The role of MOV ah, 4CH int 21

int Indicates the meaning of interrupt .

mov ah,4ch

int 21h

Express , Execute interrupt instruction 4c    ( Check the table below to see End program with return code )

AH yes ax The high

AH	function	Call parameters	Returns the parameter
00	Termination of procedure ( Same as INT 20H)	CS= Segment prefixes
01	Keyboard input and echo		AL= The input character
02	display output	DL= The output characters
03	Asynchronous communication input		AL= input data
04	Asynchronous communication output	DL= Output data
05	Printer output	DL= The output characters
06	Direct console I/O	DL=FF( Input ) DL= character ( Output )	AL= The input character
07	Keyboard entry ( No echo )		AL= The input character
08	Keyboard entry ( No echo ) testing Ctrl-Break		AL= The input character
09	display string	DS:DX= String address '$' End string
0A	Keyboard input to buffer	DS:DX= Buffer first address (DS:DX)= Maximum number of characters in buffer	(DS:DX+1)= The number of characters actually entered
0B	Verify keyboard status		AL=00 There's input AL=FF No input
0C	Clear the input buffer and Request the specified input function	AL= Enter function number (1,6,7,8,A)
0D	Disk reset		Clear file buffer
0E	Specify the current default disk drive	DL= Drive letter 0=A,1=B,...	AL= Number of drives
0F	Open file	DS:DX=FCB The first address	AL=00 File found AL=FF File not found
10	Close file	DS:DX=FCB The first address	AL=00 Directory modification succeeded AL=FF File not found in directory
11	Find the first catalog entry	DS:DX=FCB The first address	AL=00 find AL=FF Not found
12	Find the next catalog entry	DS:DX=FCB The first address ( The document contains * or ?)	AL=00 find AL=FF Not found
13	Delete file	DS:DX=FCB The first address	AL=00 Delete successful AL=FF Not found
14	Sequential reading	DS:DX=FCB The first address	AL=00 Read successfully   =01 End of file , There is no data in the record   =02 DTA Space is not enough   =03 End of file , The record is incomplete
15	Sequential writing	DS:DX=FCB The first address	AL=00 Write a successful   =01 The dish is full   =02 DTA Space is not enough
16	Build documents	DS:DX=FCB The first address	AL=00 Build success   =FF No disk space
17	Change the name of the document	DS:DX=FCB The first address (DS:DX+1)= Old filename (DS:DX+17)= New file name	AL=00 success AL=FF unsuccessful
19	Take the current default disk drive		AL= The default drive letter 0=A,1=B,2=C,...
1A	Set up DTA Address	DS:DX=DTA Address
1B	Take the default drive FAT Information		AL= Sectors per cluster DS:BX=FAT Identification bytes CX= Physical sector size DX= Default number of clusters of drives
1C	Take any drive FAT Information	DL= Drive letter	ditto
21	random block read	DS:DX=FCB The first address	AL=00 Read successfully   =01 End of file   =02 out of buffer   =03 The buffer is not full
22	Write at random	DS:DX=FCB The first address	AL=00 Write a successful   =01 The dish is full   =02 out of buffer
23	Determine file size	DS:DX=FCB The first address	AL=00 success ( Fill in the length of the document FCB) AL=FF Not found
24	Set random record number	DS:DX=FCB The first address
25	Set interrupt vector	DS:DX= Interrupt vector AL= Interrupt type number
26	Set up block prefixes	DX= New segment prefix
27	Read in random chunks	DS:DX=FCB The first address CX= Record number	AL=00 Read successfully   =01 End of file   =02 The buffer is too small , End of transmission   =03 The buffer is not full
28	Write in random blocks	DS:DX=FCB The first address CX= Record number	AL=00 Write a successful   =01 The dish is full   =02 out of buffer
29	Analysis file name	ES:DI=FCB The first address DS:SI=ASCIIZ strand AL= Control analysis flags	AL=00 Standard document   =01 A polysemous document   =02 Illegal drive letter
2A	Take the date		CX= year DH:DL= month : Japan ( Binary system )
2B	Setting date	CX:DH:DL= year : month : Japan	AL=00 success   =FF Invalid
2C	Take the time		CH:CL= when : branch DH:DL= second :1/100 second
2D	Setup time	CH:CL= when : branch DH:DL= second :1/100 second	AL=00 success   =FF Invalid
2E	Set the disk auto read / write flag	AL=00 Turn off the sign AL=01 Open the sign
2F	Take the first address of the disk buffer		ES:BX= Buffer first address
30	take DOS Version number		AH= Issue No ,AL= edition
31	End and stay	AL= Return code DX= Resident area size
33	Ctrl-Break testing	AL=00 Take the State   =01 Set state (DL) DL=00 Turn off detection   =01 Turn on detection	DL=00 close Ctrl-Break testing   =01 open Ctrl-Break testing
35	Take the interrupt vector	AL= Interrupt type	ES:BX= Interrupt vector
36	Get free disk space	DL= Drive letter   0= default ,1=A,2=B,...	success :AX= Sectors per cluster      BX= Number of effective clusters      CX= Bytes per sector      DX= The total number of clusters Failure :AX=FFFF
38	Set up / Take country information	DS:DX= First address of information area	BX= Country code ( Prefix code of international telephone ) AX= Error code
39	Create subdirectories (MKDIR)	DS:DX=ASCIIZ String address	AX= Error code
3A	delete a sub dir （RMDIR）	DS:DX=ASCIIZ String address	AX= Error code
3B	Change the current directory (CHDIR)	DS:DX=ASCIIZ String address	AX= Error code
3C	create file	DS:DX=ASCIIZ String address CX= File attribute	success :AX= Document code error :AX= Error code
3D	Open file	DS:DX=ASCIIZ String address AL=0 read   =1 Write   =3 read / Write	success :AX= Document code error :AX= Error code
3E	Close file	BX= Document code	Failure :AX= Error code
3F	Read a document or device	DS:DX= Data buffer address BX= Document code CX= Number of bytes read	Read successfully :   AX= The actual number of bytes read   AX=0 It's at the end of the file Read wrong :AX= Error code
40	Write a document or device	DS:DX= Data buffer address BX= Document code CX= Bytes written	Write a successful :   AX= The number of bytes actually written Write wrong :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= Document code CX:DX= Displacement AL= Mobile mode (0: Absolute displacement from header ,1: Relative movement from current position ,2: Absolute displacement from end of file )	success :DX:AX= New file pointer location error :AX= Error code
43	Set up / Take file properties	DS:DX=ASCIIZ String address AL=0 Take file properties AL=1 Set file properties CX= File attribute	success :CX= File attribute Failure :CX= Error code
44	Device file I/O control	BX= Document code AL=0 Take the State   =1 Set state DX   =2 Reading data   =3 Writing data   =6 Take the input status   =7 Take the output state	DX= Equipment information
45	Copy file code	BX= Document code 1	success :AX= Document code 2 Failure :AX= Error code
46	Manual copy of document code	BX= Document code 1 CX= Document code 2	Failure :AX= Error code
47	Take the path name of the current directory	DL= Drive letter DS:SI=ASCIIZ String address	(DS:SI)=ASCIIZ strand Failure :AX= Error code
48	Allocate memory space	BX= Request memory capacity	success :AX= Allocate memory first Failure :BX= Maximum available memory
49	Free up content space	ES= Memory starting segment address	Failure :AX= Error code
4A	Adjust allocated storage blocks	ES= Original memory starting address BX= Re application capacity	Failure :BX= Maximum available space      AX= Error code
4B	assembly / Execution procedure	DS:DX=ASCIIZ String address ES:BX= First address of parameter area AL=0 Load execution AL=3 Load does not execute	Failure :AX= Error code
4C	End with return code	AL= Return code
4D	Take the return code		AX= Return code
4E	Find the first matching file	DS:DX=ASCIIZ String address CX= attribute	AX= Error code (02,18)
4F	Find the next matching file	DS:DX=ASCIIZ String address ( File name with ? or *)	AX= Error code (18)
54	Automatic read / write mark for disk Retrieval		AL= Current flag value
56	Change the name of the document	DS:DX=ASCIIZ strand ( used ) ES:DI=ASCIIZ strand ( new )	AX= Error code (03,05,17)
57	Set up / Take the document date and time	BX= Document code AL=0 Read AL=1 Set up (DX:CX)	DX:CX= Date and time Failure :AX= Error code
58	take / Set allocation policy code	AL=0 Take the code AL=1 Set code (BX)	success :AX= Strategy code Failure :AX= Error code
59	Take the extended error code		AX= Extended error code BH= Wrong type BL= Recommended operation CH= Wrong place
5A	Create temporary documents	CX= File attribute DS:DX=ASCIIZ String address	success :AX= Document code Failure :AX= Error code
5B	Create new document	CX= File attribute DS:DX=ASCIIZ String address	success :AX= Document code Failure :AX= Error code
5C	Control file access	AL=00 The blockade   =01 Turn on BX= Document code CX:DX= File displacement SI:DI= file length	Failure :AX= Error code
62	Take program segment prefix		BX=PSP Address