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1985-06-27
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HOW TO USE THE ROM BIOS INTERRUPTS
by Scott Pakin
Introductory Notes:
1) It is assumed that the reader knows a little bit of assembly language
programming.
2) These charts were taken from the ROM BIOS listing in the Technical
Reference Manual.
3) All interrupts and function numbers are given in HEXADECIMAL (base 16)!!!
SEND ALL QUESTIONS AND COMMENTS TO:
Scott Pakin
6007 N. Sheridan Rd.
Chicago, IL 60660
INTERRUPT 10 -- Display (VIDEO) I/O
(AH) Operation Also on entry On return
------------------------------------------------------------------------------
0 Set video mode (AL) = 0 40x25 B/W
= 1 40x25 Color
= 2 80x25 B/W
= 3 80x25 Color
= 4 320x200 Color
= 5 320x200 B/W
= 6 640x200 B/W
1 Set cursor type CH bits 0-4 = Start line
CL bits 0-4 = End line
[Other bits must be zero
or problems may occur]
2 Position cursor (DH,DL) = Row, Column
(BH) = Page number
3 Read cursor pos. (BH) = Page number (DH,DL) = Row, Column
(CH,CL) = Cursor type
4 Read light pen pos. (AH) = 1
(DH,DL) = Row, Column
for characters
(CH,BX) = Row, Column
for pixels
[ If (AH) = 0, then
the light pen switch
is not down/triggered]
5 Select display page (AL) = New page value
[ Valid only in
text modes. ] [ (AL) must be 0-7 for
40-column and 0-3 for
80-column modes.]
6 Scroll page up (AL) = No. of lines
(CH,CL) = Upper left (R,C)
(DH,DL) = Lower right (R,C)
(BH) = Attribute to be used
on bottom lines
[ (AL) = 0 clears entire
screen. ]
INTERRUPT 10 -- Display (VIDEO) I/O
(AH) Operation Also on entry On return
------------------------------------------------------------------------------
7 Scroll page down Same as function #6
except (BH) is the
attribute to be used
on the bottom lines
8 Read attribute/ (BH) = Display page (AL) = Character read
character at current (AH) = Attribute
cursor position
[ (AH) is invalid if in
in graphics modes. ]
9 Write attribute/ (BH) = Display page
character at current (BL) = Attribute (Text)
cursor position = Char. color (Graphics)
(CX) = No. of chars. to write
(AL) = Char. to write
A Write character (BH) = Display page
only at current (CX) = No. of characters to
cursor position write
(AL) = Character to write
B Set color palette (BH) = Palette color ID (0-127)
(BL) = Color value to be used
with the color ID
C Write dot (DX,CX) = Row, Column
(AL) = Color
[ If (AH) > 127, the dot will
be exclusive-or'd with the dot
presently occupying the dot
position. ]
D Read dot (DX,CX) = Row, Column (AL) = Color of dot read
E Teletype output (AL) = Character to write
(BL) = Foreground color
(BH) = Display page
[ (BL) is only valid in
text modes. ]
INTERRUPT 10 -- Display (VIDEO) I/O
(AH) Operation Also on entry On return
------------------------------------------------------------------------------
F Read video mode (AL) = 0 40x25 B/W
= 1 40x25 Color
= 2 80x25 B/W
= 3 80x25 Color
= 4 320x200 Color
= 5 320x200 B/W
= 6 640x200 B/W
= 7 Monochrome
(AH) = No. of char.
columns on screen
(BH) = Current display
page
________________________________________________________________________________
Notes on interrupt 10:
1) Page numbers are invalid in graphics modes or when using the monochrome
monitor. If a register requires a page number, set it to zero if it's
invalid.
2) For (Row, Column), (0,0) is upper left.
3) Text modes are often called Alphanumeric (Alpha) modes; there is no
difference between the two.
4) Palette 0 Palette 1
-------------------------------------
Color 1 | Green Cyan
Color 2 | Red Magenta
Color 3 | Brown/Yellow Grey/White
[ Color 3 is dependant on the background color. ]
Background Colors
Color (dec.) (bin.)
Black 0 0000
Blue 1 0001
Green 2 0010
Cyan 3 0011
Red 4 0100
Magenta 5 0101
Brown 6 0110
Light Grey 7 0111
Dark Gray 8 1000
Light Blue 9 1001
Light Green 10 1010
Light Cyan 11 1011
Light Red 12 1100
Light Magenta 13 1101
Yellow 14 1110
White 15 1111
INTERRUPT 13 -- Diskette I/O
(AH) Operation Also on entry On return
------------------------------------------------------------------------------
0 Reset diskette
system
1 Read diskette status (AL) = Diskette status
[ See note #1 for a
diagram of this byte. ]
2 Read sectors into (AL) = Number of sectors Carry flag = 0
a buffer (CL) = Starting sector # (AL) = No. of
(CH) = Track number (0-39) sectors read
(DL) = Drive number [ If CF is 1, then
(0=A, 1=B, 2=C, 3=D) the operation was
(DH) = Head number (0 or 1) unsuccessful and
(ES:BX) = Address of buffer (AH) = Diskette
status ]
3 Write sectors from Same as function #2 Same as function #2
a buffer
4 Verify sectors Same as function #2, but Same as function #2
the buffer is not needed
5 Format a track (ES:BX) = Sector information (AL) does not return
(DL) = Drive # any informatiom,
(AL) = # of sectors to format otherwise, results
are the same as in
[ see note #2 for more function #2
information ]
________________________________________________________________________________
Notes on interrupt 13:
1) The diskette staus byte looks like this:
7 6 5 4 3 2 1 0 Bits 0 & 1 -- if 01, invalid command
if 10, address mark (to locate
sectors) was not found
if 11, write protect prevented
writing on disk
Bit Meaning (if on)
2 Sector not found
3 Direct memory access overrun, or
if bit 0 is on (1), illegal attempt to
transfer data across a 64K boundry
4 Cyclical redundancy check detected a
read error
5 Diskette controller chip returned an
error
6 Seek operation to a track failed
7 Drive failed to respond (time-out error)
2) The formatting data in the extra segment is set up like this:
Byte number Meaning
1 Track number (0-39)
2 Head number (0 or 1)
3 Sector number (1-8; 9 if DOS 2.xx)
4 Number of bytes for sector
(0=128, 1=256, 2=512, 3=1024)
Only full tracks may be formatted; not individual sectors. This means that
the above data must appear eight (nine for DOS 2.00+) times -- 32 bytes total
for an entire track. Remember to set (AL) to match the number of sectors to
be formatted.
INTERRUPT 14 -- Asyncronous Communications (RS-232) I/O
(AH) Operation Also on entry On return
------------------------------------------------------------------------------
0 Initialize (DX) = Communications (AX) = Status word
communications port adapter (0 = COM1,
1 = COM2) [ See note #2 for more
information ]
(AL) = Parameters for
initialization
[ See note #1 for more
information ]
1 Send a character (DX) = Communications If (AH) > 127, an error
over the comm. line adapter occurred; low 15 bits of
(AL) = Character to send (AX) have status word
2 Wait for character (DX) = Communications If (AH) is non-zero, an
from comm. line adapter error occurred; (AX) has
status word
3 Read status word (AX) = Status word
[ See note #2 for more
information ]
________________________________________________________________________________
Notes on interrupt 14:
1) The parameters for initializing the communications adapter are as follows:
7 6 5 4 3 2 1 0 Bits 0 & 1 = Word length; 10 = 7 bit bytes
11 = 8 bit bytes
Bit 2 = Stopbits; 0 = 1 stopbit
1 = 2 stopbits
Bits 3 & 4 = Parity; x0 = no parity
01 = odd parity
11 = even parity
[x could be 1 or 0 ]
Bits 5-7 = Speed; 000 = 110 bps
001 = 150 bps
010 = 300 bps
011 = 600 bps
100 = 1200 bps
101 = 2400 bps
110 = 4800 bps
111 = 9600 bps
2) The communications port status is returned in (AX):
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
(AH) = Line control status (AL) = MODEM status
Bit Meaning (when on)
0 Delta clear-to-send
1 Delta data set ready
2 Trailing edge ring detected
3 Delta receive line signal detected
4 Clear-to-send signal
5 Data set (MODEM) ready
6 Ring indication
7 Line signal detected
8 Data ready
9 Overrun error
10 Parity error
11 Framing error
12 Break detected
13 Transmission holding-register empty
14 Transmission shift-register empty
15 Time out
[ NOTE: Bits 15, 10, and 8 might be set incorrectly if you have a
BIOS with a release date of 4/24/81 ]
INTERRUPT 15 -- Cassete I/O
(AH) Operation Also on entry On return
------------------------------------------------------------------------------
0 Turn cassette motor on
1 Turn cassette motor off
2 Read one or more (CX) = Number of bytes (DX) = Number of bytes
256-byte blocks from to read read
cassette into a (ES:BX) = Pointer to Carry flag = 0 if
buffer buffer successful
If unsuccessful, the
carry flag = 1 and (AH)
has the error code
[ See note #1 for more
information ]
3 Write one or more (CX) = Number of bytes (CX) = 0
256-byte blocks from to write (ES:BX) = Pointer to the
a buffer to (ES:BX) = Pointer to buffer byte after the
cassette last byte
written
________________________________________________________________________________
Notes on interrupt 15:
1) Error returns for function #2:
7 6 5 4 3 2 1 0
Bit Meaning (when on)
0 Cyclical redundancy check failed
1 Data transactions were lost
2 No data was found
All other bits have no purpose
2) ROM BIOS automatically waits for the cassette motor to reach neccessary
speed, writes a leader, a syncronization bit and a syncronization byte. It
then writes the data in your buffer and forces it to take up a multiple of
256 bytes by appending zeroes. Then a 16-bit number to be used for cyclical
redundancy checks and a trailer are added to the end of the block.
INTERRUPT 16 -- Keyboard I/O
(AH) Operation Also on entry On return
------------------------------------------------------------------------------
0 Wait for keypress (AL) = Character read
(AH) = Scan code of
character read
(i.e. if it's
Ctrl, Shift, Alt,
or a combination)
1 Check for character Zero flag = 1 if buffer
ready is empty
Zero flag = 0 if buffer
is not empty. Next char.
is in (AL), scan code is
in (AH)
2 Return shift status (AL) = Shift status
[ See note #1 for more
information ]
________________________________________________________________________________
Notes on interrupt 16:
1) The shift status byte returned by function #2 looks like this:
7 6 5 4 3 2 1 0
Bit Meaning (when on)
0 Right shift key depressed
1 Left shift key is depressed
2 Ctrl key depressed
3 Alt key depressed
4 Scroll Lock state is on
5 Num Lock state is on
6 Caps Lock state in on
7 Insert mode is on
2) The shift status byte is located at memory location 0040:0017, and is called
KB_FLAG by IBM. At location 0040:0018 is KB_FLAG_1, which looks like the
following:
7 6 5 4 3 2 1 0
Bit Meaning (when on)
3 Hold state (Ctrl-Num Lock) is on
4 The Scroll Lock key is depressed
5 The Num Lock key is depressed
6 The Caps Lock key is depressed
7 The Insert key is depressed
[ Bits 0, 1, and 2 have no meaning ]
INTERRUPT 17 -- Printer I/O
(AH) Operation Also on entry On return
------------------------------------------------------------------------------
0 Print a character (AL) = Character to be (AH) = Printer status
printed byte (see note
(DX) = Printer to use #1)
(0=LPT1, 1=LPT2
2=LPT3)
1 Initialize printer (DX) = Printer to use (AH) = Printer status
byte
2 Read printer status (DX) = Printer to use (AH) = Printer status
byte byte
________________________________________________________________________________
Notes on interrupt 17:
1) The printer status byte looks like this:
7 6 5 4 3 2 1 0
Bit Meaning (when on)
0 Time out error
3 I/O error
4 Selected (off-line)
5 Out of paper
6 Acknowledge
7 Busy
[ Bits 1 and 2 have no meaning ]
INTERRUPT 1A -- Time-of-Day
(AH) Operation Also on entry On return
------------------------------------------------------------------------------
0 Read current clock (CX) = High 16 bits of
setting count
(DX) = Low 16 bits of
count
(AL) = 0 if the time was
last read within
24 hours.
Otherwise,
(AL) = 1
1 Set the system (CX) = High 16 bits of count
clock (DX) = Low 16 bits of count
________________________________________________________________________________
Notes on interrupt 1A:
1) The count is the amount of 0.05492 (65535/1193100) seconds that have passed
since 12 a.m. on January 1, 1980. Upon booting/rebooting, the count becomes
zero (12 a.m.) and the system date becomes January 1, 1980 (although these
could differ if you have a real-time clock/calendar installed).
2) The count is incremented 18.205 times per second (or, in fractional
form, 1193100/65536 times per second).
3) After CX and DX both reach 65535 (FFFF hex), they stop increasing. (You don't
have to worry about this happening, though, unless you leave your computer on
for seven years -- until the system date reaches December 31, 2099.)
SOME INTERESTING INTERRUPTS THAT ONLY HAVE SINGLE FUNCTIONS
INTERRUPT 5 -- PRINT SCREEN
Interrupt 5 prints everything on your screen. It is normally activated
when you press <shift><PrtSc>, although it can be called from an assembly
language program with an INT 5 instruction.
Interrupt 5 returns a status byte in memory location 0050:0000. If this
byte is 0, the screen was successfully printed. If an error occurred during
printing, this byte is FF (decimal 255).
________________________________________________________________________________
INTERRUPT 11 -- EQUIPMENT CHECK
Interrupt 11 reads the word at location 0040:0010, which contains some
information about the system's hardware (taken from the DIP switch settings),
and returns it in AX. AX then looks like this:
15 14 13 12 11 10 09 08 07 06 05 04 03 02 01 00
Bit 1 -- If on, computer has diskettes. Otherwise, no diskettes.
Bit 12 -- If on, computer has game I/O card. Otherwise, no game I/O card.
Bits Meaning How to Read Bits
---- ------- ----------------
2&3 Memory on system board 00 = 16K
01 = 32K
10 = 48K
11 = 64K
4&5 Original video mode 00 = None (could be a T.V. set)
01 = 40x25 B/W, (color/graphics monitor)
10 = 80x25 B/W, (color/graphics monitor)
11 = 80x25 B/W, (monochrome monitor)
6&7 Number of disk drives 00 = 1
(only valid if bit 1 01 = 2
is on) 10 = 3
11 = 4 or more
9&10 Number of RS-232 cards 00 = None
(serial ports) 01 = 1
10 = 2
11 = 3
14&15 Number of printers 00 = None
(parallel ports) 01 = 1
10 = 2
11 = 3
INTERRUPT 18 -- CASSETTE BASIC
An INT 18H instruction causes the computer to enter cassette BASIC,
which is the form of BASIC that you use if you don't have diskettes (or if you
forget to close the disk drive door when you boot up).
________________________________________________________________________________
INTERRUPT 19 -- REBOOT
When you press <CTRL-ALT-DEL>, an interrupt 19 is executed. When put in
a program, an INT 19H will reboot the system.
________________________________________________________________________________
INTERRUPT 1B -- BREAK
When you press <CTRL-Break>, the computer executes an INT 1BH
instruction, which just performs an IRET instruction and returns to DOS. The
pointer to this interrupt could be changed if you want to implement your own
<CTRL-Break> handler.
________________________________________________________________________________
INTERRUPT 1C -- TIMER TICK
18.205 times per second, an INT 18H instruction is executed. Like
interrupt 1B, this interrupt is only an IRET instruction. If you have a routine
that needs to be called 18.205 times a second, this interrupt could be helpful.
SEND ALL QUESTIONS AND COMMENTS TO:
Scott Pakin
6007 N. Sheridan Rd.
Chicago, IL 60660
