INT 21h VXL8086 linhcan87

The short list of emulated MS-DOS interrupts -- INT 21h

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DOS file system is emulated in C:\emu8086\vdrive\x (x is a drive letter)

If no drive letter is specified and current directory is not set, then C:\emu8086\MyBuild\ path is used by default. FLOPPY_0,1,2,3 files are emulated independently from DOS file system.

For the emulator physical drive A: is this file c:\emu8086\FLOPPY_0 (for BIOS interrupts: INT 13h and boot).

For DOS interrupts (INT 21h) drive A: is emulated in this subdirectory: C:\emu8086\vdrive\a\

Note: DOS file system limits the file and directory names to 8 characters, extension is limited to 3 characters;

example of a valid file name: myfile.txt (file name = 6 chars, extension - 3 chars). extension is written after the dot, no other dots are allowed.

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INT 21h / AH=1 - read character from standard input, with echo, result is stored in AL.

if there is no character in the keyboard buffer, the function waits until any key is pressed.

example:

mov ah, 1

int 21h

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INT 21h / AH=2 - write character to standard output.

entry: DL = character to write, after execution AL = DL.

example:

mov ah, 2

mov dl, 'a'

int 21h

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INT 21h / AH=5 - output character to printer.

entry: DL = character to print, after execution AL = DL.

example:

mov ah, 5

mov dl, 'a'

int 21h

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INT 21h / AH=6 - direct console input or output.

parameters for output: DL = 0..254 (ascii code)

parameters for input: DL = 255

for output returns: AL = DL

for input returns: ZF set if no character available and AL = 00h, ZF clear if character available.

AL = character read; buffer is cleared.

example:

mov ah, 6

mov dl, 'a'

int 21h ; output character.

mov ah, 6

mov dl, 255

int 21h ; get character from keyboard buffer (if any) or set ZF=1.

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INT 21h / AH=7 - character input without echo to AL.

if there is no character in the keyboard buffer, the function waits until any key is pressed.

example:

mov ah, 7

int 21h

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INT 21h / AH=9 - output of a string at DS:DX. String must be terminated by '$'.

example:

org 100h

mov dx, offset msg

mov ah, 9

int 21h

msg db "hello world $"

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INT 21h / AH=0Ah - input of a string to DS:DX, fist byte is buffer size, second byte is number of chars actually read. this function does not add '$' in the end of string. to print using INT 21h / AH=9 you must set dollar character at the end of it and start printing from address DS:DX + 2.

example:

org 100h

mov dx, offset buffer

mov ah, 0ah

int 21h

jmp print

buffer db 10,?, 10 dup(' ')

print:

xor bx, bx

mov bl, buffer[1]

mov buffer[bx+2], '$'

mov dx, offset buffer + 2

mov ah, 9

int 21h

the function does not allow to enter more characters than the specified buffer size.

see also int21.asm in c:\emu8086\examples

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INT 21h / AH=0Bh - get input status;

returns: AL = 00h if no character available, AL = 0FFh if character is available.

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INT 21h / AH=0Ch - flush keyboard buffer and read standard input.

entry: AL = number of input function to execute after flushing buffer (can be 01h,06h,07h,08h, or 0Ah - for other values the buffer is flushed but no input is attempted); other registers as appropriate for the selected input function.

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INT 21h / AH= 0Eh - select default drive.

Entry: DL = new default drive (0=A:, 1=B:, etc)

Return: AL = number of potentially valid drive letters

Notes: the return value is the highest drive present.

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INT 21h / AH= 19h - get current default drive.

Return: AL = drive (0=A:, 1=B:, etc)

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INT 21h / AH=25h - set interrupt vector;

input: AL = interrupt number. DS:DX -> new interrupt handler.

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INT 21h / AH=2Ah - get system date;

return: CX = year (1980-2099). DH = month. DL = day. AL = day of week (00h=Sunday)

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INT 21h / AH=2Ch - get system time;

return: CH = hour. CL = minute. DH = second. DL = 1/100 seconds.

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INT 21h / AH=35h - get interrupt vector;

entry: AL = interrupt number;

return: ES:BX -> current interrupt handler.

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INT 21h / AH= 39h - make directory.

entry: DS:DX -> ASCIZ pathname; zero terminated string, for example:

org 100h

mov dx, offset filepath

mov ah, 39h

int 21h

filepath DB "C:\mydir", 0 ; path to be created.

end

the above code creates c:\emu8086\vdrive\C\mydir directory if run by the emulator.

Return: CF clear if successful AX destroyed. CF set on error AX = error code.

Note: all directories in the given path must exist except the last one.

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INT 21h / AH= 3Ah - remove directory.

Entry: DS:DX -> ASCIZ pathname of directory to be removed.

Return:

CF is clear if successful, AX destroyed CF is set on error AX = error code.

Notes: directory must be empty (there should be no files inside of it).

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INT 21h / AH= 3Bh - set current directory.

Entry: DS:DX -> ASCIZ pathname to become current directory (max 64 bytes).

Return:

Carry Flag is clear if successful, AX destroyed.

Carry Flag is set on error AX = error code.

Notes: even if new directory name includes a drive letter, the default drive is not changed,

only the current directory on that drive.

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INT 21h / AH= 3Ch - create or truncate file.

entry:

CX = file attributes:

mov cx, 0 ; normal - no attributes.

mov cx, 1 ; read-only.

mov cx, 2 ; hidden.

mov cx, 4 ; system

mov cx, 7 ; hidden, system and read-only!

mov cx, 16 ; archive

DS:DX -> ASCIZ filename.

returns:

CF clear if successful, AX = file handle.

CF set on error AX = error code.

note: if specified file exists it is deleted without a warning.

example:

org 100h

mov ah, 3ch

mov cx, 0

mov dx, offset filename

mov ah, 3ch

int 21h

jc err

mov handle, ax

jmp k

filename db "myfile.txt", 0

handle dw ?

err:

; ....

k:

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INT 21h / AH= 3Dh - open existing file.

Entry:

AL = access and sharing modes:

mov al, 0 ; read

mov al, 1 ; write

mov al, 2 ; read/write

DS:DX -> ASCIZ filename.

Return:

CF clear if successful, AX = file handle.

CF set on error AX = error code.

note 1: file pointer is set to start of file.

note 2: file must exist.

example:

org 100h

mov al, 2

mov dx, offset filename

mov ah, 3dh

int 21h

jc err

mov handle, ax

jmp k

filename db "myfile.txt", 0

handle dw ?

err:

; ....

k:

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INT 21h / AH= 3Eh - close file.

Entry: BX = file handle

Return:

CF clear if successful, AX destroyed.

CF set on error, AX = error code (06h).

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INT 21h / AH= 3Fh - read from file.

Entry:

BX = file handle.

CX = number of bytes to read.

DS:DX -> buffer for data.

Return:

CF is clear if successful - AX = number of bytes actually read; 0 if at EOF (end of file) before call.

CF is set on error AX = error code.

Note: data is read beginning at current file position, and the file position is updated after a successful read the returned AX may be smaller than the request in CX if a partial read occurred.

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INT 21h / AH= 40h - write to file.

entry:

BX = file handle.

CX = number of bytes to write.

DS:DX -> data to write.

return:

CF clear if successful; AX = number of bytes actually written.

CF set on error; AX = error code.

note: if CX is zero, no data is written, and the file is truncated or extended to the current position data is written beginning at the current file position, and the file position is updated after a successful write the usual cause for AX < CX on return is a full disk.

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INT 21h / AH= 41h - delete file (unlink).

Entry:

DS:DX -> ASCIZ filename (no wildcards, but see notes).

return:

CF clear if successful, AX destroyed. AL is the drive of deleted file (undocumented).

CF set on error AX = error code.

Note: DOS does not erase the file's data; it merely becomes inaccessible because the FAT chain for the file is cleared deleting a file which is currently open may lead to filesystem corruption.

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INT 21h / AH= 42h - SEEK - set current file position.

Entry:

AL = origin of move: 0 - start of file. 1 - current file position. 2 - end of file.

BX = file handle.

CX:DX = offset from origin of new file position.

Return:

CF clear if successful, DX:AX = new file position in bytes from start of file.

CF set on error, AX = error code.

Notes:

for origins 1 and 2, the pointer may be positioned before the start of the file; no error is returned in that case, but subsequent attempts to read or write the file will produce errors. If the new position is beyond the current end of file, the file will be extended by the next write (see AH=40h).

example:

org 100h

mov ah, 3ch

mov cx, 0

mov dx, offset filename

mov ah, 3ch

int 21h ; create file...

mov handle, ax

mov bx, handle

mov dx, offset data

mov cx, data_size

mov ah, 40h

int 21h ; write to file...

mov al, 0

mov bx, handle

mov cx, 0

mov dx, 7

mov ah, 42h

int 21h ; seek...

mov bx, handle

mov dx, offset buffer

mov cx, 4

mov ah, 3fh

int 21h ; read from file...

mov bx, handle

mov ah, 3eh

int 21h ; close file...

filename db "myfile.txt", 0

handle dw ?

data db " hello files! "

data_size=$-offset data

buffer db 4 dup(' ')

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INT 21h / AH= 47h - get current directory.

Entry:

DL = drive number (00h = default, 01h = A:, etc)

DS:SI -> 64-byte buffer for ASCIZ pathname.

Return:

Carry is clear if successful

Carry is set on error, AX = error code (0Fh)

Notes:

the returned path does not include a drive and the initial backslash.

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INT 21h / AH=4Ch - return control to the operating system (stop program).

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INT 21h / AH= 56h - rename file / move file.

Entry:

DS:DX -> ASCIZ filename of existing file.

ES:DI -> ASCIZ new filename.

Return:

CF clear if successful.

CF set on error, AX = error code.

Note: allows move between directories on same logical drive only; open files should not be renamed!

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