File Operations Procedures

File Operations in MASM (Microsoft Macro Assembler)

MASM provides several functions via DOS interrupts (primarily int 21h) for performing file operations such as opening, reading, writing, and closing files. These operations are handled via system calls that allow you to interact with files on disk. Below are examples of file operations in MASM, including file opening, reading, writing, and closing using procedures.

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1. Opening a File

To open a file in MASM, you use the DOS interrupt int 21h with function 3Dh, which opens a file and returns a file handle.

MASM Code: Opening a File

; open_file.asm
.model small
.stack 100h

.data
    filename db 'testfile.txt', 0  ; Filename to open
    handle dw 0                    ; File handle
    filemode db 'r', 0             ; File access mode ('r' for read, 'w' for write)
    errorMessage db 'Error opening file.$', 0

.code
main:
    mov ax, @data            ; Initialize data segment
    mov ds, ax

    ; Call OpenFile procedure
    call OpenFile

    ; Exit program
    mov ah, 4Ch
    int 21h

; Procedure to open a file
OpenFile proc
    ; Open file using DOS function 3Dh (open)
    lea dx, filename         ; Load address of filename
    mov al, 0                ; '0' = read mode, 'w' = write mode, 'a' = append mode
    int 21h                  ; DOS interrupt 21h, function 3Dh (open)
    jc ErrorOpeningFile      ; Jump if there was an error (carry flag set)
    
    ; Store the returned file handle
    mov [handle], ax

    ; File opened successfully, return
    ret

ErrorOpeningFile:
    ; Print error message if file could not be opened
    mov ah, 09h
    lea dx, errorMessage
    int 21h                  ; DOS interrupt 21h to print string
    ret

OpenFile endp
end main

Explanation:

• int 21h, function 3Dh: Used to open the file. The filename is passed in DX, and the mode is passed in AL.
• The file handle returned by the int 21h call is stored in AX.
• If an error occurs (file not found or invalid mode), the carry flag is set, and the program will jump to ErrorOpeningFile.

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2. Writing to a File

Writing to a file in MASM involves using DOS interrupt int 21h with function 40h. This function requires the file handle, a pointer to the data, and the number of bytes to write.

MASM Code: Writing to a File

; write_to_file.asm
.model small
.stack 100h

.data
    filename db 'testfile.txt', 0  ; File name
    textToWrite db 'Hello, MASM! Writing to a file.$'
    handle dw 0                    ; File handle
    errorMessage db 'Error writing to file.$', 0

.code
main:
    mov ax, @data            ; Initialize data segment
    mov ds, ax

    ; Open the file for writing
    call OpenFile

    ; Write to the file
    call WriteToFile

    ; Close the file
    call CloseFile

    ; Exit program
    mov ah, 4Ch
    int 21h

; Procedure to open a file
OpenFile proc
    lea dx, filename         ; Load address of filename
    mov al, 'w'              ; 'w' = write mode
    int 21h                  ; DOS interrupt 21h, function 3Dh (open)
    jc ErrorOpeningFile      ; Jump if error

    mov [handle], ax         ; Store the returned file handle
    ret

ErrorOpeningFile:
    mov ah, 09h
    lea dx, errorMessage
    int 21h                  ; Print error message
    ret

OpenFile endp

; Procedure to write to a file
WriteToFile proc
    lea dx, textToWrite      ; Load address of text to write
    mov ah, 40h              ; DOS function 40h (write)
    mov bx, [handle]         ; Load the file handle into BX
    mov cx, 18               ; Number of bytes to write
    int 21h                  ; Call DOS interrupt
    jc ErrorWritingFile      ; Jump if there was an error

    ret

ErrorWritingFile:
    mov ah, 09h
    lea dx, errorMessage
    int 21h                  ; Print error message
    ret

WriteToFile endp

; Procedure to close the file
CloseFile proc
    mov ah, 3Eh              ; DOS function 3Eh (close file)
    mov bx, [handle]         ; Load file handle into BX
    int 21h                  ; Call DOS interrupt
    ret

CloseFile endp

end main

Explanation:

• OpenFile: Opens a file for writing using the 3Dh function (open file) with 'w' mode.
• WriteToFile: Writes data to the file using int 21h with function 40h. The file handle is passed through BX, the data to write is in DX, and the number of bytes to write is passed through CX.
• CloseFile: Closes the file using int 21h with function 3Eh.

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3. Reading from a File

To read from a file in MASM, you use DOS interrupt int 21h with function 3Fh. This function allows you to read data from a file into a buffer.

MASM Code: Reading from a File

; read_from_file.asm
.model small
.stack 100h

.data
    filename db 'testfile.txt', 0  ; File name to read from
    buffer db 100, 0               ; Buffer to store the data read
    handle dw 0                    ; File handle
    errorMessage db 'Error reading file.$', 0
    bytesRead dw 0                 ; Variable to store number of bytes read

.code
main:
    mov ax, @data            ; Initialize data segment
    mov ds, ax

    ; Open the file
    call OpenFile

    ; Read from the file
    call ReadFromFile

    ; Print the data read (optional)
    call PrintBuffer

    ; Close the file
    call CloseFile

    ; Exit program
    mov ah, 4Ch
    int 21h

; Procedure to open a file
OpenFile proc
    lea dx, filename         ; Load address of filename
    mov al, 'r'              ; 'r' = read mode
    int 21h                  ; DOS interrupt 21h, function 3Dh (open)
    jc ErrorOpeningFile      ; Jump if error

    mov [handle], ax         ; Store the returned file handle
    ret

ErrorOpeningFile:
    mov ah, 09h
    lea dx, errorMessage
    int 21h                  ; Print error message
    ret

OpenFile endp

; Procedure to read from a file
ReadFromFile proc
    lea dx, buffer           ; Load address of the buffer
    mov ah, 3Fh              ; DOS function 3Fh (read)
    mov bx, [handle]         ; Load the file handle into BX
    mov cx, 100              ; Number of bytes to read
    int 21h                  ; Call DOS interrupt

    mov [bytesRead], ax      ; Store the number of bytes read
    jc ErrorReadingFile      ; Jump if there was an error

    ret

ErrorReadingFile:
    mov ah, 09h
    lea dx, errorMessage
    int 21h                  ; Print error message
    ret

ReadFromFile endp

; Procedure to print the buffer (the data read)
PrintBuffer proc
    mov ah, 09h
    lea dx, buffer
    int 21h                  ; Call DOS interrupt to print string
    ret

PrintBuffer endp

; Procedure to close the file
CloseFile proc
    mov ah, 3Eh              ; DOS function 3Eh (close file)
    mov bx, [handle]         ; Load file handle into BX
    int 21h                  ; Call DOS interrupt
    ret

CloseFile endp

end main

Explanation:

• OpenFile: Opens the file in read mode ('r').
• ReadFromFile: Reads the contents of the file into the buffer using the int 21h function 3Fh. The number of bytes read is stored in the bytesRead variable.
• PrintBuffer: Prints the data from the buffer using int 21h, function 09h.
• CloseFile: Closes the file using int 21h, function 3Eh.

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4. File Operations Summary

In MASM, file operations can be done using DOS interrupts. Here’s a summary of the key functions for file manipulation:

• Opening a File: int 21h, function 3Dh
  • Mode (AL): `'