Programming Forums > Application Development > Assembly

liquidsilver · Mar 11th, 2005, 1:37 AM

I've just recently started with SMC (Self Modifying Code) and I'm having some problems:

.model tiny
.stack
.data
  key db 01h
.code
org 0100h

mov cx,3
StrLoop:
  dec cx
  mov di,(offset fix)
  add di,cx
  mov al,[di]
  xor al,key
  mov [di],al
  jcxz OutLoop
  jmp StrLoop
OutLoop:

fix db 0B9h, 036h, 003h ;mov ax,0237h
  ret
end main

This is my SMC.COM source. This works, but when I convert it to a EXE then it goes haywire. The correct data seems to not get targeted. I use MASM, please help.

Ooble · Mar 11th, 2005, 10:38 AM

I hardly know any ASM, but IIRC, you may have to change .model tiny to .model small or something.

liquidsilver · Mar 11th, 2005, 2:13 PM

Maybe you misunderstood me. This is the code that works, when I change it from tiny to small model then it doesn't work.

Ooble · Mar 11th, 2005, 3:26 PM

I see. In that case, I can't help you - distinct lack of knowledge here.

liquidsilver · Mar 12th, 2005, 12:44 PM

Thanx anyway for trying. Anyone else?

Madman · Mar 15th, 2005, 10:53 AM

Ok, I havent played with asm in a while but here's some key concepts to
remember. DOS or windows for that matter always loads a .com file so that the code segment and data segment is the same causing .com files never to
use more than 64kb without using special tricks

Things a bit different in a .exe file and ds and cs isn't nessecary the same
anymore. I myself havent really tried self modifying code but I can clearly see
how your prog works.

What probably happening is that your [di] not pointing to the right place
in memory from the start and then the wrong bytes get modified somewhere
else in memory.

Just make sure your exe starts like this :

(Toggle Plain Text)

dosseg

BUFSIZE EQU 1024

.stack
.data
 key db 01h
.code
main proc                
 mov ax,@data         ; load ds,es 
 mov ds,ax
 mov es,ax

 mov cx,3
 StrLoop:
  dec cx
  mov di,(offset fix)
  add di,cx
  mov al,[di]
  xor al,key
  mov [di],al
  jcxz OutLoop
  jmp StrLoop
OutLoop:
fix db 0B9h, 036h, 003h ;mov ax,0237h 

  mov ax,4C00h  ; exit to dos
  int 21h
 main endp

; if it still doesn't work then rather try define key here
; key db 01h
; and then set your ds = cs instead making it behave more like a .com file :
; mov ax,cs
; mov ds,ax 

end main

Hope this helps.

Anyway if someone else want to correct me on something here, please
do

( I'm still a bit rusty after not touching asm for 7 years )

Actually did some fun stuff back then like playing around with vga registers
and doing 3d stuff in asm ( definitely needs that putpixel routine as
fast as possible )

liquidsilver · Mar 16th, 2005, 1:54 PM

Thanx, I'll try when I'm free.

Mar 11th, 2005, 1:37 AM	#1
liquidsilver Newbie Join Date: Mar 2005 Location: South Africa Posts: 21 Rep Power: 0	Self Modifying Code I've just recently started with SMC (Self Modifying Code) and I'm having some problems: (Toggle Plain Text) .model tiny .stack .data key db 01h .code org 0100h mov cx,3 StrLoop: dec cx mov di,(offset fix) add di,cx mov al,[di] xor al,key mov [di],al jcxz OutLoop jmp StrLoop OutLoop: fix db 0B9h, 036h, 003h ;mov ax,0237h ret end main .model tiny .stack .data key db 01h .code org 0100h mov cx,3 StrLoop: dec cx mov di,(offset fix) add di,cx mov al,[di] xor al,key mov [di],al jcxz OutLoop jmp StrLoop OutLoop: fix db 0B9h, 036h, 003h ;mov ax,0237h ret end main This is my SMC.COM source. This works, but when I convert it to a EXE then it goes haywire. The correct data seems to not get targeted. I use MASM, please help. __________________ Small is beautiful

Mar 11th, 2005, 10:38 AM	#2
Ooble I eat cake for breakfast. Join Date: Jul 2004 Location: In my box. Posts: 4,434 Rep Power: 9	I hardly know any ASM, but IIRC, you may have to change .model tiny to .model small or something. __________________ Me :: You :: Them

Mar 11th, 2005, 2:13 PM	#3
liquidsilver Newbie Join Date: Mar 2005 Location: South Africa Posts: 21 Rep Power: 0	Maybe you misunderstood me. This is the code that works, when I change it from tiny to small model then it doesn't work. __________________ Small is beautiful

Mar 11th, 2005, 3:26 PM	#4
Ooble I eat cake for breakfast. Join Date: Jul 2004 Location: In my box. Posts: 4,434 Rep Power: 9	I see. In that case, I can't help you - distinct lack of knowledge here. __________________ Me :: You :: Them

Mar 12th, 2005, 12:44 PM	#5
liquidsilver Newbie Join Date: Mar 2005 Location: South Africa Posts: 21 Rep Power: 0	Thanx anyway for trying. Anyone else? __________________ Small is beautiful Last edited by liquidsilver; Mar 12th, 2005 at 1:10 PM.

Mar 15th, 2005, 10:53 AM	#6
Madman Newbie Join Date: Mar 2005 Posts: 4 Rep Power: 0	Ok, I havent played with asm in a while but here's some key concepts to remember. DOS or windows for that matter always loads a .com file so that the code segment and data segment is the same causing .com files never to use more than 64kb without using special tricks Things a bit different in a .exe file and ds and cs isn't nessecary the same anymore. I myself havent really tried self modifying code but I can clearly see how your prog works. What probably happening is that your [di] not pointing to the right place in memory from the start and then the wrong bytes get modified somewhere else in memory. Just make sure your exe starts like this : (Toggle Plain Text) dosseg BUFSIZE EQU 1024 .stack .data key db 01h .code main proc mov ax,@data ; load ds,es mov ds,ax mov es,ax mov cx,3 StrLoop: dec cx mov di,(offset fix) add di,cx mov al,[di] xor al,key mov [di],al jcxz OutLoop jmp StrLoop OutLoop: fix db 0B9h, 036h, 003h ;mov ax,0237h mov ax,4C00h ; exit to dos int 21h main endp ; if it still doesn't work then rather try define key here ; key db 01h ; and then set your ds = cs instead making it behave more like a .com file : ; mov ax,cs ; mov ds,ax end main dosseg BUFSIZE EQU 1024 .stack .data key db 01h .code main proc mov ax,@data ; load ds,es mov ds,ax mov es,ax mov cx,3 StrLoop: dec cx mov di,(offset fix) add di,cx mov al,[di] xor al,key mov [di],al jcxz OutLoop jmp StrLoop OutLoop: fix db 0B9h, 036h, 003h ;mov ax,0237h mov ax,4C00h ; exit to dos int 21h main endp ; if it still doesn't work then rather try define key here ; key db 01h ; and then set your ds = cs instead making it behave more like a .com file : ; mov ax,cs ; mov ds,ax end main Hope this helps. Anyway if someone else want to correct me on something here, please do ( I'm still a bit rusty after not touching asm for 7 years ) Actually did some fun stuff back then like playing around with vga registers and doing 3d stuff in asm ( definitely needs that putpixel routine as fast as possible ) Last edited by Madman; Mar 15th, 2005 at 11:14 AM.

Mar 16th, 2005, 1:54 PM	#7
liquidsilver Newbie Join Date: Mar 2005 Location: South Africa Posts: 21 Rep Power: 0	Thanx, I'll try when I'm free. __________________ Small is beautiful

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