Interrupt Assembly

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Appendix A - List of Interrupts

The following is a small portion of thesoftware interrupts available up to DOS 6.22. This list contains the

interrupts most useful in this course. Students are not restricted to this list, of course ...

INT 10h, Service 00h - Set Video Mode

Note that many applications ignore the video interrupts and deal directly with the video hardware installed

in a particular machine (by means of video drivers supplied by manufacturer). One can do many clever

and fun things with this interrupt, but none of that is really required in this course. The simple I/O of INT

21h, service 1,2,8,9,0Bh, etc. will probably do everything required for simple data entry and display.

INT 11h - Get Equipment Status

(Seldom used, since the amount of equipment available has outstripped this interrupt's capability.)

INT 12h - Get Memory Size

Exit

On exit, AX = # memory blocks (1K each)

INT 13h, Service 00h - Reset Disk Drives

Entry:

AH = 00h

DL = Drive number, zero based (0 = A:, 1=B:, 2=C:, etc.)

Exit

Registers unchanged

Comments

Drive head moved out to track 0 (outer track).

INT 13h, Service 01h - Get Disk Drive Status

Assemblers http://physics.concordia.ca/~eddy/391/A

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Entry:

AH = 01h

DL = Drive number (0 based)

Exit

AH = status byte - floppy disk drive only (see following table). 00 = no error.

Hard drives generate a somewhat diff erent error set.

Floppy Status Byte

Bits Set Hex Value Description

7 80Drive did not complete handshaking procedure in required time

(time-out error)

6 40 Seek failure - unable to access requested track/sector

5 20 Controller unable to complete command (possible chip malfunction)

4 10 Bad CRC detected (read only) - bits lost (media damage ordemagnitization)

3,0 09 DMA error, segment register must be changed

3 08 DMA overrun

3,2 06 Floppy disk drive empty or door not closed

2 04 Bad sector/sector not found

1,0 03 Write attempt on write-protected disk

1 02 Bad sector ID (address mark)

0 01 Bad controller chip command

INT 13h, Service 02h - Read Disk Sector(s)

INT 13h, Service 03h - Write Sectors to Disk

INT 13h, Service 04h - Verify Disk Sectors

INT 13h, Service 05h - Format Disk Tracks

Entry:

AH = 02, 03, 04 or 05

AL = number of sectors to read

CH = Track (cylinder) number

CL = Sector number (unused if AH=05)

DH = head number (side of diskette, 0 or 1)

DL = Drive number, zero based

ES:BX = buffer address of DTA (disk transfer area). Unused if AH=04.


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Exit

CF=0 if no error, AH = status byte (see above table).

If AH=05, ES:BX = 4-byte address field entries, 1 per sector:

byte

numberdata

0 track number

1 head number

2 sector number

3 sector-size code: bytes/sector

00 128

01 256

02512

(standard)

03 1024

Comments

Always check status byte before reading/writing data.

INT 14h, Service 00h - Initialize Communications Port

Entry:

AH = 00h

AL = Initialization data (see Comments below)

DX = Communications port number (0 = COM1:, 1 = COM2:, etc.)

Exit

AH = Line status*

AL = Modem status**

Comments/Warning

A typical setting is AL = E3h (9600 bd, no parity, 1 stop bit and an 8-bit data character length).

*Line Status Register (8 bits)

b7 = 1: Time-out error

b6 = 1: Transfer Shift Register (TSR) empty

b5 = 1: Transfer Holding Register (THR) empty

b4 = 1: Break Interrupt Detected (data line is held in non-idle state for several character times)


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b3 = 1: Framing error (character does not have start bit and stop bit)

b2 = 1: Parity error (if even parity, character did not have an even number of bits set to one)

b1 = 1: Overrun error (character not read out of UART before next character's arrival)

b0 = 1: Data ready to be read out.

**Modem Status Register (8 bits)

b7 = 1: Data Terminal Ready (DTR) detected

b6 = 1: Ring indicator

b5 = 1: Data Set Ready (DSR)

b4 = 1: Clear To Send (CTS)

b3 - b0: Seldom used in normal operation

INT 14h, Service 01h - Send (Write) Character

Entry:

AH = 01hAL = ASCII character

DX = Communications port number

Exit

AH = Error code (MSB clear means transmission successful)

Comments/Warnings

UART must have been initialized before transmission of first character. Check Line Status Register (03FD forCOM1:) for value 60h, which indicates no character is waiting to be read, the Transmit Holding Register is

awaiting data, and the Transmitter is empty (last character has been sent). 61h means a character has arrived

and needs to be read. Reading data from UART has the highest priority, because a second character could

overwrite the first.

Details of error code following transmission attempt may be found in Wyatt, for example (p.532).

INT 14h, Service 02h - Receive Character

Entry:

AH = 02h


Exit

AH = Error code

AL = Incoming character (ASCII)


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Comments/Warning

Line status register with b0 = 1 means a character has arrived. The LSR data is placed in AH, so the

instruction BT AH,1 sets the carry flag (CF = 1) if the LSR = 61h.

INT 14h, Service 03h - Get Communications Port StatusEntry:

AH = 03h


Exit

AH = Line status

AL = Modem status

Comments/Warning

INT 21h, Service 3Ch - Create File

Entry:

AH = 3Ch

CX = File attribute (cfINT 21/ service 43)

DX = Offset address of path name (label used in .DATA segment to specify file name)

DS = Segment address of path name (.DATA address)

Exit

AX = error code (use INT 21h, AH=59 for display of error codes)

Comments/Warning

This interrupt function will destroy an existing file. Use service 5Bh if you wish to avoid this possibility. 5B

(not available before DOS 3.0) is identical to 3C, but returns an error code if the file already exists. Bothinterrupts (3C and 5B) leave the file open (contents partially in RAM). Remember to close the file (INT

21/3E) before terminating the program.

INT 21h, Service 3Dh - Open File

Entry:

AH = 3Dh


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AL = File attribute

DX = Offset address of path name

DS = Segment address of path name (.DATA address)

Exit

Success: AX = file handle (simple digit)

Failure: AX = error code (use INT 21h, AH=59 for display of error codes)

Comments

DS:DX point to label used in .DATA segment to specify file name as ASCIZ (null-terminated ASCII) string.

This opens file (contents partially in RAM). Remember to close file (INT 21/3E) before terminating

program.

INT 21h, Service 3Eh - Close File

Entry:

AH = 3Eh

BX = File handle (simple digit produced in AX by Open File command)

Exit

Success: carry bit clear

Failure: carry bit set, AX = error code (use INT 21h, AH=59 for display of error codes)

Comments

Failure to close file may result in its complete destruction! Always place Close File command as close behind

last file operation (read or write) as possible, in case program fails to exit properly while file open

INT 21h, Service 3Fh - Read File

Entry:

AH = 3Fh

BX = File handle

CX = # bytes to readDX = Offset address buffer

DS = Segment address of buffer (.DATA address, presumably)

Exit

Success: AX = number of bytes read. If AX < CX or AX = 0, one has hit end-of-file.

Failure: AX = error code (use INT 21h, AH=59 for display of error codes)

Comments


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.Data read from file begins at location pointed to by DS:DX. For this reason, one must be sure number of

bytes read does not exceed memory allocation made in data segment.

INT 21h, Service 40h - Write File

Entry:

AH = 40hBX = File handle

CX = # bytes to write

DX = Offset address buffer

DS = Segment address of buffer (.DATA address, presumably)

Exit

Success: CF=0, AX = CX = number of bytes read.

Failure: CF = 1, AX = error code. If AX != CX an error (e.g. disk full) has occurred, even though CF=0.

Comments

.Data read from file begins at location pointed to by DS:DX. For this reason, one must be sure number of

bytes read does not exceed memory allocation made in data segment.


Interrupt Assembly

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