Assembly Int21h,dos Call

Table: DOS system function tuning int 21H
The AH function call parameter returns the parameter
00 program termination (same as int 20H) cs= program segment Prefix
01 keyboard input and echo al= input characters
02 Display output dl= output characters
03 Asynchronous communication input Al= input data
04 Asynchronous communication output dl= output data
05 Printer output dl= output character
06 Direct Console I/O dl=ff (Input)
dl= character (output) al= input character
07 keyboard input (no echo) al= input characters
08 Keyboard input (no echo)
Detecting Ctrl-break al= Input characters
09 Displaying strings ds:dx= string addresses
' $ ' End string
0A keyboard input to buffer ds:dx= buffer first address
(DS:DX) = maximum number of characters in buffer (ds:dx+1) = number of characters actually entered
0B Verify keyboard status al=00 has input
AL=FF No input
0C clears the input buffer and
Request the specified input function al= input function number
(1,6,7,8,a)
0D disk reset purge file buffers
0E Specifies the current default disk drive dl= drive letter 0=a,1=b,... Number of al= drives
0F Open File DS:DX=FCB first address al=00 file found
AL=FF File not found
10 Close file DS:DX=FCB first address al=00 directory modified successfully
File not found in Al=ff directory
11 Find First directory entry DS:DX=FCB first address al=00 found
Al=ff not found
12 Find Next Directory entry DS:DX=FCB first address
(file with * or?) Al=00 found
Al=ff not found
13 Delete file DS:DX=FCB First address al=00 Delete succeeded
Al=ff not found
14

Sequential read

DS:DX=FCB First Address

AL=00 Read Success
= 01 end of file, no data in record
=02 DTA space is not enough
= 03 end of file, incomplete record
15 Sequential write DS:DX=FCB first address al=00 write success
=01 full
=02 DTA space is not enough
16 built file DS:DX=FCB first address al=00 established successfully
=FF No disk space
17 File name DS:DX=FCB first address
(ds:dx+1) = old file name
(ds:dx+17) = new file name al=00 successful
Al=ff not successful
19 take the current default disk drive al= the default drive letter 0=a,1=b,2=c,...
1 A DTA address DS:DX=DTA address
1 b

Take the default drive fat information

Al= number of sectors per cluster
Ds:bx=fat Identity Byte
cx= Physical Sector size
dx= the number of clusters for the default drive
1C take any drive fat information dl= drive letter ibid.
21st

Random Read

DS:DX=FCB First Address

AL=00 Read Success
= 01 End of File
=02 Buffer Overflow
=03 Buffer Dissatisfaction
22 Random Write DS:DX=FCB first address al=00 write success
=01 full
=02 Buffer Overflow
23 Determining File Size DS:DX=FCB First address al=00 success (file length fill in FCB)
Al=ff not found
24 set random record number DS:DX=FCB first address
25 Set interrupt vector ds:dx= interrupt vector
Al= Interrupt Class Model
26 Creating a program segment prefix dx= new program segment prefix
27

Random block Reading

DS:DX=FCB First Address
cx= Record Count al=00 read success
= 01 End of File
=02 buffer too small, transfer end
=03 Buffer Dissatisfaction
28 Random chunked Write DS:DX=FCB first address
cx= Record Count al=00 write success
=01 full
=02 Buffer Overflow
29 Parse file name ES:DI=FCB first address
Ds:si=asciiz string
al= control Analysis Flag al=00 standard file
= 01 Multi-semantic files
=02 Illegal drive letter
2 a take date cx= year
Dh:dl= Month: day (binary)
2 B Set Date cx:dh:dl=: Month: Day al=00 success
=FF invalid
2 c Take time ch:cl=: minutes
Dh:dl= seconds: 1/100 seconds
Set time ch:cl=: Min
Dh:dl= seconds: 1/100 seconds al=00 Success
=FF invalid
2E disk Automatic read-write flag al=00 off flag
AL=01 Open Flag
2F fetch the first address of the disk buffer es:bx= buffer first address
30 Take DOS version number ah= release number, al= version
31 end and reside al= return code
dx= Dwell Area Size
33

Ctrl-break detection

al=00 Take status
=01 status (DL)
Dl=00 off detection
=01 Open detection dl=00 off Ctrl-break detection
=01 Open Ctrl-break Detection
35 take interrupt Vector al= interrupt type es:bx= interrupt vector
36


Take free disk space


Dl= Drive Letter
0= default, 1=a,2=b,...

Success: Number of sectors per cluster ax=
bx= number of active clusters
cx= number of bytes per sector
dx= total number of clusters
Failure: AX=FFFF
38 Place/Fetch national information ds:dx= first address bx= Country code (international telephone prefix code)
ax= error code
39 Creating a subdirectory (MKDIR) Ds:dx=asciiz string address ax= error code
3 a Delete subdirectory (RMDIR) Ds:dx=asciiz string address ax= error code
3 b Change current directory (CHDIR) Ds:dx=asciiz string address ax= error code
3C Create file Ds:dx=asciiz string address
cx= file attribute succeeded: ax= file code
Error: ax= error code
3D

Open File

Ds:dx=asciiz string Address
Al=0 Read
=1 Write
=3 Read/write success: ax= file code
Error: ax= error code
3E closing file bx= file code failure: ax= error code
3F

Read a file or device

ds:dx= Data Buffer Address
bx= File Code
Cx= read the number of bytes read successfully:
ax= number of bytes actually read in
Ax=0 to end of file
Read error: ax= error code
40 Write file or device ds:dx= data buffer address
bx= File Code
Cx= writes the number of bytes written successfully:
ax= the number of bytes actually written
Write error: ax= error code
41 Delete File Ds:dx=asciiz string address success: ax=00
Error: ax= error code (2,5)
42


Move file pointer


bx= File Code
cx:dx= Displacement Amount
Al= Move mode (0: Absolute shift from File header, 1: Relative move from current position, 2: absolute shift from end of file) success: dx:ax= new file pointer position
Error: ax= error code


43

Place/Fetch file properties

Ds:dx=asciiz string Address
al=0 fetching file attributes
Al=1 file Properties
Cx= File Property succeeded: cx= file property
Failure: cx= error code
44




Device file I/O control




bx= File Code
Al=0 Take status
=1 Status DX
=2 reading data
=3 Writing data
=6 Take input status
=7 fetching output Status dx= device information





45 Copy file Code bx= file code 1 success: ax= file code 2
Failure: ax= error code
46 Manual Copy file code bx= file code 1
cx= File Code 2 failed: ax= error code
47 take the current directory pathname dl= drive letter
Ds:si=asciiz string Address (ds:si) =asciiz string
Failure: ax= error code
48 Allocating memory space bx= request memory capacity success: ax= allocating memory first place
Failure: bx= Maximum available memory
49 Free content Space es= memory start segment address failed: ax= error code
4A Adjust allocated storage Block es= original memory start address
Bx= re-requested capacity failed: bx= Maximum free space
ax= error code
4 b

Assembly/Execution Procedures

Ds:dx=asciiz string Address
Es:bx= the first address of the parameter area
Al=0 Loading execution
al=3 mount no execution failure: ax= error code


4C with return code end al= return code
4D fetch return code ax= return code
4E find first matching file Ds:dx=asciiz string address
cx= Property ax= error code (02,18)
4F Find Next matching file Ds:dx=asciiz string address
(file name with the? or *) Ax= error code (18)
54 Fetch Disk Automatic read-write flag al= Current flag value
56 File Renaming Ds:dx=asciiz string (old)
Es:di=asciiz string (new) ax= error code (03,05,17)
57 Place/Fetch file date and time bx= file code
Al=0 Read
Al=1 Set (DX:CX) dx:cx= Date and time
Failure: ax= error code
58 Pick/Place allocation policy code al=0 fetch code
Al=1 Code (BX) Success: ax= Strategy code
Failure: ax= error code
59

Fetch extension Error code

Ax= Expansion Error code
Bh= Error Type
Bl= Recommended Actions
Ch= wrong place
5A Creating temporary files cx= file properties
Ds:dx=asciiz string Address success: ax= file code
Failure: ax= error code
5B creating a new file Cx= file properties
Ds:dx=asciiz string Address success: ax= file code
Failure: ax= error code
5C


Controlling file access


al=00 blockade
=01 Open
bx= File Code
cx:dx= File Displacement
si:di= file length failed: ax= error code



62 Fetch Program Segment prefix BX=PSP address

Assembly Int21h,dos Call (GO)