INT 21H Functions

In DOS, interrupt 21H provides the main interface to the operating system, and most system calls are invoked through it. This page describes

DOS 1.0 Functions

AH=00h - Terminate

Parameters

• CS - Segment of PSP

Return

None (the program terminates).

Description

Legacy exit system call. This function resets the terminate, ctrl-break, and critical error addresses in the PSP. All buffers are flushed, memory used by the application is freed, and the process is terminated via INT 22h.

Notes

This is a legacy function and should be mostly avoided unless its specific behavior is required.

AH=01h - Character Input

AH=02h - Character Output

AH=03h - Auxilliary Input

AH=04h - Auxilliary Output

AH=05h - Printer Output

AH=06h - Direct Console I/O

AH=07h - Direct Console Input without Echo

AH=08h - Console Input without Echo

AH=09h - Display Cash-Terminated String

AH=0Ah - Buffered Keyboard Input

AH=0Bh - STDIN Input Status

AH=0Ch - Clear Keyboard Buffer and Invoke Keyboard Function

AH=0Dh - Disk Reset

AH=0Eh - Select Disk

AH=0Fh - Open FCB

Parameters

• DS:DX - far pointer to an unopened FCB

Return

• AL - 00h on success; otherwise, FFh.

Description

This function is the DOS 1.0 method of opening a file. The FCB shall have been previously initialized with valid device number, filename, and extension values; it must also refer to a file that already exists prior to invocation. On success, the FCB refers to an open file and will have its fields set accordingly.

Note

This is a legacy function and should be avoided. FCBs are a holdover from CP/M which were replaced with proper file handles in DOS 2.0. The responsibility of managing FCBs is delegated to the programmer and the FCBs themselves must reside in conventional memory, making them inconvenient for the programmer. Additionally, neither function AH=00h or AH=4Ch will close open FCBs, leading to the possibility that these files will remain open.

This function yields version-specific behavior that must be taken into account when working with this function. In particular, DOS 2.0 will allow directories to be opened (which DOS 3.0 later forbids) and the values in the reserved area change between DOS versions.

AH=10h - Close FCB

AH=11h - Search for First Entry with FCB

AH=12h - Search for Next Entry with FCB

AH=13h - Delete File with FCB

AH=14h - Sequential Read with FCB

AH=15h - Sequential Write with FCB

AH=16h - Create a File with FCB

AH=17h - Rename a File with FCB

AH=19h - Get Current Default Drive

AH=1Ah - Set Disk Transfer Address

AH=1Bh - Get Allocation Table for Default Drive

AH=1Ch - Get Allocation Table for Specified Drive

AH=1Fh - Get Current Drive Parameter Table

AH=21h - Random Read with FCB

AH=22h - Random Write with FCB

AH=23h - Get File Size with FCB

AH=24h - Set Relative Record Field for FCB

AH=25h - Set Interrupt Vector

AH=26h - Create PSP

AH=27h - Random Block Read with FCB

AH=28h - Random Block Write with FCB

AH=29h - Parse Filename with FCB

AH=2Ah - Get Date

AH=2Bh - Set Date

AH=2Ch - Get Time

AH=2Dh - Set Time

AH=2Eh - Set Verify Flag

DOS 2.0 Functions

AH=2Fh - Get Disk Transfer Address

AH=30h - Get DOS Version

AH=31h - Terminate and Stay Resident

AH=32h - Get Drive Parameter Table

AH=33h - Manipulate System Values

AH=34h - Get Address of Critical Flag

AH=35h - Get Interrupt Vector

AH=36h - Get Free Disk Space

AH=37h - Manipulate Switch Dispatcher

AH=38h - Manipulation Country Information

AH=39h - Create Subdirectory

AH=3Ah - Delete Subdirectory

AH=3Bh - Change Subdirectory

AH=3Ch - Create File Handle

AH=3Dh - Open File Handle

AH=3Eh - Close File Handle

AH=3Fh - Read File Handle

AH=40h - Write File Handle

AH=41h - Delete File

AH=42h - Seek File Handle

AH=43h - Change File Mode

AH=44h - Handle I/O Control

AH=45h - Duplicate Handle

AH=46h - Redirect Handle

AH=47h - Get Current Directory

AH=48h - Allocate Memory

AH=49h - Free Memory

AH=4Ah - Reallocate Memory

AH=4Bh - Execute Process

AH=4Ch - Terminate Process with Return Code

AH=4Dh - Get Return Code

AH=4Eh - Find First Matching File with Handle

AH=4Fh - Find Next Matching File with Handle

AH=50h - Set PSP

AH=51h - Get PSP

AH=52h - Get SYSVARS

AH=53h - Generate Drive Parameter Table

AH=54h - Get Verification Flag

AH=55h - Create PSP

AH=56h - Rename File

AH=57h - Get or Set File Timestamp

DOS 2.11 Functions

AH=58h - Get or Set Memory Allocation Strategy

DOS 3.0 Functions

AH=59h - Get Extended Error Information

AH=5Ah - Create Temporary File

AH=5Bh - Create New File (DOS 3.0)

AH=5Ch - Lock or Unlock File

AH=5Dh - Critical Error Information

AH=5Eh - Network Services

AH=5Fh - Network Redirection

AH=60h - Get Fully-Qualified File Name

AH=62h - Get Address of Current PSP

AH=63h - Get Lead Byte Table

DOS 3.2 Functions

AH=64h - Set External Event Flag

DOS 3.3 Functions

AH=65h - Get Extended Country Info

AH=66h - Manipulate Global Code Page

AH=67h - Set Handle Limit

AH=68h - Flush Buffer

DOS 4.0 Functions

AH=69H - Manipulate Disk Serial Number

AH=6Ah - Commit File

AH=6Ch - Extended Open