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1989-02-04
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CHAPTER 2
Programming Technical Reference - IBM
Copyright 1988, Dave Williams
CPU Port Assignments, System Memory Map, BIOS Data Area, Interrupts 00h to 09h
SYSTEM MEMORY MAP - OVERALL
The IBM PC handles its address space in 64k segments, divided into 16k
fractions and then further as nescessary.
start start end
addr. addr. addr. usage
(dec) (hex)
00000 **** 640k *************** system data, drivers....
0000:0000 hardware interrupt vectors
0000:0040 BIOS interrupt vectors
0k start of RAM | 0000:0080 DOS interrupt vector table
16k 00000-03FFF | 0000:0300 Stack area during POST and bootstrap routine
32k 04000-07FFF | 0000:0400 BIOS Data Area
48k 08000-0BFFF | 0000:04F0 Intra-Application Communications Area
|
64k 10000-13FFF | 0000:0500 DOS reserved communication area
80k 14000-17FFF | xxxx:0000 IO.SYS - DOS interface to ROM I/O routines
96k 18000-1BFFF | xxxx:0000 MSDOS.SYS - DOS interrupt handlers, service
112k 1C000-1FFFF | routines (int 21 functions)
|
128k 20000-23FFF | xxxx:xxxx DOS buffers, control areas, and installed
144k 24000-27FFF | device drivers.
160k 28000-2BFFF | xxxx:0000 resident portion of COMMAND.COM, interrupt
176k 2C000-2FFFF | handlers for int 22h, 23h,24h, and code to
| reload the transient portion
192k 30000-33FFF | xxxx:0000 master environment block, default 64 bytes
208k 34000-37FFF | xxxx:0000 environment for next program
224k 38000-3BFFF | xxxx:0000 external commands or utilities (COM or EXE
240k 3C000-3FFFF | files)
|
256k 40000-43FFF | ----:---- application programs
272k 44000-47FFF | xxxx:0000 user stack for COM files (256 bytes)
288k 48000-4BFFF | xxxx:0000 transient portion of COMMAND.COM
304k 4C000-4FFFF |
|
320k 50000-53FFF |
336k 54000-57FFF |
352k 58000-5BFFF |
368k 5C000-5FFFF |
|
384k 60000-63FFF |
400k 64000-67FFF |
416k 68000-6BFFF |
432k 6C000-6FFFF |
|
448k 70000-73FFF |
464k 74000-77FFF |
480k 78000-7BFFF |
496k 7C000-7FFFF |
|
512k 80000-83FFF |
528k 84000-87FFF |
544k 88000-8BFFF | original IBM PC-1 BIOS limited memory to 544k
560k 8C000-8FFFF |
|
576k 90000-93FFF |
592k 94000-97FFF |
609k 98000-9BFFF |
624k 9C000-9FFFF | to 640k (top of RAM address space)
A0000 ***** 64k *************** EGA address
640k A0000-A95B0 MCGA 320x200 256 color video buffer
AF8C0 MCGA 640x480 2 color video buffer
-A3FFF
656k A4000-A7FFF
672k A8000-ABFFF
688k AC000-AFFFF
B0000 ***** 64k *************** mono and CGA address
704k B0000-B3FFF mono uses only 4k | The PCjr and early Tandy 1000
720k B4000-B7FFF | BIOSs revector direct writes to
736k B8000-BBFFF CGA uses entire 16k | the B8 area to the Video Gate
756k BC000-BFFFF | Array and reserved system RAM
C0000 ***** 64k *************** expansion ROM
768k C0000-C3FFF 16k EGA BIOS C000:001E EGA BIOS signature (the letters IBM)
784k C4000-C5FFF
C6000-C63FF 256 bytes Professional Graphics Display communication area
C6400-C7FFF
800k C8000-CBFFF 16k hard disk controller BIOS, drive 0 default
816k CC000-CDFFF 8k IBM PC Network NETBIOS
CE000-CFFFF
D0000 ***** 64k *************** expansion ROM | PCjr first ROM cartridge
832k D0000-D7FFF 32k IBM Cluster Adapter | address area.
DA000 voice communications |
848k D4000-D7FFF | Common expanded memory board
864k D8000-DBFFF | paging area.
880k DC000-DFFFF |
E0000 ***** 64k *************** expansion ROM | PCjr second ROM
896k E0000-E3FFF | cartridge address
912k E4000-E7FFF | area
928k E8000-EBFFF |
944k EC000-EFFFF |
F0000 ***** 64k *************** system | PCjr optional ROM
960k F0000-F3FFF reserved by IBM | cartridge address
976k F4000- | area (cartridge
F6000 ROM BASIC Begins | BASIC)
992k F8000-FB000 |
1008k FC000-FFFFF ROM BASIC and original |
BIOS (Compatibility BIOS |
in PS/2) |
1024k FFFFF end of memory (1024k) for 8088 machines
F000:FFF5 BIOS release date
F000:FFFE PC model identification
384k 100000-15FFFF 80286/AT extended memory area, 1Mb motherboard
15Mb 100000-FFFFFF 80286/AT extended memory address space
15Mb 160000-FDFFFF Micro Channel RAM expansion (15Mb extended memory)
128k FE0000-FFFFFF system board ROM (PS/2 Advanced BIOS)
PC Port Assignment, Intel 8088, 80C88, 8086, 80286, 80386 CPUs
hex addr. Function
0000-000F 8237 DMA controller
0010-001F 8237 DMA controller (AT, PS/2)
0020-0027 8259A interrupt controller
0020-003F 8259A interrupt controller (AT)
0040-005F 8253-5 programmable timers
(note: 0041 was memory refresh in PCs. Not used in PS/2)
0060-0067 8255 peripheral interface
0060-006F 8042 keyboard controller (AT)
0200-020F game-control adapter
0210-0217 expansion box (PC, XT)
0278-027F LPT3
02F8-02FF COM2
0300-031F prototype card
0320-032F hard disk controller
0378-037F LPT2
03BC-03BF LPT1
03D0-03DF CGA, MCGA, VGA adapter control
03F0-03F7 floppy disk controller
03F8-03FF COM1
note: These are functions common across the IBM range. The PCjr, PC
Convertible and PS/2 (both buses) have enhancements. In some cases, the
AT and PS/2 series ignore, duplicate, or reassign ports arbitrarily. If
your code incorporates specific port addresses for video or system board
control it would be wise to have your application determine the machine
type and video adapter and address the ports as required.
Reserved Memory Locations in the IBM PC
addr. size description
000h-3FFh DOS interrupt vector table
30:00h- used as a stack area during POST and bootstrap routines. This
3F:FFh stack area may be revectored by an application program.
The BIOS Data Area addr. from 400h to 4FFh
40:00 word COM1 port address | These addresses are zeroed out in the
40:02 word COM2 port address | OS/2 DOS Compatibility Box if any of
40:04 word COM3 port address | the OS/2 COM??.SYS drivers are loaded.
40:06 word COM4 port address |
40:08 word LPT1 port address
40:0A word LPT2 port address
40:0C word LPT3 port address
40:0E word LPT4 port address (not valid in PS/2 machines)
40:0E word PS/2 pointer to 1k extended BIOS Data Area at top of RAM
40:10 word equipment flag (see int 11h)
bits:
0 1 if floppy drive present (see bits 6&7) 0 if not
1 1 if 80x87 installed (not valid in PCjr)
2,3 system board RAM (not used on AT or PS/2)
00 16k
01 32k
10 48k
11 64k
4,5 initial video mode
00 no video adapter
01 40 column color (PCjr)
10 80 column color
11 MDA
6,7 number of diskette drives
00 1 drive
01 2 drives
10 3 drives
11 4 drives
8 0 DMA present
1 DMA not present (PCjr)
9,A,B number of RS232 serial ports
C game adapter (joystick)
0 no game adapter
1 if game adapter
D serial printer (PCjr only)
0 no printer
1 serial printer present
E,F number of parallel printers installed
note 1) The IBM PC and AT store the settings of the system board
switches or CMOS RAM setup information (as obtained by the BIOS
in the Power-On Self Test (POST)) at addresses 40:10h and
40:13h. 00000001b indicates "on", 00000000b is "off".
2) CMOS RAM map, PC/AT:
offset contents
00h Seconds
01h Second Alarm
02h Minutes
03h Minute Alarm
04h Hours
05h Hour Alarm
06h Day of the Week
07h Day of the Month
08h Month
09h Year
0Ah Status Register A
0Bh Status Register B
0Ch Status Register C
0Dh Status Register D
0Eh Diagnostic Status Byte
0Fh Shutdown Status Byte
10h Disk Drive Type for Drives A: and B:
The drive-type bytes use bits 0:3 for the first
drive and 4:7 for the other
Disk drive types:
00h no drive present
01h double sided 360k
02h high capacity (1.2 meg)
03h-0Fh reserved
11h (AT):Reserved (PS/2):drive type for hard disk C:
12h (PS/2):drive type for hard disk D:
(AT, XT/286):hard disk type for drives C: and D:
Format of drive-type entry for AT, XT/286:
0 number of cyls in drive (0-1023 allowed)
2 number of heads per drive (0-15 allowed)
3 starting reduced write compensation (not
used on AT)
5 starting cylinder for write compensation
7 max. ECC data burst length, XT only
8 control byte
Bit
7 disable disk-access retries
6 disable ECC retries
5-4 reserved, set to zero
3 more than 8 heads
2-0 drive option on XT (not used by AT)
9 timeout value for XT (not used by AT)
12 landing zone cylinder number
14 number of sectors per track (default 17,
0-17 allowed)
13h Reserved
14h Equipment Byte (corresponds to sw. 1 on PC and XT)
15h-16h Base Memory Size (low,high)
17h-18h Expansion Memory Size (low,high)
19h-20h Reserved
(PS/2) POS information Model 50 (60 and 80 use a 2k
CMOS RAM that is not accessible through software)
21h-2Dh Reserved (not checksumed)
2Eh-2Fh Checksum of Bytes 10 Through 20 (low,high)
30h-31h Exp. Memory Size as Det. by POST (low,high)
32h Date Century Byte
33h Information Flags (set during power-on)
34h-3Fh Reserved
3) The alarm function is used to drive the BIOS wait function (int
15h function 90h).
4) To access the configuration RAM write the byte address (00-3Fh)
you need to access to I/O port 70h, then access the data via I/O
port 71h.
5) CMOS RAM chip is a Motorola 146818
6) The equipment byte is used to determine the configuration for the
power-on diagnostics.
7) Bytes 00-0Dh are defined by the chip for timing functions, bytes
0Eh-3Fh are defined by IBM.
40:12 byte number of errors detected by infrared keyboard link (PCjr only)
40:13 word availible memory size in Kbytes (less display RAM in PCjr)
this is the value returned by int 12h
40:17 byte keyboard flag byte 0 (see int 9h)
bit 7 insert mode on 3 alt pressed
6 capslock on 2 ctrl pressed
5 numlock on 1 left shift pressed
4 scrollock on 0 right shift pressed
40:18 byte keyboard flag byte 1 (see int 9h)
bit 7 insert pressed 3 ctrl-numlock (pause) toggled
6 capslock pressed 2 PCjr keyboard click active
5 numlock pressed 1 PCjr ctrl-alt-capslock held
4 scrollock pressed 0
40:19 byte storage for alternate keypad entry (not normally used)
40:1A word pointer to keyboard buffer head character
40:1C word pointer to keyboard buffer tail character
40:1E 32bytes 16 2-byte entries for keyboard circular buffer, read by int 16h
40:3E byte drive seek status - if bit=0, next seek will recalibrate by
repositioning to Track 0.
bit 3 drive D bit 2 drive C
1 drive B 0 drive A
40:3F byte diskette motor status
bit 7 1, write in progress 3 1, D: motor on (floppy 3)
6 2 1, C: motor on (floppy 2)
5 1 1, B: motor on
4 0 1, A: motor on
40:40 byte motor off counter
starts at 37 and is decremented 1 by each system clock tick.
motor is shut off when count = 0.
40:41 byte status of last diskette operation where:
bit 7 timeout failure bit 3 DMA overrun
6 seek failure 2 sector not found
5 controller failure 1 address not found
4 CRC failure 0 bad command
40:42 7 bytes NEC status
40:49 byte current CRT mode (hex value)
00h 40x25 BW (CGA) 01h 40x25 color (CGA)
02h 80x25 BW (CGA) 03h 80x25 color (CGA)
04h 320x200 color (CGA) 05h 320x200 BW (CGA)
06h 640x200 BW (CGA) 07h monochrome (MDA)
extended video modes (EGA/MCGA/VGA or other)
08h lores,16 color 09h med res,16 color
0Ah hires,4 color 0Bh n/a
0Ch med res,16 color 0Dh hires,16 color
0Eh hires,4 color 0Fh hires,64 color
40:4A word number of columns on screen, coded as hex number of columns
20 col = 14h (video mode 8, low resolution 160x200 CGA graphics)
40 col = 28h
80 col = 46h
40:4C word screen buffer length in bytes
(number of bytes used per screen page, varies with video mode)
40:4E word current screen buffer starting offset (active page)
40:50 8 words cursor position pages 1-8
the first byte of each word gives the column (0-19, 39, or 79)
the second byte gives the row (0-24)
40:60 byte end line for cursor (normally 1)
40:61 byte start line for cursor (normally 0)
40:62 byte current video page being displayed (0-7)
40:63 word base port address of 6845 CRT controller or equivalent
for active display 3B4h=mono, 3D4h=color
40:65 byte current setting of the CRT mode register
40:66 byte current palette mask setting (CGA)
40:67 5 bytes temporary storage for SS:SP during shutdown (cassette interface)
40:6C word timer counter low word
40:6E word timer counter high word
40:69 byte HD_INSTALL (Columbia PCs) (not valid on most clone computers)
bit 0 = 0 8 inch external floppy drives
1 5-1/4 external floppy drives
1,2 = highest drive address which int 13 will accept
(since the floppy drives are assigned 0-3, subtract
3 to obtain the number of hard disks installed)
4,5 = # of hard disks connected to expansion controller
6,7 = # of hard disks on motherboard controller
(if bit 6 or 7 = 1, no A: floppy is present and
the maximum number of floppies from int 11 is 3)
40:70 byte 24 hour timer overflow 1 if timer went past midnight
it is reset to 0 each time it is read by int 1Ah
40:71 byte BIOS break flag (bit 7 = 1 means break key hit)
40:72 word reset flag (1234 = soft reset, memory check will be bypassed)
PCjr keeps 1234h here for softboot when a cartridge is installed
40:74 byte status of last hard disk operation; PCjr special diskette control
40:75 byte # of hard disks attached (0-2) ; PCjr special diskette control
40:76 byte hd control byte; temporary holding area for 6th param table entry
40:77 byte port offset to current hd adapter ; PCjr special diskette control
40:78 4 bytes timeout value for LPT1,LPT2,LPT3,LPT4
40:7C 4 bytes timeout value for COM1,COM2,COM3,COM4 (0-FFh seconds, default 1)
40:80 word pointer to start of circular keyboard buffer, default 03:1E
40:82 word pointer to end of circular keyboard buffer, default 03:3E
40:84 byte rows on the screen (EGA only)
40:84 byte PCjr interrupt flag; timer channel 0 (used by POST)
40:85 word bytes per character (EGA only)
40:85 2 bytes (PCjr only) typamatic char to repeat
40:86 2 bytes (PCjr only) typamatic initial delay
40:87 byte mode options (EGA only)
Bit 1 0 = EGA is connected to a color display
1 = EGA is monochrome.
Bit 3 0 = EGA is the active display,
1 = "other" display is active.
Mode combinations:
Bit3 Bit1 Meaning
0 0 EGA is active display and is color
0 1 EGA is active display and is monochrome
1 0 EGA is not active, a mono card is active
1 1 EGA is not active, a CGA is active
40:87 byte (PCjr only) current Fn key code
40:88 byte feature bit switches (EGA only) 0=on, 1=off
bit 3 = switch 4
bit 2 = switch 3
bit 1 = switch 2
bit 0 = switch 1
40:88 byte (PCjr only) special keyboard status byte
bit 7 function flag 3 typamatic (0=enable,1=disable)
6 Fn-B break 2 typamatic speed (0=slow,1=fast)
5 Fn pressed 1 extra delay bef.typamatic (0=enable)
4 Fn lock 0 write char, typamatic delay elapsed
40:89 byte PCjr, current value of 6845 reg 2 (horiz.synch) used by
ctrl-alt-cursor screen positioning routine in ROM
40:8A byte PCjrCRT/CPU Page Register Image, default 3Fh
40:8B byte last diskette data rate selected
40:8C byte hard disk status returned by controller
40:8D byte hard disk error returned by controller
40:8E byte hard disk interrupt (bit 7=working int)
40:90 4 bytes media state drive 0, 1, 2, 3
40:94 2 bytes track currently seeked to drive 0, 1
40:96 byte keyboard flag byte 3 (see int 9h)
40:97 byte keyboard flag byte 2 (see int 9h)
40:98 dword pointer to users wait flag
40:9C dword users timeout value in microseconds
40:A0 byte real time clock wait function in use
40:A1 byte LAN A DMA channel flags
40:A2 2 bytes status LAN A 0,1
40:A4 dword saved hard disk interrupt vector
40:A8 dword EGA pointer to parameter table
40:B4 byte keyboard NMI control flags (Convertible)
40:B5 dword keyboard break pending flags (Convertible)
40:B9 byte port 60 single byte queue (Convertible)
40:BA byte scan code of last key (Convertible)
40:BB byte pointer to NMI buffer head (Convertible)
40:BC byte pointer to NMI buffer tail (Convertible)
40:BD 16bytes NMI scan code buffer (Convertible)
40:CE word day counter (Convertible and after)
to -04:8F end of BIOS Data Area
40:90-40:EF reserved by IBM
04:F0 16 bytes Intra-Application Communications Area (for use by applications
04:FF to transfer data or parameters to each other)
05:00 byte DOS print screen status flag
00h not active or successful completion
01h print screen in progress
0FFh error during print screen operation
05:01 Used by BASIC
05:02-03 PCjr POST and diagnostics work area
05:04 byte Single drive mode status byte
00 logical drive A
01 logical drive B
05:05-0E PCjr POST and diagnostics work area
05:0F BASIC: SHELL flag (set to 02h if there is a current SHELL)
05:10 word BASIC: segment address storage (set with DEF SEG)
05:12 4 bytes BASIC: int 1Ch clock interrupt vector segment:offset storage
05:16 4 bytes BASIC: int 23h ctrl-break interrupt segment:offset storage
05:1A 4 bytes BASIC: int 24h disk error interrupt vector segment:offset storage
05:1B-1F Used by BASIC for dynamic storage
05:20-21 Used by DOS for dynamic storage
05:22-2C Used by DOS for diskette parameter table. See int 1Eh for values
05:30-33 Used by MODE command
05:34-FF Unknown - Reserved for DOS
At Absolute Addresses:
0008:0047 IO.SYS or IBMBIO.COM IRET instruction. This is the dummy routine that
interrupts 01h, 03h, and 0Fh are initialized to during POST.
C000:001E EGA BIOS signature (the letters IBM)
F000:FFF5 BIOS release date
F000:FFFE PC model identification
date model byte submodel byte revision
04/24/81 FF = PC-0 (16k) -- --
10/19/81 FF = PC-1 (64k) -- --
08/16/82 FF = PC, XT, XT/370 -- --
(256k motherboard)
10/27/82 FF = PC, XT, XT/370 -- --
(256k motherboard)
11/08/82 FE = XT, Portable PC -- --
XT/370, 3270PC
01/10/86 FB = XT 00 01
01/10/86 FB = XT-2 (early)
05/09/86 FB = XT-2 (640k) 00 02
06/01/83 FD = PCjr -- --
01/10/84 FC = AT -- --
06/10/85 FC = AT 00 01
11/15/85 FC = AT 01 00
04/21/86 FC = XT/286 02 00
09/13/85 F9 = Convertible 00 00
09/02/86 FA = PS/2 Model 30 00 00
11/15/86 FC = AT, Enhanced 8mHz
02/13/87 FC = PS/2 Model 50 04 00
02/13/87 FC = PS/2 Model 60 05 00
1987 F8 = PS/2 Model 80 00 00
2D = Compaq PC (4.77) -- --
9A = Compaq Plus (XT) -- --
00FC 7531/2 Industrial AT
06FC 7552 Gearbox
The IBM PC System Interrupts (Overview)
The interrupt table is stored in the very lowest location in memory, starting
at 0000:0000h. The locations are offset from segment 0, ie location 0000h has
the address for int 0, etc. Each address is four bytes long and its location in
memory can be found by multiplying the interrupt number by 4. For example, int
7 could be found by (7x4=28) or 1Bh (0000:001Bh).
These interrupt vectors normally point to ROM tables or are taken over by DOS
when an application is run. Some applications revector these interrupts to
their own code to change the way the system responds to the user.
Interrupt Address Function
Number (Hex)
0 00-03 CPU Divide by Zero
1 04-07 CPU Single Step
2 08-0B CPU Nonmaskable
3 0C-0F CPU Breakpoint
4 10-13 CPU Overflow
5 14-17 BIOS Print Screen
6 18-1B hdw Reserved
7 1C-1F hdw Reserved
8 20-23 hdw Time of Day
9 24-27 hdw Keyboard
A 28-2B hdw Reserved
B 2C-2F hdw Communications [8259]
C 30-33 hdw Communications
D 34-37 hdw Disk
E 38-3B hdw Diskette
F 3C-3F hdw Printer
10 40-43 BIOS Video
11 44-47 BIOS Equipment Check
12 48-4B BIOS Memory
13 4C-4F BIOS Diskette/Disk
14 50-53 BIOS Serial Communications
15 54-57 BIOS Cassette, System Services
16 58-5B BIOS Keyboard
17 5C-5F BIOS Parallel Printer
18 60-63 BIOS Resident BASIC
19 64-67 BIOS Bootstrap Loader
1A 68-6B BIOS Time of Day
1B 6C-6F BIOS Keyboard Break
1C 70-73 BIOS Timer Tick
1D 74-77 BIOS Video Initialization
1E 78-7B BIOS Diskette Parameters
1F 7C-7F BIOS Video Graphics Characters, second set
20 80-83 DOS General Program Termination
21 84-87 DOS DOS Services Function Request
22 88-8B DOS Called Program Termination Address
23 8C-8F DOS Control Break Termination Address
24 90-93 DOS Critical Error Handler
25 94-97 DOS Absolute Disk Read
26 98-9B DOS Absolute Disk Write
27 9C-9F DOS Terminate and Stay Resident
28-3F A0-FF DOS Reserved for DOS
40-43 100-115 BIOS Reserved for BIOS
44 116-119 BIOS First 128 Graphics Characters
45-47 120-131 BIOS Reserved for BIOS
48 132-135 BIOS PCjr Cordless Keyboard Translation
49 136-139 BIOS PCjr Non-Keyboard Scancode Translation Table
50-5F 140-17F BIOS Reserved for BIOS
60-67 180-19F Reserved for User Software Interrupts
68-7F 1A0-1FF Reserved by IBM
80-85 200-217 ROM BASIC
86-F0 218-3C3 Used by BASIC Interpreter When BASIC is Running
F1-FF 3C4-3FF Reserved by IBM
For consistency in this volume, all locations and offsets are in hexadecimal
unless otherwise specified. All hex numbers are prefaced with a leading zero
if they begin with an alphabetic character, and are terminated with a
lowercase H (h). The formats vary according to common usage.
The IBM-PC System Interrupts (in detail)
Interrupt 00h Divide by Zero (processor error). Automatically called at end
(0:0000h) of DIV or IDIV operation that results in error. Normally set by
DOS to display an error message and abort the program.
Interrupt 01h Single step - Taken after every instruction when CPU Trap Flag
(0:0004h) indicates single-step mode (bit 8 of FLAGS is 1). This is what
makes the T command of DEBUG work for single stepping. Is not
generated after MOV to segment register or POP of segment
register. (unless you have a very early 8088 with the microcode
bug).
Interrupt 02h Non-maskable interrupt - Vector not disabled via CLI. Used by
(0:0008h) parity check routine in POST, 8087 coprocessor, PCjr infrared
keyboard link.
Interrupt 03h Breakpoint - Taken when CPU executes the 1-byte int 3 (0CCh).
(0:000Ch) Generated by opcode 0CCh. Similar to 8080's RST instruction.
Generally used to set breakpoints for DEBUG.
Interrupt 04h Divide overflow - Generated by INTO instruction if OF flag is
(0:0010h) set. If flag is not set, INTO is effectively a NOP. Used to trap
any arithmetic errors when program is ready to handle them rather
than immediately when they occur.
Interrupt 05h Print Screen - service dumps the screen to the printer. Invoked
(0:0014h) by int 9 for shifted key 55 (PrtSc). Automatically called by
keyboard scan when PrtSc key is pressed. Normally executes
routine to print the screen, but may call any routine that can
safely be executed from inside the keyboard scanner. Status and
result byte are at address 0050:0000.
entry AH 05h
return absolute address 50:0
00h print screen has not been called, or upon return from a call
there were no errors.
01h print screen is already in progress.
0FFh error encountered during printing.
note 1) Uses BIOS services to read the screen
2) Output is directed to LPT1
3) Revectored into GRAPHICS.COM if GRAPHICS.COM is loaded
Interrupt 06h Reserved by IBM
(0:0018h)
Interrupt 07h Reserved by IBM
(0:00C0h)
Interrupt 08h Timer - 55ms timer "tick" taken 18.2 times per second. Updates
(0:0020h) (IRQ0) BIOS clock and turns off diskette drive motors after 2
seconds of inactivity.
entry AH 08h
return absolute addresses:
40:6C number of interrupts since power on (4 bytes)
40:70 number of days since power on (1 byte)
40:67 day counter on all products after AT
40:40 motor control count - gets decremented and shuts off diskette
motor if zero
note Int 1Ch invoked as a user interrupt.
Interrupt 09h Keyboard - taken whenever a key is pressed or released.
(0:0024h) (IRQ1) Stores characters/scan-codes in status at [0040:0017,18]
entry AH 09h
return at absolute memory addresses:
40:17 bit
0 right shift key depressed
1 left shift key depressed
2 control key depressed
3 alt key depressed
4 ScrollLock state has been toggled
5 NumLock state has been toggled
6 CapsLock state has been toggled
7 insert state is active
40:18 bit
0 left control key depressed
1 left alt key depressed
2 SysReq key depressed
3 Pause key has been toggled
4 ScrollLock key is depressed
5 NumLock key is depressed
6 CapsLock key is depressed
7 Insert key is depressed
40:96 bit
0 last code was the E1h hidden code
1 last code was the E0h hidden code
2 right control key down
3 right alt key down
4 101 key Enhanced keyboard installed
5 force NumLock if rd ID & kbx
6 last character was first ID character
7 doing a read ID (must be bit 0)
40:97 bit
0 ScrollLock indicator
1 NumLock indicator
2 CapsLock indicator
3 circus system indicator
4 ACK received
5 resend received flag
6 mode indicator update
7 keyboard transmit error flag
40:1E keyboard buffer (20h bytes)
40:1C buffer tail pointer
40:72 1234h if ctrl-alt-del pressed on keyboard
AL scan code
note 1) Int 05h invoked if PrtSc key pressed
2) Int 1Bh invoked if Ctrl-Break key sequence pressed
3) Int 15h, AH=85h invoked on AT and after if SysReq key is pressed
4) Int 15h, AH=4Fh invoked on machines after AT
Interrupt 0Ah EGA Vertical Retrace
(0:0028h) (IRQ2) used by EGA vertical retrace, hard disk
Interrupt 0Bh Communications Controller (serial port) hdw. entry
(0:002Ch) (IRQ3) Serial Port 2 (com2)
note IRQ 3 may be used by SDLC (synchronous data-link control) or
bisynchronous communications cards instead of a serial port.
Interrupt 0Ch Communications Controller (serial port) hdw. entry
(0:0030h) (IRQ4) Serial Port 1 (com1)
note IRQ 4 may be used by SDLC (synchronous data-link control) or
bisynchronous communications cards instead of a serial port.
Interrupt 0Dh Alternate Printer, PC/AT 80287
(0:0034h) (IRQ5) used by hard disk, 60 Hz RAM refresh, LPT2 on AT, XT/286,
and PS/2, dummy CRT vertical retrace on PCjr
Interrupt 0Eh Diskette - indicates that a seek is in progress
(0:0038h) (IRQ6) (sets bit 8 of 40:3E)
Interrupt 0Fh Reserved by IBM
(0:003Ch) (IRQ7) IRQ7 used by PPI interrupt (LPT1, LPT2)
