At my university, I was given the following task:
Compare character by character the contents of two text files. Write to a new text file only those characters from the first file and their position number in the file that do not match the corresponding characters from the second. Using the TASM assembler.
Well, I wrote the program, ran it through the Turbo assembler, but nothing is written to the output.txt file. All files are in the same directory.
The contents of file1.txt are abobabiba1, the contents of file2.txt are abobapipa1.
In theory, something like this should be written to a file:
5 7
bb
.model large
.stack 1000h
.data
infile1 db "file1.txt", 0
infile2 db "file2.txt", 0
outfile db "output.txt", 0
buffer1 db 1000h dup(?)
buffer2 db 1000h dup(?)
outfile_char db 0
position db 0
end_of_file db 0
.code
main proc
; open the first input file
mov ah, 3dh
mov al, 0
lea dx, infile1
int 21h
mov bx, ax ; bx contains the first file descriptor
; open second input file
mov ah, 3dh
mov al, 0
lea dx, infile2
int 21h
mov cx, ax ; cx contains the second file descriptor
; create output file
mov ah, 3ch
mov al, 0
lea dx, outfile
int 21h
mov dx, ax ; dx contains the output file descriptor
; read the contents of the first file into buffer1
mov ah, 3fh
mov bx, ax
lea dx, buffer1
mov cx, 1000h
int 21h
; read the contents of the first file into buffer2
mov ah, 3fh
mov bx, cx
lea dx, buffer2
mov cx, 1000h
int 21h
; compare the contents of the buffers character by character
mov si, offset buffer1
mov di, offset buffer2
mov cx, 1000h
mov position, 0
loop_start:
mov al, [si]
mov bl, [di]
cmp al, bl
je skip
; set the file pointer to the beginning of the file
mov ah, 42h
mov al, 0
mov bx, dx
int 21h
; write information to the output file
mov ah, 02h
mov dl, al
mov ah, 40h
mov bx, dx
mov cx, 1
mov dx, offset outfile_char
int 21h
mov ah, 02h
mov dl, position
add dl, '0'
mov ah, 40h
mov bx, dx
mov cx, 1
mov dx, offset outfile_char
int 21h
mov ah, 40h
mov bx, dx
mov cx, 1000h
lea dx, buffer1
int 21h
mov ah, 40h
mov bx, dx
mov cx, 1000h
lea dx, buffer2
int 21h
skip:
; increment the position and move on to the next character
inc si
inc di
inc position
; check for end of file
cmp position, 10h
jne loop_start
; close the output file
mov ah, 3eh
mov bx,dx
int 21h
; close the second input file
mov ah, 3eh
mov bx, cx
int 21h
; close the first input file
mov ah, 3eh
mov bx, ax
int 21h
; we complete the program
mov ah, 4ch
int 21h
main endp
end main
Are you sure that DS points to your .data section? Don't remember if .code takes care of that...
The most important error in your program is that you are using several registers for multiple purposes each, and that you do this without taking precautions like preserving their pre-existing values on the stack.
Another approach would be to store some data in memory-based variables. You definitely should choose this for your file handles, like in next code:
; open the first input file
mov ah, 3dh
mov al, 0
lea dx, infile1
int 21h
jc SomeError
mov handle1, ax ; the first file descriptor
; open second input file
mov ah, 3dh
mov al, 0
lea dx, infile2
int 21h
jc SomeError
mov handle2, ax ; the second file descriptor
; create output file
mov ah, 3ch
mov al, 0 <<<< THIS IS AN ERROR: NEEDS TO BE CX=0
lea dx, outfile
int 21h
jc SomeError
mov handle3, ax ; the output file descriptor
; read the contents of the first file into buffer1
mov ah, 3fh
mov bx, handle1
mov cx, 16
lea dx, buffer1
int 21h
jc SomeError
; read the contents of the first file into buffer2
mov ah, 3fh
mov bx, handle2
mov cx, 16
lea dx, buffer2
int 21h
jc SomeError
...
; close the output file
mov ah, 3eh
mov bx, handle3
int 21h
; close the second input file
mov ah, 3eh
mov bx, handle2
int 21h
; close the first input file
mov ah, 3eh
mov bx, handle1
int 21h
; set the file pointer to the beginning of the file mov ah, 42h mov al, 0 mov bx, dx int 21h
In the middle part of the program where you compare buffers, you are also trying to reset the output filepointer. I say 'trying' because you don't setup the parameters correctly! But why would you need to reset anything? Right after a file is created, the filepointer is already at the start. Moreover resetting the filepointer with each find will keep overwriting the previous results!
mov ah, 02h mov dl, al ... mov ah, 02h mov dl, position add dl, '0'
In the part that 'writes information to the output file', you are mixing character output to the screen (but incomplete) with buffer output to a file. If the character output to the screen was there for testing purposes, then document this with suitable comments in the text, else remove these lines as they are very disturbing.
This is how you would output the character. Notice that the character is at the address SI:
; write information to the output file
mov ah, 40h
mov bx, handle3
mov cx, 1
mov dx, si
int 21h
jc SomeError
And here you can learn how to write the position:
mov al, position ; [0,9] will work fine for your input files
add al, '0'
mov outfile_char, al
mov ah, 40h
mov bx, handle3
mov cx, 1
mov dx, offset outfile_char
int 21h
jc SomeError
Processing input files 'abobabiba1' and 'abobapipa1', I would expect a 4-byte output file 'b5b7'.