 if drive
    CX = parameter to save for caller
    DS:SI = far pointer to ASCIIZ local device name
    ES:DI = far pointer to ASCIIZ network name followed
        by ASCIIZ password

    on return
    AX = error code if CF is set
       = 1    invalid function code or
        invalid source or destination string format or
        source device already redirected
       = 3    path not found
       = 5    access denied
       = 8    insufficient memory

    - parameter passed in CX can later be retrieved by calls
      to INT 21,5F subfunction 2
    - local device name in DS:SI can be a drive designator
      (D:), a printer name (PRN or LPTx), or a null string
    - when a null string is followed by a password is used, DOS
      tries granting access to the network directory using the
      given password
    - fails if file sharing is not loaded


^INT 21,5F  AL=04  Cancel Device Redirection  (DOS 3.1+)


    AH = 5F
    AL = 04
    DS:SI = far pointer to ASCIIZ local device name


    on return
    AX = error code if CF set
       = 01  function code invalid
         ASCIIZ string is not an existing source device
       = 0F  redirection paused on server


    - local device name in DS:SI can be a drive designator
      (D:), a printer name (PRN or LPTx), or a string beginning
      with 2 backslashes "\\"
    - when two backslashes are used for the string the connection
      between the local machine and network directory terminates
    - fails if file sharing is not loaded

^INT 21,5F  AL=05  Get Redirection List Extended Entry (DOS 4.0+)

    AH = 5F
    AL = 05
    BX = redirection list index
    DS:SI = pointer to buffer to receive ASCIIZ source device name
    ES:DI = pointer to buffer to receive ASCIIZ network path

    on return
    AX = error code if CF set
    BH = device status flag (bit 0 clear if valid)
    BL = device type
       = 03  printer device
       = 04  disk device
    CX = stored user parameter value
    BP = NETBIOS local session number
    DS:SI = pointer to buffer containing ASCIIZ source device name
    ES:DI = pointer to buffer containing ASCIIZ network path


    - retrieves the status, type, source and target of a given redirection
    - fails if file sharing is not loaded


^INT 21,5F  AL=06  Get Redirection List (DOS 4.0+)


    AH = 5F
    AL = 06

    - no information is currently available for this subfunction
:int 21,60
^INT 21,60 - Get Fully Qualified File Name (Undocumented 3.x+)


    AH = 60h
    DS:SI = pointer to ASCIIZ string containing unqualified filename
    ES:DI = pointer to 128 byte buffer to contain fully qualified filename


    on return:
    ES:DI = address of fully qualified filename string
    AH = error code if CF set


    - undocumented, available in DOS 3.x+
    - does not check for file existence, simple provides a fully
      qualified filename for the given filespec
    - this function ignores ASSIGN'ed, JOIN'ed, SUBST'ed and network
      redirections and returns the true path name
    - returned string is in upper case and has forward slashes replaced
      by back slashes;  filename always begins with "d:" or "\\" format
    - AL register is destroyed
    - supported by OS/2 1.1 compatibility box
:int 21,62
^INT 21,62 - Get PSP address  (DOS 3.x)


    AH = 62h


    on return:
    BX = segment address of current process



    - this is useful for TSR's to access their own file handles
    - for DOS 2.x use ~INT 21,51~

    - see also  ~INT 21,50~
:int 21,63
^INT 21,63 - Get Lead Byte Table  (MSDOS 2.25 only)

%    See below for information on the Asian INT 21,63 specifications

    AH = 63h
    AL = 00  get address of system lead byte table
       = 01  set or clear interim console flag
       = 02  get value of interim console flag
    DL = 00  clear interim console flag (when AL=01)
       = 01  set interim console flag (when AL=01)

    on return
    AX = error code if CF set
    DS:SI = far pointer to lead byte table (AL=00)
    DL = value of interim console flag (AL=02)

    - available only in MSDOS 2.25
    - provides multiple byte character support
    - function 00, returns the address of system table containing
      legal lead byte ranges
    - function 01 & 02 set and get the interim console flag which
      determines whether interim bytes are returned on some console
      functions
    - all registers except CS:IP and SS:SP may be destroyed


^INT 21,63 - Get Lead Byte Table  (Asian DOS 3.2+)

    AH = 63h
    AL = 00  get address of Double Byte Character Set (DBCS) lead table
       = 01  set Hongeul (Korean) input mode
       = 02  get Hongeul (Korean) input mode
    DL = new keyboard input mode
       = 00  return completely formed characters only
       = 01  return partially formed characters also


    on return
    AL = 00 on success
       = FF if function not supported
    DS:SI = pointer to DBCS table  (function 00)
    DL = current keyboard input mode (function 02)
       = 00  only completely formed characters are accepted
       = 01  partially formed characters are accepted


    - available only in Asian MSDOS 3.2+
    - provides Asian multiple byte character support
    - all registers except CS:IP and SS:SP may be destroyed
    - DOS 4.0 returns an empty list
:int 21,64
^INT 21,64 - Set Device Driver Look Ahead  (Undocumented)


    AH = 64h
    AL = value for switch
         00   causes DOS to perform look ahead to the device driver
          before execution of ~INT 21,1~, ~INT 21,8~ and ~INT 21,A~
         other - no look ahead is performed


    returns ???


    - DOS internal; should not be called by user program
    - switch defaults to zero
    - undocumented; since DOS 3.3
    - some other subfunctions are know to have existed since DOS 3.2 but
      details on their functionality are incomplete
:int 21,65
^INT 21,65 - Get Extended Country Information (DOS 3.3+)

    AH = 65h
    AL = 01  get extended country information
       = 02  get pointer to character translation table
       = 04  get pointer to filename character translation table
       = 05  get pointer to filename terminator table
       = 06  get pointer to collating sequence
       = 07  get segment of DCBS vector (DOS 4.x)
       = 20  country dependant character capitalization (DOS 4+)
       = 21  country dependant string capitalization (DOS 4+)
       = 22  country dependant ASCIIZ string capitalization (DOS 4+)
       = 23  determine if character represents country relative
         Yes or No response (DOS 4+)
       = A0  country dependant filename character capitalization (DOS 4+)
       = A1  country dependant filename string capitalization (DOS 4+)
       = A2  country dependant ASCIIZ filename capitalization (DOS 4+)
    BX = code page (-1 = current global code page, AL < 20h)
    CX = amount of data to return
       = length of string (AL=21h, A1h)
    DX = country ID (-1 = current country, AL < 20h)
    DL = character to capitalize or test (AL=20h, 23h, A0h)
    DH = second character if double byte character (AL=23h)
    ES:DI = pointer to output buffer (AL < 20h, see tables below)
    DS:DX = pointer to string to capitalize (AL=21h, 22h, A1h, A2h)


    on return
    AX = error code if CF is set  (see ~DOS ERROR CODES~)
       = 00  NO response if (AL was 23h)
       = 01  YES response if (AL was 23h)
       = 02  not a yes or no response (AL was 23h)
    CX = amount of data returned
    DL = capitalize character (AL=20)
    ES:DI pointer to returned table of the forms:


%    Offset Size    Table for Function 01

      00   byte   country id = 01
      01   word   size (38 or less)
      03   word   country id
      05   word   code Page
      07   word   date Format
      09  5bytes  currency symbol string (ASCIIZ)
      0E  2bytes  thousands separator string (ASCIIZ)
      10  2bytes  decimal separator string (ASCIIZ)
      12  2bytes  date separator string (ASCIIZ)
      14  2bytes  time Separator string (ASCIIZ)
      16   byte   currency symbol location
      17   byte   currency decimal places
      18   byte   time format
      19   dword  extended ASCII map call address
      1D  2bytes  list separator string (ASCIIZ)
      1F  10bytes reserved


%    Offset Size    Table for Function 2
      00   byte   country Id = 02
      01   dword  pointer to uppercase table


%    Offset Size    Table for Function 4
      00   byte   country Id = 04
      01   dword  pointer to filename character translation table


%    Offset Size    Table for Function 5
      00   byte   country Id = 05
      01   dword  pointer to filename uppercase table


%    File Terminator Table for Function 5
      00   word   table size
      02  7bytes  unknown
      09   byte   length of following data
      0A  nbytes  list of filename termination characters, ex:
              < > . " / \ [ ] : | + = ; ,

%    Offset Size    Table for Function 6
      00   byte   country Id = 06
      01   dword  pointer to collating table


%    Offset Size    Table for Function 7
      00   byte   country Id = 07
      01   dword  pointer to byte pair table


    - functions A0, A1, A2 are implemented in DOS 4.x but don't work
:int 21,66
^INT 21,66 - Get/Set Global Code Page  (DOS 3.3+)


    AH = 66h
    AL = 01  get global code page
       = 02  set global code page
    BX = active code page if setting
    DX = system code page if setting


    on return
    AX = error code if CF set  (see ~DOS ERROR CODES~)
    BX = active code page  (for AL=1 only)
    DX = system code page  (for AL=1 only)


    - moves country data from COUNTRY.SYS to country memory table

:int 21,67
^INT 21,67 - Set Handle Count  (DOS 3.3+)

    AH = 67h
    BX = new maximum open handles allowed

    on return
    CF = 0 if successful
       = 1 if error
    AX = error code if CF is set  (see ~DOS ERROR CODES~)


    - gives program control of the number of files simultaneously open
    - if CX is less than the current number of open files the change
      will take effect when the number of open handles falls below
      the new limit
    - this function allows the application to use more than 20 files,
      up to the FILES=N limit
    - earlier copies of IBM DOS 3.3 sometimes incorrectly allocates
      memory (up to 64K) if an even number of handles is requested
    - only the first 20 files handles are copied to a child process
      regardless of the max number of files
    - it is possible to allocate more than 255 file handles but it is
      difficult to use more than 255

    - see  ~SFT~   ~FILE HANDLES~
:int 21,68
^INT 21,68 - Flush Buffer Using Handle    (DOS 3.3+)


    AH = 68h
    BX = file handle


    on return
    AX = error code if CF set  (see ~DOS ERROR CODES~)


    - flushes DOS buffers to disk, does not update directory entry

:int 21,69
^INT 21,69 - Get/Set Disk Serial Number  (Undocumented DOS 4+)

    AH = 69h
    AL = 00 get serial number
       = 01 set serial number
    BL = drive (0 = default, 1=A:, 2 = B:, ... )
    DS:DX = buffer to contain extended BIOS Parameter Block (~BPB~, AL=0)
          = disk information to set of the form:

%      Offset  Size        Description
        00      word      info level (zero)
        02      dword   disk serial number (binary)
        06    11bytes   volume label or "NO NAME    " if not present
        11     8bytes   filesystem type string "FAT12   " or "FAT16    "

    returns:
    CF = set on error
       AX = error code if CF set
          = 01  network drive not supported
          = 05  no extended BPB found on disk
    CF = clear if successful and value in AX is destroyed
       AL = 00  buffer filled from Extended BPB on disk
          = 01  disk Extended BPB updated from buffer
    DS:DX = pointer to returned data, when AL = 0


    - copies data from Extended BIOS Parameter Block to buffer
    - no critical error is generated on error
    - does not work on network drives (returns 01)
    - returned buffer (after offset 2) is similar to offsets 27 through
      3D of the Extended BPB found in the boot record

    - see  ~INT 21,59~
:int 21,6c
^INT 21,6C - Extended Open/Create  (DOS 4.x+)

    AH = 6C
    AL = 00
    BX = open mode (see below)
    CX = file attribute (see ~FILE ATTRIBUTES~, bits 15-8 are zero)
    DX = function control indicator (see below)
    DS:SI = ASCIIZ filespec


    returns:
    AX = handle if CF clear
       = error code if CF set  (see ~DOS ERROR CODES~)
    CX = 01  file opened
       = 02  file created and opened
       = 03  file truncated and opened (replaced)
    CF = 0 if successful
       = 1 if error


%    Open Mode in BX:

    │F│E│D│C│B│A│9│8│7│6│5│4│3│2│1│0│  BX  (Open Mode)
     │ │ │ │ │ │ │ │ │ │ │ │ │ └─┴─┴──── read/write access mode
     │ │ │ │ │ │ │ │ │ │ │ │ └──────── reserve (must be 0)
     │ │ │ │ │ │ │ │ │ └─┴─┴──────── sharing mode
     │ │ │ │ │ │ │ │ └──────────── 1 = private, 0 = inheritable
     │ │ │ └─┴─┴─┴─┴──────────── reserved
     │ │ └──────────────────── 0 = call INT 24, 1 = ignore INT 24
     │ └──────────────────── 0 = normal write, 1 = flush each write
     └──────────────────── reserved

%    Read/Write Access bits:     Sharing Mode bits:
%    210                  654
    000  read access          000  compatibility mode
    001  write access          001  read/write access
    010  read/write access          010  write access denied
                      011  read access denied
                      100  full access

%    Function Control Indicator in DX:

    │F│E│D│C│B│A│9│8│7│6│5│4│3│2│1│0│  DX  (Function Control)
     │ │ │ │ │ │ │ │ │ │ │ │ └─┴─┴─┴──── file existence behavior
     │ │ │ │ │ │ │ │ └─┴─┴─┴─────────── creation behavior
     └─┴─┴─┴─┴─┴─┴─┴────────────────── reserved

%    File Existence Behavior bits:
%    3210
    0000  if file exist, generate error, take no action
    0001  if file exists, open file
    0010  if file exists, truncate and open


%    Creation Behavior bits:
%    7654
    0000  if file does not exist, generate error, take no action
    0001  if file does not exist, create


:int 21,f8
^INT 21,F8 - Set OEM Int 21 Handler (functions F9-FF, Undocumented)


    AH = F8
    DS:DX = pointer to OEM handler for INT 21H calls F9 through FF
          = FFFF:FFFF to reset to original handlers


    - DOS is set up to allow ONE handler for all 7 function calls.
      Any call to these handlers will set the CF and AX
      will contain 1 if they are not initialized.    The handling
      routine is passed all registers just as the user set them.
      The handler should exit with an ~IRET~.
:int 22
^INT 22 - Program Terminate


    no input data


    - not an interrupt, but a vector to the terminate address
    - copied to ~PSP~ offset 0Ah during program load
    - do not execute this interrupt directly
:int 23:ctrl-c
^INT 23 - Control-Break Exit Address

    no input data

    - not an interrupt but a pointer to a routine that is called when a
      DOS function detects a ~Ctrl-Break~ or Ctrl-C has been pressed
      resulting in a "Break condition"
    - a Break condition is detected if DOS's internal Break flag is
      found set by ~INT 1B~ or the next word in the BIOS keyboard buffer
      is a scan code representing one of the Break key combinations.
      If a matching scan code is found, the keyboard buffer head pointer
      is incremented by 2 (which effectively removes the ^C keycode), the
      ^C is printed and then INT 23 is called.
    - disabling the abort is easy, but to disable the ^C from appearing
      you must either not use any of the DOS I/O functions that check
      for Break or you must trap the Ctrl-C keypress from within INT 9.
      Another method is to scan the BIOS keyboard buffer and delete any
      Break key combinations.
    - Ctrl-Break empties the keyboard buffer by resetting the keyboard
      head and tail pointers then places a 0000h at the queue head.  It
      then sets an internal "Break" flag.  DOS subfunctions that check
      for Break see this and then issue INT 23h.  DOS does not detect
      INT 1B using the keyboard buffer, but uses it's own internal flag.
    - Alt-Keypad-3 and Ctrl-2 also result in this interrupt
    - Ctrl-C places 2E03h in the BIOS keyboard buffer while Ctrl-2
      places 0300h and Alt-Keypad-3 places 0003h;  none of these key
      combinations empty the keyboard like Ctrl-Break but all result
      in a ^C being displayed;  note that all three produce scan codes
      containing '03'
    - since DOS checks only the first word of the keyboard buffer
      Ctrl-C, Ctrl-2 and Alt-Keypad-3 are only detected if they are the
      first word in the buffer while Ctrl-Break is detected via the
      internal flag and takes effect as soon as it's detected.
    - do not execute this interrupt directly
    - see also   ~INT 9~
:int 24:critical error handler
^INT 24 - Critical Error Handler

    no input data

    on entry to INT 24:
    AH = bit 7 of register AH is set to one if other than disk error
       = bit 7 of register AH is set to zero if disk error
         AL = failing drive number
         AH = bits 0-2 indicate affected disk area and read/write status

          │5│4│3│2│1│0│  AH
           │ │ │ │ │ └──── read = 0, write = 1
           │ │ │ └─┴───── 00=DOS, 01=FAT, 10=DIR, 11=data area
           │ │ └──────── 0=FAIL not allowed, 1=FAIL allowed
           │ └───────── 0=RETRY not allowed, 1=RETRY allowed
           └────────── 0=IGNORE not allowed, 1=IGNORE allowed

    DI = error code in lower byte
    BP:SI = pointer to device header control block where additional
        information about the error can be found


    on exit:
    AL = action code determining exit behavior
       = 00  ignore error
       = 01  retry
       = 02  terminate through ~INT 23~
       = 03  fail system call (DOS 3.x+)


%    Error Codes in low order byte of DI:

      00  write protect error
      01  unknown unit
      02  drive not ready
      03  unknown command
      04  data error (bad CRC)
      05  bad request structure length
      06  seek error
      07  unknown media type
      08  sector not found
      09  printer out of paper
      0A  write fault
      0B  read fault
      0C  general failure

%    Users Stack at Entry to Error Handler:

    (top of stack)
        IP           DOS code next instruction pointer
        CS
        FLAGS      DOS's flags
        AX -\
        BX     \
        CX      \
        DX       \
        SI        |- User's registers at entry to INT 21 function
        DI       /
        BP      /
        DS     /
        ES -/
        IP           User code next instruction pointer
        CS
        FLAGS

    - on entry registers are setup for a retry operation
    - user routine must issue an IRET or simulate an IRET
    - ~INT 21,0~ through ~INT 21,C~ and ~INT 21,59~ can safely be invoked
      from the handler.  Other calls may destroy DOS's stack
    - handler must preserve register SS,SP,DS,ES,BX,CX,DX
    - choosing ignore can show side effects, since it causes DOS
      to continue as if it the call were successful
    - if an improper action code is specified in DOS 3.x it is changed:
      if IGNORE is invalidly specified, action is converted to FAIL
      if RETRY is invalidly specified, action is converted to FAIL
      if FAIL is invalidly specified, action is converted to ABORT
    - IGNORE requests are converted to FAIL for ~FAT~ and ~DIR~ disk
      errors and network critical errors
    - if the user routine wishes to handle the error instead of passing
      it to DOS, it must restore the user program registers from the
      stack and remove all but the last 3 words from the stack (FLAGS,
      CS, IP) and issue an ~IRET~
    - do not execute this interrupt directly
    - ~INDOS~ flag is cleared on INT 24 (see ~INT 21,34~)
:int 25
^INT 25 - Absolute Disk Read

    AL = logical drive number (0=A:, 1=B:, 2=C:, ...)
    CX = number of sectors to read
       = -1 if DOS 4.x (control block pointer is in DS:BX)
    DX = starting logical sector number (see below for DOS 4.x+)
    DS:BX = pointer to data buffer
          = pointer to control block (DOS 4.x+, see below)

    on return:
    AH = error code if CF set:
         01  bad command
         02  bad address mark
         03  write protect
         04  sector not found
         08  DMA failure
         10  data error (bad CRC)
         20  controller failed
         40  seek failed
         80  attachment failed to respond
    AL = BIOS error code if CF set
         00  write protect error
         01  unknown unit
         02  drive not ready
         03  unknown command
         04  data error (bad CRC)
         05  bad request structure length
         06  seek error
         07  unknown media type
         08  sector not found
         0A  write fault
         0B  read fault
         0C  general failure

^Control Block Format (DOS 4.x):

%    Offset Size       Description

      00   dword  starting sector
      04   word   number of sectors to read
      06   dword  pointer to buffer


    - reads disk sectors into buffer at DS:BX or DS:[BX+6]
    - after calling this interrupt the flags register remains on the
      stack and must be popped manually
    - sectors are logical sectors starting at the beginning of a
      logical disk; each DOS partition on a drive unit is considered
      one logical drive and has it's own logical sector numbers with
      track 0 starting at the first track in the partition
    - this function uses logical drives, and is susceptible to ASSIGN
    - physical sector numbers can be converted to and from DOS sector
      numbers with the following formulas:

      dos_sector = (sector - 1) + (head * sectors_per_track) +
               (track * sectors_per_track * num_heads)

      physical_sector = 1 + (dos_sector  MOD  sectors_per_track)
      physical_head = (dos_sector / sectors_per_track)  MOD  num_heads
      physical_track = dos_sector / (sectors_per_track * num_heads)

    - see    ~INT 13,STATUS~
:int 26
^INT 26 - Absolute Disk Write

    AL = logical drive number (0=A:, 1=B:, 2=C:, ...)
    CX = number of sectors to write
       = -1 if DOS 4.x (control block pointer is in DS:BX)
    DX = starting logical sector number (see below for DOS 4.x+)
    DS:BX = pointer to data buffer
          = pointer to control block (DOS 4.x+, see below)

    on return:
    AH = error code if CF set:
         01  bad command
         02  bad address mark
         03  write protect
         04  sector not found
         08  DMA failure
         10  data error (bad CRC)
         20  controller failed
         40  seek failed
         80  attachment failed to respond
    AL = BIOS error code if CF set
         00  write protect error
         01  unknown unit
         02  drive not ready
         03  unknown command
         04  data error (bad CRC)
         05  bad request structure length
         06  seek error
         07  unknown media type
         08  sector not found
         0A  write fault
         0B  read fault
         0C  general failure

^Control Block Format (DOS 4.x):

%    Offset Size     Description

      00   dword  starting sector
      04   word   number of sectors to write
      06   dword  pointer to buffer


    - writes disk sectors from buffer at DS:BX or DS:[BX+6]
    - after calling this interrupt the flags register remains on the
      stack and must be popped manually
    - sectors are logical sectors starting at the beginning of a
      logical disk; each DOS partition on a drive unit is considered
      one logical drive and has it's own logical sector numbers with
      track 0 starting at the first track in the partition
    - this function uses logical drives, and is susceptible to ASSIGN
    - physical sector numbers can be converted to and from DOS sector
      numbers with the following formulas:

      dos_sector = (sector - 1) + (head * sectors_per_track) +
               (track * sectors_per_track * num_heads)

      physical_sector = 1 + (dos_sector  MOD  sectors_per_track)
      physical_head = (dos_sector / sectors_per_track)  MOD  num_heads
      physical_track = dos_sector / (sectors_per_track * num_heads)

    - see    ~INT 13,STATUS~
:int 27
^INT 27 - Terminate and Stay Resident


    DX = offset of last byte in program to remain resident plus 1
    CS = segment of ~PSP~


    returns nothing


    - terminates process without closing open files
    - memory beyond the offset in DX is freed


    - see    ~INT 21,31~

:int 28:DOS scheduler
^INT 28 - DOS Idle Loop / Scheduler  (Undocumented)




    - issued by DOS during keyboard poll loop
    - indicates DOS may be carefully re-entered by TSR
    - used by TSR programs to popup and make DOS disk I/O calls
    - supported by OS/2 compatibility box
    - default behavior is to simply perform an ~IRET~
    - any DOS functions above 0Ch may be called while in this handler
    - ~INT 21,3F~ and ~INT 21,40~ may not use a handle that refers to
      CON while in this handler
    - see also  ~INDOS~
:int 29
^INT 29 - DOS Fast Character I/O  (Undocumented 2.x+)


    AL = character to write


    returns nothing ???


    - used by DOS device drivers to output data to the screen if
      bit 4 of the device attribute word is set
    - similar to ~INT 21,2~ and ~INT 21,9~ but much faster
    - simply calls the BIOS ~INT 10,E~ in DOS versions 2.x and 3.x
    - advances the cursor after character output
:int 2a
^INT 2A - DOS Network Critical Section and NETBIOS

%    For more information see the following topics:

     ~INT 2A,0~  Network installation query
     ~INT 2A,1~  Execute NETBIOS request with no error retry
     INT 2A,2  Set network printer mode (not listed)
     ~INT 2A,3~  Check if direct I/O allowed
     ~INT 2A,4~  Execute NETBIOS request
     ~INT 2A,5~  Get network resource information
     ~INT 2A,6~  Network print stream control
    ~INT 2A,80~  Begin DOS critical section
    ~INT 2A,81~  End DOS critical section
    ~INT 2A,82~  End DOS critical sections 0 through 7
    ~INT 2A,84~  Keyboard busy loop
    ~INT 2A,87~  Critical section

    - functions requests are supplied in AH
:int 2a,0
^INT 2A,0 - Network Installation Query


    AH = 00


    on return:
    AH = zero if not installed
       = nonzero if installed


    - used to detemine if a Microsoft compatible network is present
:int 2a,1
^INT 2A,1 - Execute NETBIOS Request With No Error Retry


    AH = 01
    ES:BX = pointer to Network Control Block


    on return:
    AL = NETBIOS error code
    AH = zero if success
       = nonzero if error


    - similar to INT 5C
:int 2a,3
^INT 2A,3 - Check if Direct I/O Allowed


    AH = 03
    DS:SI = pointer to ASCIIZ disk device name or pathname (including ":")


    on return:
    CF = set if disk access is NOT allowed
       = clear if disk access is allowed


    - do not use ~INT 13~, ~INT 25~ or ~INT 26~ if this function returns
      with the CF set
    - see  ~INT 5F,2~
:int 2a,4
^INT 2A,4 - Execute NETBIOS Request


    AH = 04
    AL = 00  allow retry
       = 01  no retry allowed
    ES:BX = pointer to Network Control Block (NCB)


    on return:
    AX = 00 if successful
    AH = 01 if error
    AL = error code if AH = 01


    - invokes NETBIOS handler and allows retry of operation if specified
:int 2a,5
^INT 2A,5 - Get Network Resource Information


    AX = 0500


    on return:
    AX = destroyed
    BX = number of available network names
    CX = number of available NCB commands
    DX = number of available sessions


    - returns information on the number of available network resources
:int 2a,6
^INT 2A,6 - Network Print Stream Control


    AH = 06
    AL = 01 set concatenation mode where all printer output is placed
        in one job
       = 02 set truncation mode where each printer open and printer
        close starts a new job
       = 03 flush printer output then start a new print job (similar to
        pressing Ctrl-Alt-Kp-* )


    on return:
    AX = error code if CF set


    - allows control of network printers
:int 2a,80
^INT 2A,80 - Begin DOS Critical Section

    AH = 80h
    AL = critical section identifier (00-0F)
       = 01  DOS kernel, SHARE.EXE and network data structure integrity
       = 02  DOS kernel, device driver exclusivity
       = 05  IFSFUNC (Installable File System, IFS, DOS 4.0+)
       = 06  IFSFUNC (Installable File System, IFS, DOS 4.0+)
       = 08  ASSIGN.COM

    returns nothing


    - indicates that the region of code being entered should not be
      interrupted and identifies the critical region
    - this function is not normally called by user applications but is
      often hooked to monitor if DOS is in a critical section
    - this information is necessary to reenter DOS using the swappable
      data area returned by ~INT 21,5D~ subfunction 0B
    - see  ~Bibliography~ reference to "Undocumented DOS"
:int 2a,81
^INT 2A,81 - End DOS Critical Section

    AH = 81h
    AL = critical section identifier (00-0F)
       = 01  DOS kernel, SHARE.EXE and network data structure integrity
       = 02  DOS kernel, device driver exclusivity
       = 05  IFSFUNC (Installable File System, IFS, DOS 4.0+)
       = 06  IFSFUNC (Installable File System, IFS, DOS 4.0+)
       = 08  ASSIGN.COM


    returns nothing


    - indicates that a critical code section is being exited
    - this function is usually hooked rather then called directly from a
      user application
    - see  ~Bibliography" reference to "Undocumented DOS"
:int 2a,82
^INT 2A,82 - End DOS Critical Sections 0 thru 7


    AH = 82h


    returns nothing


    - the DOS function dispatcher calls this function during DOS
      function 00, functions above 0C (excluding function 59h) and
      during process termination
    - this function is usually hooked rather then called directly from
      a user application
    - see  ~Bibliography~ reference to "Undocumented DOS"
:int 2a,84
^INT 2A,84 - Keyboard Busy Loop


    AH = 84h


    returns nothing


    - allows other processes to continue while waiting for keyboard
      input, similar to ~INT 28~
    - see  BIBLIO reference to "Undocumented DOS"
:int 2a,87
^INT 2A,87 - Critical Section


    AH = 87h
    AL = 00  start critical section
       = 01  end critical section


    returns nothing


    - specifies the start or end of a critical section
    - called by PRINT.COM
    - see  ~Bibliography~ reference to "Undocumented DOS"
:int 2e
^INT 2E - Execute Command Using Base Level Command Interpreter
^(Undocumented DOS 2.0+)

    DS:SI = pointer to command string to be executed

    returns nothing ???

    - causes the base level COMMAND.COM to execute a command as if it
      were typed from the keyboard
    - the transient portion of COMMAND.COM must be loaded if not resident
    - first byte of the string contains the string length, inclusive
    - command string must be terminated with a CR
    - all registers including SS & SP are destroyed;  SS & SP should be
      preserved in your own code segment
    - can be used to modify parents environment with SET command
    - incompatible under some software multitasking systems
    - before returning to the parent program this interrupt will
      attempt to execute the "current" batch file (if the parent or
      any ancestors were invoked from a batch file.   This can cause
      remaining lines in a batch file to execute before the parent
      program finishes and fragment memory
:int 2f:multiplex interrupt
^INT 2F - DOS Multiplex Interrupt

%    For more information see the following topics:

                         ┌────── ERROR CODES ────
    ~INT 2F,0~  Get installed state         │ 01    Invalid function
    ~INT 2F,1~  Submit file             │ 02    File not found
    ~INT 2F,2~  Cancel file             │ 03    Path not found
    ~INT 2F,3~  Cancel all files         │ 04    Too many files
    ~INT 2F,4~  Pause / return status      │ 05    Access denied
    ~INT 2F,5~  End of status           │ 08    Queue full
    ~INT 2F,8~  DRIVER.SYS support         │ 09    Busy
    ~INT 2F,2E~ Error Translation Tables   │ 0C    Name too long
                         │ 0F    Invalid drive

     - function request is specified in AL or AH
    - see  ~INT 2F,0~  for installation checks
    - INT 2F uses AL to specify most documented functions but occasionally
      uses AH also.  This can be quite confusing so INT 2F functions using
      AH are not currently listed if the conflict but will be added at a
      later time.
:int 2f,0
^INT 2F,0 - DOS Multiplex Interrupt - Get Installed State

    AL = 00
    AH = 00  DOS 2.x PRINT.COM (undocumented, see note below)
         01  PRINT    (undocumented behavior also, see note below)
         05  Critical error handler (undocumented DOS 3.0+)
         06  ASSIGN  (see note)
         08  DRIVER.SYS (undocumented DOS 3.2+)
         10  SHARE
         11  Redirector/IFS is resident
         12  MultiDOS is resident
         13  Swap 13h and 19h
         14  NLSFUNC
         15  GRAPHICS.COM (see note below)
         MS CD-ROM extension (MSCDEX) (see note below)
         16  Windows 386
         17  DOS shell
         19  SHELLB.COM (DOS 4.0 only)
         1A  ANSI.SYS (DOS 4.0+)
         1B  XMA2EMS.SYS
         40  OS/2 compatibility box
         43  Himem XMS driver
         7A  Novell Netware IPX
         AD  DISPLAY.SYS internal (DOS 3.3+)
         AE  DOS installable command extension (DOS 3.3+)
         B0  GRAFTABL.COM (DOS 3.3+)
         B4  IBM 3270 emulation
         B7  APPEND  (DOS 3.3+)
         B8  LAN existence
         B9  LAN RECEIVER.COM
         BF  LAN REDIRIFS.EXE
         C0-FF reserved for user applications


    on return:
    AL = 00  not installed, ok to install
       = 01  not installed, do NOT install
       = FF  installed


    - the installed test for append returns non-zero if installed;
      zero if not installed
    - AH = 00 and 01 are used by DOS 2.x PRINT.COM and is not a check
      for installed state, see BIBLIO reference to "Undocumented DOS"
    - ASSIGN returns non-zero if installed
    - installation checks with AH=15 do not follow the standard format;
    - see  ~Bibliography~ reference to "Undocumented DOS"
:int 2f,1:DOS multiplex
^INT 2F,1 - Multiplex Interrupt - Submit file


    AL = 01
    AH = 01  resident portion of PRINT
         02  resident portion of ASSIGN
         06  get pointer to ASSIGN drive translation table (undoc DOS 3+)
         10  resident portion of SHARE
         B7  resident portion of APPEND
         C0-FF reserved for user applications
    DS:DX = pointer to a 5 byte packet containing an priority byte
        and a DWORD pointer to an ASCIIZ filename
    DS:DI

    on return:
    ES = segment of ASSIGN work area and assignment table (AH=6 only)

:int 2f,2
^INT 2F,2 - Multiplex Interrupt - Cancel file


    AL = 02
    AH = 01  resident portion of PRINT
         02  resident portion of ASSIGN
         10  resident portion of SHARE
         B7  resident portion of APPEND
         C0-FF reserved for user applications
    DS:DX = pointer to ASCIIZ filename to cancel


    returns nothing


    - accepts wildcards

:int 2f,3
^INT 2F,3 - Multiplex Interrupt - Cancel all files


    AL = 03
    AH = 01  resident portion of PRINT
         02  resident portion of ASSIGN
         10  resident portion of SHARE
         B7  resident portion of APPEND
         C0-FF reserved for user applications


    returns nothing

:int 2f,4
^INT 2F,4 - Multiplex Interrupt - Pause / return status


    AL = 04
    AH = 01  resident portion of PRINT
         02  resident portion of ASSIGN
         10  resident portion of SHARE
         B7  resident portion of APPEND
         C0-FF reserved for user applications


    on return:
    DX = error count
    DS:SI = pointer to queue of files


    - each file in the queue is represented by 64 byte filename
    - the file currently being printed is the first entry in the queue
    - the last entry in the queue is a null string

:int 2f,5
^INT 2F,5 - Multiplex Interrupt - End of Status


    AL = 05
    AH = 01  resident portion of PRINT
         02  resident portion of ASSIGN
         10  resident portion of SHARE
         B7  resident portion of APPEND
         C0-FF reserved for user applications


    on return:
    AH = error code  (see ~INT 2F~)

:int 2f,6
^INT 2F,6 - PRINT.COM Check for Error on Output Device (Undocumented)


    AL = 06
    AH = 01


    on return:
    AX = error code if CF set
       = 00 on success
    DS:SI = device driver header if CF set


    - checks the output device for an error condition
    - undocumented, available since DOS 3.3+
:int 2f,8
^INT 2F,8 - DRIVER.SYS Support    (Undocumented DOS 3.2+)


    AH = 08
    AL = 00  installation check
       = 01  add new block device
       = 02  execute device driver request
       = 03  get drive data table list (DOS 4.0+)
    DS:DI = pointer to drive data table  (AL=01)
    ES:BX = pointer to device driver request header (AL=02)


    on return:
    AL = 00  not installed, ok to install (AL=00)
       = 01  not installed, do NOT install (AL=00)
       = FF  installed (AL=00)
    ES:BX = updated device request header (AL=02)

    - function 01 scans DOS's drive data tables and appends the new
      data table to to the chain.  All tables referencing the same
      physical drive are updated

:int 2f,2e
^INT 2F,2E - Setup Error Code Translation Tables  (undoc. DOS 4.x+)


    - discovered in DOS 4.x by Jim Kyle, this function provides more
      detailed information on the pending critical error.

:int 2f,80
^INT 2F,80 - PRINT.COM Time Slice Allotment  (Undocumented DOS 3.1+)

    AL = 80h
    AH = 00


    returns nothing


    - DOS PRINT.COM is given a time slice
:int 33:mouse interrupt:mouse services
^INT 33 - Mouse Function Calls

%    For more information see the following topics:

    ~INT 33,0~   Mouse Reset/Get Mouse Installed Flag
    ~INT 33,1~   Show Mouse Cursor
    ~INT 33,2~   Hide Mouse Cursor
    ~INT 33,3~   Get Mouse Position and Button Status
    ~INT 33,4~   Set Mouse Cursor Position
    ~INT 33,5~   Get Mouse Button Press Information
    ~INT 33,6~   Get Mouse Button Release Information
    ~INT 33,7~   Set Mouse Horizontal Min/Max Position
    ~INT 33,8~   Set Mouse Vertical Min/Max Position
    ~INT 33,9~   Set Mouse Graphics Cursor
    ~INT 33,A~   Set Mouse Text Cursor
    ~INT 33,B~   Read Mouse Motion Counters
    ~INT 33,C~   Set Mouse User Defined Subroutine and Input Mask
    ~INT 33,D~   Mouse Light Pen Emulation On
    ~INT 33,E~   Mouse Light Pen Emulation Off
    ~INT 33,F~   Set Mouse Mickey Pixel Ratio
    ~INT 33,10~  Mouse Conditional OFF
    ~INT 33,13~  Set Mouse Double Speed Threshold
    ~INT 33,14~  Swap interrupt subroutines
    ~INT 33,15~  Get mouse driver state and memory requirements
    ~INT 33,16~  Save mouse driver state
    ~INT 33,17~  Restore mouse driver state
    ~INT 33,18~  Set alternate subroutine call mask and address
    ~INT 33,19~  Get user alternate interrupt address
    ~INT 33,1A~  Set mouse sensitivity
    ~INT 33,1B~  Get mouse sensitivity
    ~INT 33,1C~  Set mouse interrupt rate (InPort only)
    ~INT 33,1D~  Set mouse CRT page
    ~INT 33,1E~  Get mouse CRT page
    ~INT 33,1F~  Disable mouse driver
    ~INT 33,20~  Enable mouse driver
    ~INT 33,21~  Reset mouse software
    ~INT 33,22~  Set language for messages
    ~INT 33,23~  Get language number
    ~INT 33,24~  Get driver version, mouse type & ~IRQ~ number

    - function is specified in AX
    - a mickey is 1/200 inches
    - for additional information see your vendor documentation
    - function number occupies all of AX rather than AH


^Mouse functions can be broken down into the following classes:

%    Mouse Driver Control / Feedback Functions
    ~INT 33,0~   Mouse Reset/Get Mouse Installed Flag
    ~INT 33,15~  Get Mouse Driver State and Memory Requirements
    ~INT 33,16~  Save Mouse Driver State
    ~INT 33,17~  Restore Mouse Driver State
    ~INT 33,1C~  Set Mouse Interrupt Rate (InPort only)
    ~INT 33,1F~  Disable Mouse Driver
    ~INT 33,20~  Enable Mouse Driver
    ~INT 33,21~  Reset Mouse Software
    ~INT 33,24~  Get Driver Version, Mouse Type & IRQ Number

%    Mouse Cursor Control Functions
    ~INT 33,1~   Show Mouse Cursor
    ~INT 33,2~   Hide Mouse Cursor
    ~INT 33,4~   Set Mouse Cursor Position
    ~INT 33,7~   Set Mouse Horizontal Min/Max Position
    ~INT 33,8~   Set Mouse Vertical Min/Max Position
    ~INT 33,9~   Set Mouse Graphics Cursor
    ~INT 33,A~   Set Mouse Text Cursor
    ~INT 33,F~   Set Mouse Mickey Pixel Ratio
    ~INT 33,10~  Mouse Conditional OFF
    ~INT 33,13~  Set Mouse Double Speed Threshold
    ~INT 33,1A~  Set Mouse Sensitivity
    ~INT 33,1B~  Get Mouse Sensitivity

%    Mouse Button and Position Feedback Functions
    ~INT 33,3~   Get Mouse Position and Button Status
    ~INT 33,5~   Get Mouse Button Press Information
    ~INT 33,6~   Get Mouse Button Release Information
    ~INT 33,B~   Read Mouse Motion Counters

%    Video Control and Feedback Functions
    ~INT 33,1D~  Set Mouse CRT Page
    ~INT 33,1E~  Get Mouse CRT Page

%    Mouse Interrupt Setup Functions
    ~INT 33,C~   Set Mouse User Defined Subroutine and Input Mask
    ~INT 33,14~  Swap Interrupt Subroutines

%    Alternate Mouse Interrupt Setup Functions
    ~INT 33,18~  Set Alternate Subroutine Call Mask and Address
    ~INT 33,19~  Get User Alternate Interrupt Address

%    Light Pen Emulation Functions
    ~INT 33,D~   Mouse Light Pen Emulation On
    ~INT 33,E~   Mouse Light Pen Emulation Off

%    International Language Support Functions
    ~INT 33,22~  Set Language for Messages
    ~INT 33,23~  Get Language Number

:int 33,0
^INT 33,0 - Mouse Reset/Get Mouse Installed Flag

    AX = 00


    on return:
    AX = 0000  mouse driver not installed
         FFFF  mouse driver installed
    BX = number of buttons


    - resets mouse to default driver values:

      .  mouse is positioned to screen center
      .  mouse cursor is reset and hidden
      .  no interrupts are enabled (mask = 0)
      .  double speed threshold set to 64 mickeys per second
      .  horizontal mickey to pixel ratio (8 to 8)
      .  vertical mickey to pixel ratio (16 to 8)
      .  max width and height are set to maximum for video mode
:int 33,1
^INT 33,1 - Show Mouse Cursor


    AX = 01


    returns nothing


    - increments the cursor flag;  the cursor is displayed if flag
      is zero;  default flag value is -1

:int 33,2
^INT 33,2 - Hide Mouse Cursor


    AX = 02


    returns nothing


    - decrements cursor flag; hides cursor if flag is not zero

:int 33,3
^INT 33,3 - Get Mouse Position and Button Status


    AX = 03


    on return:
    CX = horizontal (X) position  (0..639)
    DX = vertical (Y) position  (0..199)
    BX = button status:

        │F-8│7│6│5│4│3│2│1│0│  Button Status
          │  │ │ │ │ │ │ │ └──── left button (1 = pressed)
          │  │ │ │ │ │ │ └───── right button (1 = pressed)
          └──┴─┴─┴─┴─┴─┴────── unused


    - values returned in CX, DX are the same regardless of video mode

:int 33,4
^INT 33,4 - Set Mouse Cursor Position


    AX = 4
    CX = horizontal position
    DX = vertical position


    returns nothing


    - default cursor position is at the screen center
    - the position must be within the range of the current video mode
    - the position may be rounded to fit screen mode resolution

:int 33,5
^INT 33,5 - Get Mouse Button Press Information


    AX = 5
    BX = 0    left button
         1    right button


    on return:
    BX = count of button presses (0-32767), set to zero after call
    CX = horizontal position at last press
    DX = vertical position at last press
    AX = status:

        │F-8│7│6│5│4│3│2│1│0│  Button Status
          │  │ │ │ │ │ │ │ └──── left button (1 = pressed)
          │  │ │ │ │ │ │ └───── right button (1 = pressed)
          └──┴─┴─┴─┴─┴─┴────── unused

:int 33,6
^INT 33,6 - Get Mouse Button Release Information


    AX = 6
    BX = 0    left button
         1    right button


    on return:
    BX = count of button releases (0-32767), set to zero after call
    CX = horizontal position at last release
    DX = vertical position at last release
    AX = status

        │F-8│7│6│5│4│3│2│1│0│  Button status
          │  │ │ │ │ │ │ │ └──── left button (1 = pressed)
          │  │ │ │ │ │ │ └───── right button (1 = pressed)
          └──┴─┴─┴─┴─┴─┴────── unused

:int 33,7
^INT 33,7 - Set Mouse Horizontal Min/Max Position


    AX = 7
    CX = minimum horizontal position
    DX = maximum horizontal position


    returns nothing


    - restricts mouse horizontal movement to window
    - if min value is greater than max value they are swapped

:int 33,8
^INT 33,8 - Set Mouse Vertical Min/Max Position


    AX = 8
    CX = minimum vertical position
    DX = maximum vertical position


    returns nothing


    - restricts mouse vertical movement to window
    - if min value is greater than max value they are swapped

:int 33,9
^INT 33,9 - Set Mouse Graphics Cursor


    AX = 9
    BX = horizontal hot spot (-16 to 16)
    CX = vertical hot spot (-16 to 16)
    ES:DX = pointer to screen and cursor masks (16 byte bitmap)


    returns nothing


    - screen mask is AND'ed to screen Cursor Mask is XOR'ed
    - bytes 0-7 form the screen mask bitmap
    - bytes 8-F form the cursor mask bitmap

:int 33,a
^INT 33,A - Set Mouse Text Cursor


    AX = 0A
    BX = 00  software cursor
         01  hardware cursor
    CX = start of screen mask or hardware cursor scan line
    DX = end of screen mask or hardware cursor scan line


    returns nothing

:int 33,b
^INT 33,B - Read Mouse Motion Counters


    AX = 0B


    on return:
    CX = horizontal mickey count (-32768 to 32767)
    DX = vertical mickey count (-32768 to 32767)


    - count values are 1/200 inch intervals (1/200 in. = 1 mickey)

:int 33,c
^INT 33,C - Set Mouse User Defined Subroutine and Input Mask

    AX = 0C
    ES:DX = far pointer to user interrupt
    CX = user interrupt mask:

       │F-5│4│3│2│1│0│ user interrupt mask in CX
         │    │ │ │ │ └─── cursor position changed
         │    │ │ │ └──── left button pressed
         │    │ │ └───── left button released
         │    │ └────── right button pressed
         │    └─────── right button released
         └───────── unused

    returns nothing


    - routine at ES:DX is called if an event occurs and the
      corresponding bit specified in user mask is set
    - routine at ES:DX receives parameters in the following
      registers:

      AX = condition mask causing call
      CX = horizontal cursor position
      DX = vertical cursor position
      DI = horizontal counts
      SI = vertical counts
      DS = mouse driver data segment
      BX = button state:

         │F-2│1│0│
           │  │ └─── left button (1 = pressed)
           │  └──── right button (1 = pressed)
           └────── unused

    - initial call mask and user routine should be restore on exit
      from user program
    - user program may need to set DS to it's own segment
    - see    ~INT 33,14~

:int 33,d
^INT 33,D - Mouse Light Pen Emulation On


    AX = 0D


    returns nothing


    - turns on light pen emulation;  the light pen is considered
      down when both buttons are down;   when both buttons are
      up the pen is considered off screen

    - see    ~INT 33,E~

:int 33,e
^INT 33,E - Mouse Light Pen Emulation Off


    AX = 0E


    returns nothing


    - disables light pen emulation

    - see    ~INT 33,D~

:int 33,f
^INT 33,F - Set Mouse Mickey Pixel Ratio


    AX = 0F
    CX = horizontal ratio (1..32767, default 8)
    DX = vertical ratio (1..32767, default 16)


    returns nothing


    - sets the ratio between physical cursor movement (mickeys) and
      screen coordinate changes
    - CX and DX must be unsigned (high bit must be 0)

:int 33,10
^INT 33,10 - Mouse Conditional OFF


    AX = 10h
    CX = upper X screen coordinate
    DX = upper Y screen coordinate
    SI = lower X screen coordinate
    DI = lower Y screen coordinate


    returns nothing


    - defines screen region for updating in which the mouse is hidden
      if found these coordinates (INT 33,1 must be used to turn cursor
      on again)

    - see  ~INT 33,1~

:int 33,13
^INT 33,13 - Set Mouse Double Speed Threshold


    AX = 13h
    DX = threshold speed (mickeys per second, default 64)


    returns nothing


    - cursor speed is doubled when the cursor moves across the screen
      at the threshold speed

:int 33,14
^INT 33,14 - Swap Interrupt Subroutines

    AX = 14h
    ES:DX = far pointer to user routine
    CX = user interrupt mask:

        │F-8│7│6│5│4│3│2│1│0│ user interrupt mask in CX
          │  │ │ │ │ │ │ │ └─── cursor position changed
          │  │ │ │ │ │ │ └──── left button pressed
          │  │ │ │ │ │ └───── left button released
          │  │ │ │ │ └────── right button pressed
          │  │ │ │ └─────── right button released
          └──┴─┴─┴──────── unused

    on return:
    CX = previous user interrupt mask
    ES:DX = far pointer to previous user interrupt

    - routine at ES:DX is called if an event occurs and the
      corresponding bit specified in user mask is set
    - routine at ES:DX receives parameters in the following
      registers:

      AX = condition mask causing call
      CX = horizontal cursor position
      DX = vertical cursor position
      DI = horizontal counts
      SI = vertical counts
      DS = mouse driver data segment
      BX = button state:

         │F-2│1│0│
           │  │ └─── left button (1 = pressed)
           │  └──── right button (1 = pressed)
           └────── unused

    - initial call mask and user routine should be restore on exit
      from user program
    - user program may need to set DS to it's own segment
    - see    ~INT 33,C~

:int 33,15
^INT 33,15 - Get Mouse Driver State and Memory Requirements


    AX = 15h


    on return
    BX = buffer size need to hold current mouse state


    - used before mouse functions 16h and 17h to determine memory
      needed to save mouse state before giving up control of mouse
      to another program

:int 33,16
^INT 33,16 - Save Mouse Driver State


    AX = 16h
    ES:DX = far pointer to mouse state save buffer


    returns nothing


    - used to save mouse information before relinquishing control
      to another programs mouse handler
    - see ~INT 33,15~   ~INT 33,17~

:int 33,17
^INT 33,17 - Restore Mouse Driver State


    AX = 17h
    ES:DX = far pointer to mouse state save buffer


    returns nothing


    - used to restore mouse information after regaining control
      from another programs mouse handler
    - see ~INT 33,15~   ~INT 33,16~

:int 33,18
^INT 33,18 - Set alternate subroutine call mask and address

    AX = 18h
    DX = offset to function
    CX = user interrupt mask:

        │F-8│7│6│5│4│3│2│1│0│ user interrupt mask in CX
          │  │ │ │ │ │ │ │ └─── alt key pressed during event
          │  │ │ │ │ │ │ └──── ctrl key pressed during event
          │  │ │ │ │ │ └───── shift key pressed during event
          │  │ │ │ │ └────── right button up event
          │  │ │ │ └─────── right button down event
          │  │ │ └──────── left button up event
          │  │ └───────── left button down event
          │  └────────── cursor moved
          └──────────── unused

    - entire mask is set to zero when ~INT 33,0~ is called
    - up to three handlers may be defined with this call
    - mask should be cleared before program exit
    - when handler is called it receives parameters in the following
      registers:

      AX = condition mask causing call
      CX = horizontal cursor position
      DX = vertical cursor position
      DI = horizontal counts
      SI = vertical counts
      DS = mouse driver data segment
      BX = button state:

         │F-2│1│0│
           │  │ └─── left button (1 = pressed)
           │  └──── right button (1 = pressed)
           └────── unused

:int 33,19
^INT 33,19 - Get User Alternate Interrupt Address

    AX = 19h
    CX = user interrupt call mask (see below)


    on return:
    BX:DX = user interrupt vector
    CX = user interrupt call mask or zero if not found

        │F-8│7│6│5│4│3│2│1│0│ user interrupt mask in CX
          │  │ │ │ │ │ │ │ └─── alt key pressed during event
          │  │ │ │ │ │ │ └──── ctrl key pressed during event
          │  │ │ │ │ │ └───── shift key pressed during event
          │  │ │ │ │ └────── right button up event
          │  │ │ │ └─────── right button down event
          │  │ │ └──────── left button up event
          │  │ └───────── left button down event
          │  └────────── cursor moved
          └──────────── unused

    - returns vector to function defined by ~INT 33,18~
    - searches the event handlers defined by INT 33,18 for a routine
      with a call mask matching CX

:int 33,1a
^INT 33,1A - Set Mouse Sensitivity


    AX = 1A
    BX = horizontal coordinates per pixel  (≤ 100)
    CX = vertical coordinates per pixel  (≤ 100)
    DX = double speed threshold


    returns nothing


    - sets mouse sensitivity by setting the ratio of the mouse
      coordinates per screen pixel
    - provides same results as calls to both ~INT 33,F~ and ~INT 33,13~
    - these values are not reset by ~INT 33,0~
:int 33,1b
^INT 33,1B - Get Mouse Sensitivity


    AX = 1B


    on return:
    BX = horizontal coordinates per pixel  (≤ 100)
    CX = vertical coordinates per pixel  (≤ 100)
    DX = double speed threshold


    - returns mouse sensitivity information as the number of mouse
      coordinates per screen pixel

:int 33,1c
^INT 33,1C - Set Mouse Interrupt Rate  (InPort only)


    AX = 1C
    BX = rate code
       = 0    no interrupts
       = 1    30 interrupts per second
       = 2    50 interrupts per second
       = 3    100 interrupts per second
       = 4    200 interrupts per second



    - work with the InPort mouse only
    - sets the rate the mouse status is polled by the mouse driver
    - faster rates provide better resolution but take away CPU time
    - values in BX > 4 can cause unpredicatable results

:int 33,1d
^INT 33,1D - Set Mouse CRT Page


    AX = 1D
    BX = CRT page number


    returns nothing


    - sets the CRT page which the mouse cursor is displayed
    - see ~VIDEO PAGES~

:int 33,1e
^INT 33,1E - Get Mouse CRT Page


    AX = 1E


    on return:
    BX = CRT page number cursor is displayed on


    - see ~VIDEO PAGES~

:int 33,1f
^INT 33,1F - Disable Mouse Driver


    AX = 1F


    on return:
    AX = 001F if successful
         FFFF if error
    ES:BX = previous ~INT 33~ vector


    - restores vectors for ~INT 10~ and INT 71 (8088/86) or INT 74 (286+)
    - ~INT 33~ interrupt vector (software) is not affected
    - use of the vector returned in ES:BX to restore the previous INT 33
      vector can cause problems since it contains the value of INT 33
      before the driver was installed; any other hooks into INT 33 set
      after driver installation will not receive service

:int 33,20
^INT 33,20 - Enable Mouse Driver


    AX = 20h


    returns nothing


    - reinstalls the mouse drivers interrupt vectors for INT 10 and
      INT 71 (8088/86) and INT 74 (286/386)
    - see ~INT 33,1F~

:int 33,21
^INT 33,21 - Reset Mouse Software

    AX = 21h

    on return:
    AX = 0021  mouse driver not installed
         FFFF  mouse driver installed
    BX = 2    mouse driver installed

    - similar to ~INT 33,0~ but does not reset the mouse hardware or
      display variables
    - both AX and BX must be correct for successful reset
    - interrupt routines set through ~INT 33,18~ are preserved
    - resets the mouse to the following defaults:

      .  mouse is positioned to screen center
      .  mouse cursor is reset and hidden
      .  no interrupts are enabled (mask = 0)
      .  double speed threshold set to 64 mickeys per second
      .  horizontal mickey to pixel ratio (8 to 8)
      .  vertical mickey to pixel ratio (16 to 8)
      .  max width and height are set to maximum for video mode

:int 33,22
^INT 33,22 - Set Language for Messages

    AX = 22h
    BX = language number (with /L switch value):
       = 0    English     n/a
       = 1    French        F
       = 2    Dutch        NL
       = 3    German        D
       = 4    Swedish     S
       = 5    Finnish     SF
       = 6    Spanish     E
       = 7    Portuguese  P
       = 8    Italian     I


    returns nothing


    - only works with international version of the mouse driver
    - see    ~INT 33,23~
:int 33,23
^INT 33,23 - Get Language Number

    AX = 23h


    on return:
    BX = language number (with /L switch value):
       = 0    English     n/a
       = 1    French        F
       = 2    Dutch        NL
       = 3    German        D
       = 4    Swedish     S
       = 5    Finnish     SF
       = 6    Spanish     E
       = 7    Portuguese  P
       = 8    Italian     I

    - only works with international version of the mouse driver
    - returns English (0) if not international version

    - see    ~INT 33,22~
:int 33,24
^INT 33,24 - Get Driver Version, Mouse Type & IRQ Number

    AX = 24h


    on return:
    BH = major version (see below)
    BL = minor version (see below)
    CH = mouse type:
       = 1    bus mouse
       = 2    serial mouse
       = 3    InPort mouse
       = 4    PS/2  mouse
       = 5    Hewlett Packard mouse
    CL = IRQ number:
       = 0    PS/2
       = 2    ~IRQ~ 2
       = 5    IRQ 5
       = 7    IRQ 7

    - version 6.1 would be represented as BH = 06h, BL = 10h
:int 4a
^INT 4A - RTC Alarm Handler Vector


    - not a true interrupt, but a pointer to an alarm handler routine
    - called by  ~INT 70~  and enabled by ~INT 1A,6~
    - routine must exit via IRET

:int 67:EMS services
^INT 67 - Expanded Memory Specification

%    For more information, see the following topics:

    ~INT 67,40~  Get EMM Status
    ~INT 67,41~  Get Page Frame Base Address
    ~INT 67,42~  Get Page Counts
    ~INT 67,43~  Get Handle and Allocate Pages
    ~INT 67,44~  Map Logical Page Into Physical Page Window
    ~INT 67,45~  Release Handle and Memory Pages
    ~INT 67,46~  Get EMM Version
    ~INT 67,47~  Save Page Map Context
    ~INT 67,48~  Restore Page Map Context
    ~INT 67,49~  Get I/O Port Addresses
    ~INT 67,4A~  Get Logical to Physical Page Mapping
    ~INT 67,4B~  Get Handle Count
    ~INT 67,4C~  Get Page Count for Handle
    ~INT 67,4D~  Get Page Count for All Handles
    ~INT 67,4E~  Get/Set Page Map Context
    ~INT 67,4F~  Get/Set Partial Page Map
    ~INT 67,50~  Map/Unmap Multiple Handle Pages
    ~INT 67,51~  Reallocate Pages
    ~INT 67,52~  Get/Set Handle Attributes
    ~INT 67,53~  Get/Set Handle Name
    ~INT 67,54~  Get Handle Directory
    ~INT 67,55~  Alter Page Map and Jump
    ~INT 67,56~  Alter Page Map and Call
    ~INT 67,57~  Move/Exchange Memory Region
    ~INT 67,58~  Get Mappable Physical Address Array
    ~INT 67,59~  Get Expanded Memory Hardware Information
    ~INT 67,5A~  Allocate Standard/Raw Pages
    ~INT 67,5B~  Alternate Map Register Set DMA Registers
    ~INT 67,5C~  Prepare Expanded Memory for Warm Boot
    ~INT 67,5D~  Enable/Disable OS Functions
    ~INT 67,60~  Get Physical Window Array

    - function number is specified in AH
    - see    ~EMS STATUS~

:int 67,40:EMS status
^INT 67,40 - Get EMM Status  (LIM EMS 3.0+)

    AH = 40h

    on return:
    AH = status

         00  success
         80  internal software error
         81  hardware malfunction
         83  unallocated or invalid handle
         84  undefined function requested
         85  no handles available
         86  error in save or restore of mapping context
         87  more pages requested than physically exist
         88  more pages requested than currently available
         89  zero pages requested
         8A  invalid logical page number
         8B  illegal physical page number
         8C  context stack out of memory
         8D  context save failed, handle already has context stack
         8E  context restore failed, no context stack for handle
         8F  invalid subfunction
         90  undefined attribute type
         91  feature not supported
         92  success, portion of the source region was overwritten
         93  length of source or destination region exceeds length of
         region allocated to either source or destination handle
         94  conventional and expanded memory regions overlap
         95  offset within logical page exceeds size of logical page
         96  region length exceeds 1M
         97  source and destination regions have same handle and overlap
         98  memory source or destination type undefined
         9A  specified DMA register set not supported ???
         specified alternate map register set not supported
         9B  all DMA register sets currently allocated ???
         all alternate map register sets currently allocated
         9C  alternate DMA sets not supported ???
         alternate map register sets not supported
         9D  undefined or unallocated DMA register set ???
         undefined or unallocated alternate map register set
         9E  dedicated DMA channels not supported
         9F  specified dedicated DMA channel not supported
         A1  duplicate handle name
         A2  attempted to wrap around 1Mb conventional address space
         A3  contents of partial page map corrupted or count of
         mappable segments exceeds total mappable segments
         A4  operating system denied access


    - registers not listed are preserved
    - should only be used after establishing that the EMS driver
      is present
:int 67,41
^INT 67,41 - Get Page Frame Base Address  (LIM EMS 3.0+)


    AH = 41h


    on return:
    AH = 00 success
       = error code (see ~EMS STATUS~)
    BX = segment of page frame (PFBA)


    - use this to determine where in the 1Mb memory address the page
      frame will be mapped
    - registers not listed are preserved

:int 67,42
^INT 67,42 - Get Page Counts  (LIM EMS 3.2+)


    AH = 42h


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)
    BX = number of unallocated or available pages
    DX = total number of pages in EMM system


    - registers not listed are preserved

:int 67,43
^INT 67,43 - Get Handle and Allocate Pages  (LIM EMS 3.2+)


    AH = 43h
    BX = number of logical pages to allocate


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)
    DX = EMM handle


    - handles not explicitly closed by the application are not
      recoverable on exit from the program
    - registers not listed are preserved

:int 67,44
^INT 67,44 - Map Logical Page Into Physical Page Window (LIM EMS)


    AH = 44h
    AL = physical page number (0-3)
    BX = logical page number (0 to total allocated minus 1)
    DX = EMM handle


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)


    - registers not listed are preserved

:int 67,45
^INT 67,45 - Release Handle and Memory Pages  (LIM EMS)


    AH = 45h
    DX = EMM handle


    on return:
    AH = 00 success
       = error code (see ~EMS STATUS~)


    - handles must be explicitly freed by an application or the memory
      will not be available to the following applications
    - if unsuccessful, the operation should be retried
    - registers not listed are preserved

:int 67,46
^INT 67,46 - Get EMM Version  (LIM EMS)


    AH = 46h


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)
    AL = EMM version number in BCD


    - upper four bits of AL contain the BCD major version
    - lower four bits of AL contain the BCD minor version
    - registers not listed are preserved

:int 67,47
^INT 67,47 - Save Page Map Context  (LIM EMS 3.0+)


    AH = 47h
    DX = EMM handle assigned to the interrupt service routine
         servicing the interrupt


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)


    - any TSR, ISR or device driver using EMS should save contexts
      before manipulating EMS memory and restore contexts afterwards
    - registers not listed are preserved
    - see ~INT 67,48~

:int 67,48
^INT 67,48 - Restore Page Map Context  (LIM EMS 3.0+)


    AH = 48h
    DX = EMM handle assigned to the interrupt service routine
         servicing the interrupt


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)


    - any TSR, ISR or device driver using EMS should save contexts
      before manipulating EMS memory and restore contexts afterwards
    - registers not listed are preserved

:int 67,49
^INT 67,49 - Get I/O Port Addresses  (LIM EMS ≤ 3.0)


    AH = 49h
    ES:DI = far pointer to storage array


    on return:
    AL = board count  (0 < AL <= 4)
    AH = 00 success
       = error code (see ~EMS STATUS~)


    - defined in EMS 3.0, but undocumented in EMS 3.2;  new software
      should not use this function, but old software still works
    - registers not listed are preserved

:int 67,4a
^INT 67,4A - Get Logical to Physical Page Mapping  (LIM EMS ≤ 3.0)


    AH = 4A
    DX = EMS handle
    ES:DI far pointer to storage array


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)
    BX = contains count of entries placed in array at ES:DI


    - defined in EMS 3.0, but undocumented in EMS 3.2;  new software
      should not use this function, but old software still works
    - registers not listed are preserved

:int 67,4b
^INT 67,4B - Get Handle Count  (LIM EMS)


    AH = 4Bh


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)
    BX = number of currently active EMM handles (0-256)
         zero indicates EMS not in use


    - to determine the number of handles available subtract BX from 255
    - registers not listed are preserved

:int 67,4c
^INT 67,4C - Get Page Count for Handle    (LIM EMS)


    AH = 4Ch
    DX = EMM handle


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)
    BX = total allocated page count for handle (1-512)


    - a handle can refer to up to 512 logical pages
    - registers not listed are preserved

:int 67,4d
^INT 67,4D - Get Page Count for All Handles  (LIM EMS)

    AH = 4Dh
    ES:DI = pointer to handle array


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)
    BX = number of active EMM handles (0..255)
    ES:DI = pointer to an array of entries of the format:

       Offset  Size     Description
         00    word   EMS handle number
         02    word   number of pages

    - buffer at ES:DI should be able to hold an array of at least 4*BX
    - an error will occur if ES:DI points to a buffer that will
      cause a segment swap
    - registers not listed are preserved
:int 67,4e
^INT 67,4E - Get/Set Page Map Context  (LIM EMS 3.2+)

    AH = 4Eh
    AL = 00  get page mapping context into array
         ES:DI = pointer to destination array
         01  set page mapping context from array
         DS:SI = pointer to context source array
         02  get and set page mapping registers at once
         ES:DI = pointer to destination array
         DS:SI = pointer to context source array
         03  get size of page mapping array
         04-31  reserved


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)
    AL = bytes in pagemapping array (when AL=3)
    ES:DI = pointer to array with mapping info (AL=00/02)

    - an error will occur if ES:DI points to a buffer that will
      cause a segment swap
    - designed for use by multitasking operating systems only

:int 67,4f
^INT 67,4F - Get/Set Partial Page Map  (LIM EMS 4.0+)

    AH = 4Fh

    AL = 00  get partial page map
         DS:SI = pointer to structure containing list of segments whose
             mapping contexts are to be saved
         ES:DI = pointer to array to receive page map

    AL = 01  set partial page map
         DS:SI = pointer to structure containing saved partial page map

    AL = 02  get size of partial page map
         BX = number of mappable segments in the partial map to be saved


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)
    AL = size of partial page map (if AL=2)

:int 67,50
^INT 67,50 - Map/Unmap Multiple Handle Pages  (LIM EMS 4.0+)


    AH = 50h
    AL = 00 Map/unmap pages
       = 01 map/unmap segments
    DX = EMM handle
    CX = number of entries in array
    DS:SI = pointer to mapping array


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)

:int 67,51
^INT 67,51 - Reallocate Pages  (LIM EMS 4.0+)


    AH = 51h
    DX = EMM handle
    BX = number of pages to be allocated to handle


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)
    BX = actual number of pages allocated to handle

:int 67,52
^INT 67,52 - Get/Set Handle Attributes    (LIM EMS 4.0+)


    AH = 52h
    AL = 00  get handle attributes
         01  set handle attributes
         02  get attribute capability
    BL = new attribute (if AL=1)
    DX = EMM handle


    on return:
    AH = 00  success
       = error code (see ~EMS STATUS~)
    AL = attribute (input AL=0)
         00  handle is volatile
         01  handle is nonvolatile
    AL = attribute capability (input AL=2)
         00  only volatile handles supported
         01  both volatile and nonvolatile supported

:int 67,53
^INT 67,53 Get/Set Handle Name    (LIM EMS 4.0+)


    AH = 53h
    AL = 00 get handle name
        ES:DI = pointer to 8byte handle name array

         01 set handle name
        DS:SI = pointer to 8byte handle name
        DX = EMM handle


    on return:
    AH = status  (see ~EMS STATUS~)

:int 67,54
^INT 67,54 - Get Handle Directory  (LIM EMS 4.0+)


    AH = 54h
    AL = 00  get handle directory
         ES:DI = pointer to buffer for handle directory

         01  search for named handle
         DS:SI = pointer to 8byte name

         02  get total number of handles


    on return:
    AL = number of entries in handle directory (AL = 00h)
    DX = value of named handle (if AH was 01)
    BX = total number of handles (if AH was 02)
    AH = status  (see ~EMS STATUS~)

:int 67,55
^INT 67,55 - Alter Page Map and Jump  (LIM EMS 4.0+)


    AH = 55h
    AL = 00  physical page numbers provided by caller
         01  segment addresses provided by caller
    DX = EMM handle
    DS:SI = pointer to structure with map and jump address


    on return:
    AH = status  (see ~EMS STATUS~)

:int 67,56
^INT 67,56 - Alter Page Map and Call  (LIM EMS 4.0+)


    AH = 56h
    AL = 00  physical page numbers provided by caller
         DX = EMM handle
         DS:SI = pointer to structure with page map and call address
         01  segment addresses provided by caller
         DX = EMM handle
         DS:SI = pointer to structure with page map and call address
         02  get page map stack space required


    on return:
    BX = stack space required (AL = 02)
    AH = status  (see ~EMS STATUS~)


    - if successful, the target address is called
    - use a RETF to return and restore mapping context

:int 67,57
^INT 67,57 - Move/Exchange Memory Region  (LIM EMS 4.0+)


    AH = 57h
    AL = 00  move memory region
         01  exchange memory region
    DS:SI = pointer to structure describing source and destination


    on return:
    AH = status  (see ~EMS STATUS~)

:int 67,58
^INT 67,58 - Get Mappable Physical Address Array  (LIM EMS 4.0+)


    AH = 58h
    AL = 00  get mappable physical address array
         ES:DI = pointer to buffer to be filled with array
         01  get number of entries in m.p.a. array


    on return:
    CX = number of entries in array
    AH = status  (see ~EMS STATUS~)

:int 67,59
^INT 67,59 - Get Expanded Memory Hardware Information (LIM EMS 4.0+)


    AH = 59h
    AL = 00  get hardware configuration array
         ES:DI = pointer to buffer to be filled with array
         01  get unallocated raw page count


    on return:
    BX = unallocated raw pages (AL = 01)
    DX = total raw pages (AL = 01)
    AH = status  (see ~EMS STATUS~)


    - subfunction 00 is for use by operating systems only; can be
      enabled or disabled at any time by the operating system

:int 67,5a
^INT 67,5A - Allocate Standard/Raw Pages  (LIM EMS 4.0+)


    AH = 5A
    AL = 00  allocate standard pages
         01  allocate raw pages
    BX = number of pages to allocate


    on return:
    DX = EMM handle
    AH = status  (see ~EMS STATUS~)

:int 67,5b
^INT 67,5B - Alternate Map Register Set  (LIM EMS 4.0+)

    AH = 5B
    AL = 00  get alternate map register set
         01  set alternate map register set
         BL = new alternate map register set number
         ES:DI = pointer to map register context save area if BL=0
         02  get alternate map save array size
         03  allocate alternate map register set
         04  deallocate alternate map register set
         BL = number of alternate map register set
         05  allocate DMA register set
         06  enable DMA on alternate map register set
         BL = DMA register set number
         DL = DMA channel number
         07  disable DMA on alternate map register set
         BL = DMA register set number
         08  deallocate DMA register set
         BL = DMA register set number

    on return:
    AH = status  (see ~EMS STATUS~)
    BL = active alternate map register set number if nonzero (AL=0)
       = number of alternate map register set; 0 if not supported (AL=3)
       = DMA register set number; zero if not supported (AL = 05)
    DX = array size in bytes (AL = 02)
    ES:DI = pointer to a map register context save area if BL=0 (AL=0)


    - for use by operating systems only; can be enabled or disabled
      at any time by the operating system

:int 67,5c
^INT 67,5C - Prepare Expanded Memory for Warm Boot  (LIM EMS 4.0+)

    AH = 5C


    on return:
    AH = status  (see ~EMS STATUS~)

:int 67,5d
^INT 67,5D - Enable/Disable OS Functions  (LIM EMS 4.0+)


    AH = 5D
    AL = 00  enable OS function Set
         01  disable OS function Set
         02  return access key
    BX,CX = access key returned by first invocation


    on return:
    AH = status  (see ~EMS STATUS~)
    BX,CX = access key, returned only on first invocation of function


    - function 2 resets memory manager, returns access key at next
      invocation

:int 67,60
^INT 67,60 - LIM EMS Get Physical Window Array


    AH = 60h
    ES:DI = pointer to physical window array


    on return:
    AH = 00  success
       = error status  (see ~EMS STATUS~)

:int 70
^INT 70 - Real Time Clock Interrupt  (XT 286,AT,PS/2)


    - called 1024 times per second for periodic and alarm functions
    - decrements a DWORD counter by 976µ sec (1/1024)
    - when DWORD reaches zero, bit 7 of designated wait flag is set
    - if alarm was enabled by  ~INT 1A,6~,  ~INT 4A~ is called when
      counter reaches zero to activate alarm handler
    - not available in model 30 PS/2


    - see    ~INT 15,83~   ~INT 15,86~