CODEWITHSUNDEEP
assemblydos
Adam
Adam

Reputation: 361

Writing to a file in Assembly x86

I need to write data to a txt file in assembly, without erasing the current data (that is, to add data, not to replace it). Right now whenever I write I replace everything currently in the file.

Here is the code I am using:

writeFile Proc  
            push ax bx cx dx

            ;open file
            mov dx, offset fileName
            mov cx, 0
            mov ah,3ch
            int 21h
            mov filePointer, ax

            ;write to file
            mov dx, offset dataToWrite ;data
            mov cx, DATA_TO_WRITE_LEN  ;size of data
            mov bx, filePointer
            mov ah, 40h
            int 21h

            ;close file
            mov bx, filePointer
            mov ah, 3eh
            int 21h 
            pop dx cx bx ax
            ret
writeFile endp

Is there any way to do this? Thanks:)

Upvotes: 0

Views: 3539

Answers (1)

fvu
fvu

Reputation: 32953

You are calling

        mov ah,3ch
        int 21h

Ie, create a file. If you use this call on an existing file that file will be truncated. You should use

        mov ah,3dh
        int 21h

with the appropriate flags in AL to open the file.

Upvotes: 3

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