Why doesn't my jump work?

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2

Joined: 26 Sep 2006
Posts: 92

2 21 Nov 2006, 10:09

I want it to print 'what ' to a file 4 times.

Code:

include 'win32ax.inc'

IO:



invoke CreateFile,f,GENERIC_WRITE,0,0,CREATE_ALWAYS,FILE_ATTRIBUTE_NORMAL,0
mov [h],eax

xor ecx,ecx

p0:

invoke WriteFile,[h],b,6,r,0

inc ecx

cmp ecx,4
jb p0 ;this should jump back to p0: but it isn't!

invoke CloseHandle,[h]
invoke MessageBox,HWND_DESKTOP,"I want the string written four times!","problem",MB_OK
invoke ExitProcess,0
.end IO;
f db '10binary.txt',0
b db 'What',13,10
h rd 1
r rd 1

It only does it once. I thought I did it right.

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Those who know binary and those who haven't met me.

21 Nov 2006, 10:09

arafel

Joined: 29 Aug 2006
Posts: 131
Location: Jerusalem, Israel

arafel 21 Nov 2006, 10:43

Windows API functions don't preserve registers other than ebx, esi, edi, and ebp. Therefore your ecx gets corrupted during the WriteFile call.

push ecx
invoke WriteFile, ...
pop ecx

21 Nov 2006, 10:43

2

Joined: 26 Sep 2006
Posts: 92

2 21 Nov 2006, 22:44

THANK YOU! I WILL write this down!
That's definitely what must be causing the problem.

21 Nov 2006, 22:44

vid
Verbosity in development

Joined: 05 Sep 2003
Posts: 7105
Location: Slovakia

vid 21 Nov 2006, 23:13

it's simpler to remember that winAPI trashes ECX and EDX Smile

21 Nov 2006, 23:13

LocoDelAssembly
Your code has a bug

Joined: 06 May 2005
Posts: 4624
Location: Argentina

LocoDelAssembly 21 Nov 2006, 23:32

And EAX even if the function does not return any value. Modifies EFLAGS too (except some bits like DF).

21 Nov 2006, 23:32

2

Joined: 26 Sep 2006
Posts: 92

2 22 Nov 2006, 02:52

Yeah<i wrote down that eax,ecx,and edx get messed with by Windows.
It's sad.

22 Nov 2006, 02:52

RedGhost

Joined: 18 May 2005
Posts: 443
Location: BC, Canada

RedGhost 22 Nov 2006, 03:42

2 wrote:

Yeah<i wrote down that eax,ecx,and edx get messed with by Windows.
It's sad.

You can't expect every api to preserve every register. cdecl and other formats aren't even so liberal in saving registers, and fastcall uses registers as arguments.

You can do

Code:

pushad
;....
popad

If you must save all GPR.

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22 Nov 2006, 03:42

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