textfiles/uploads/fassembly.txt

                                             
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File Operations in Assembly Language
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by Jakash3
February 20, 2010

All of which can be and was done using debug.exe

The following all use the MS-DOS API (int 21) to do these file operations.

(MKDIR) Create Folder
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C:\>debug
-a 0100
xxxx:0100 mov ah,39                ;Create folder func.
xxxx:0102 mov dx,010c              ;Points to pathname
xxxx:0105 int 21                   ;Do it
xxxx:0107 mov ax,4c00              ;AH=Exit, AL=Errorlevel
xxxx:010A int 21                   ;Do it
xxxx:010C db "C:\NewFolder1",00    ;Null terminated pathname
xxxx:011A
-n makef.com
-r cx
CX 0000
:001a
-w
Writing 001A bytes.
-q
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Remove Folder
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C:\>debug
-a 0100
xxxx:0100 mov ah,3a               ;Remove folder func.
xxxx:0102 mov dx,010C             ;points to pathname
xxxx:0105 int 21                  ;Do it
xxxx:0107 mov ax,4c00             ;Exit func., AL=Errorlevel
xxxx:010A int 21                  ;Do it
xxxx:010C db "C:\OldFolder1",00   ;Null terminated pathname
xxxx:011A
-n rmvf.com
-r cx
CX 0000
:001a
-w
Writing 001A bytes.
-q
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Create/Truncate File
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C:\>debug
-a 0100
xxxx:0100 mov ah,3c     ;Create file func.
xxxx:0102 mov cx,2      ;Archive file attribute
xxxx:0105 mov dx,010f   ;Points to filename
xxxx:0108 int 21        ;Do it
xxxx:010A mov ax,4c00
xxxx:010D int 21
xxxx:010F db "C:\Users\jakash3\newfile1.txt",00
-n mkf.com
-r cx
CX 0000
:002D
-w
Writing 001A bytes.
-q
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Note:
File attributes are specified by the hex value of a bit array.
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  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
| ? | ? | ? | 0 | 0 | ? | 0 | 0 |
+---+---+---+---+---+---+---+---+
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0 - Read Only
1 - Hidden
2 - System
3 - Volume label (ignored)
4 - Reserved, must be zero (directory)
5 - Archive
6 - Unused
7 - Unused

A Read-Only Archive file: 10000100
In hex that would be: 84
So 84 would be the value for CX in the 3C function of Int 21.

Open File
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C:\>debug
-a 0100
xxxx:0100 jmp 011c
xxxx:0102 db "C:\Users\jakash3\data.txt",00
xxxx:011C mov ah,3d     ;Open file func.
xxxx:011E mov al,40     ;Bit field for read/write access
xxxx:0120 mov dx,0102   ;Points to filename
xxxx:0123 int 21        ;Do it
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File access mode bit fields for this function:
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  0   1   2   3   4   5   6   7
+---+---+---+---+---+---+---+---+
| ? | ? | ? | 0 | 0 | 0 | 0 | 0 |
+---+---+---+---+---+---+---+---+
----------------------------------------
000 - Read Only (mov cl,0)
001 - Write Only (mov cl,20)
010 - Read/Write (mov cl,40)

After the interrupt call, AX should contain the file handle if no errors occurred.

Close File
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C:\>debug
-a 0100
xxxx:0100 jmp 011c
xxxx:0102 db "C:\Users\jakash3\data.txt",00
xxxx:011C mov ah,3d     ;Open file function
xxxx:011E mov al,40     ;Bit field for read/write access
xxxx:0120 mov dx,0102   ;Points to filename
xxxx:0123 int 21        ;Do it
xxxx:0125 mov bx,ax     ;Move file handle to BX for 3e function -------+
xxxx:0127 mov ax,3e00   ;Close file function <------------------------/
xxxx:012A int 21        ;Do it
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Read from file
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C:\>debug
xxxx:0100 jmp 012b
xxxx:0102 db "C:\Users\jakash3\ab.txt",00 ;filename to read from
xxxx:011A db 00,00,00,00,00,00,00,00,00,00,00,00,00,00,00,00,24 ;'$' terminated 10 byte buffer
xxxx:012B mov ah,3d     ;Open file function
xxxx:012D mov al,00     ;Read-Only file access
xxxx:012F mov dx,0102   ;Points to filename
xxxx:0132 int 21        ;Do it
xxxx:0134 mov bx,ax     ;Move retrieved file handle into BX
xxxx:0136 mov ax,3f00   ;Read from file function
xxxx:0139 mov cx,10     ;Read the first 0x10 bytes of file
xxxx:013C mov dx,011a   ;Address of buffer to store bytes read
xxxx:013F int 21        ;Do it
xxxx:0141 mov ax,0900   ;Print string function
xxxx:0144 mov dx,011a   ;Points to '$' terminated string to print
xxxx:0147 int 21        ;Do it
xxxx:0149 mov ax,4c00   ;Exit function (AL=Errorlevel to return)
xxxx:014C int 21        ;Do it
xxxx:014E
-rcx
CX 0000
:004e
-n readfile.com
-w
Writing 0004E bytes
-q
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Write to file:
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C:\>debug
xxxx:0100 jmp 0101
xxxx:0102 db "C:\Users\jakash3\ab.txt",00
xxxx:011A db "hello world!",00
xxxx:0127 mov ah,3d   ;Open file function
xxxx:0129 mov al,20   ;Write-Only file access
xxxx:012B mov dx,0102 ;Points to filename
xxxx:012E int 21      ;Do it
xxxx:0130 mov bx,ax   ;Move retrieved file handle into BX
xxxx:0132 mov ax,4000 ;Write to File function
xxxx:0135 mov dx,011a ;Points to data to write
xxxx:0138 mov cx,0d   ;Number of bytes to write
xxxx:013A int 21      ;Do it
xxxx:013C mov ax,4c00 ;Exit function
xxxx:013F int 21      ;Do it
xxxx:0140
-r cx
CX 0000
:0040
-n wfile.com
-w
Writing 0040 bytes.
-q
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Delete file:
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C:\>debug
-a 0100
xxxx:0100 mov ah,40   ;Delete file function
xxxx:0102 mov dx,010c ;Points to filename
xxxx:0105 int 21      ;Do it
xxxx:0107 mov ax,4c00 ;Exit function
xxxx:010A int 21      ;Do it
xxxx:010C db "C:\Users\jakash3\ab.txt",00
xxxx:0124
-r cx
CX 0000
:0024
-n delf.com
-w
Writing 0024 bytes.
-q
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