.model small
.286
.stack 256
.data
msgPrompt db "1>Add",0Dh,0Ah,"2>Subtract",0Dh,0Ah,'$'
.code
Template:
add ax, 1
sub ax, 1
SketchSpace:
nop
db 01h
db 00h
ret
main proc
mov ax, seg msgPrompt
mov ds, ax
mov ax, 0900h
lea dx, msgPrompt
int 21h
mov ax, 0800h
int 21h
mov al, 61h
cmp al, 61h
jnz setSubtract
setAdd:
mov byte ptr [SketchSpace], 05h
jmp setInstruction
setSubtract:
mov byte ptr [SketchSpace], 02Dh
setInstruction:
mov ax, 04Dh
call SketchSpace
; now display the result!
mov ah, 02h
mov dl, al
int 21h
mov ax, 4c00h
int 21h
main endp
end main
Self-modifying code
Author
I am trying to make self-modifying code with x86 assembly. I am running the following thru Virtual PC MS-DOS 6.22 machine.
The way I think my code should work, is that if the user presses '1', 'N' should be displayed ('M' + 1); whereas if the user presses '2', 'L' ('M' - 1) should be displayed.
When I run it thru the "debug" command, everything goes fine, however, run straight from the prompt, the output is not modified in any way (it simply displays 'M').
Any suggestions?
Thx in advance.
More likely then not your modifying the instruction after it's been fetched into the instruction queue so the processor only sees the version in the queue not the modified version, when you run the debugger the debugger code is being run in between the two instructions so it is only being loaded into the queue after it has been modified. Any way self modifying code on modern hardware is a really bad idea.
This topic is closed to new replies.
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