A loop question

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bolzano_1989

Joined: 06 Oct 2011
Posts: 4

bolzano_1989 07 Oct 2011, 05:44

I'm trying to create a loop in FASM.
Could you help me identify why the following code gives me an infinite loop and what I should do to fix this error?

Code:

include '%fasminc%/win32ax.inc'

.data
  endline DB 10,13,0
  numwritten DD ?
  inchar     DB ?
  outhandle  DD ?
  inhandle   DD ?
  numread    DD ?

.code

  start:
        invoke  AllocConsole
        invoke  GetStdHandle,STD_OUTPUT_HANDLE
        mov [outhandle],eax                   
        xor eax, eax
        mylabel:
                inc eax
                invoke  WriteConsole,[outhandle],"Hello",5,numwritten,0
                invoke  WriteConsole,[outhandle],endline,1,numwritten,0
                cmp eax,10
                jne mylabel
        invoke  GetStdHandle,STD_INPUT_HANDLE
        mov [inhandle],eax
        invoke  ReadConsole,[inhandle],inchar,1,numread,0
        invoke  ExitProcess,0
.end start

07 Oct 2011, 05:44

goldenspider

Joined: 16 May 2011
Posts: 38

goldenspider 07 Oct 2011, 06:06

Aha , Because EAX will be change. So,

xor eax, eax
mylabel:
inc eax
push eax
invoke WriteConsole,[outhandle],"Hello",5,numwritten,0
invoke WriteConsole,[outhandle],endline,1,numwritten,0
pop eax
cmp eax,10
jne mylabel

07 Oct 2011, 06:06

Madis731

Joined: 25 Sep 2003
Posts: 2139
Location: Estonia

Madis731 07 Oct 2011, 06:08

invoke and usually every other code uses eax as return value therefore its trashed every time.
You have two options:
1) use a different register (esi, edi, ebx) that should be preserved
2) after inc eax use push eax and just before cmp eax,10 use pop eax

The second method is the safest and you can even use eax as a scratchpad between the push/pop. There are other methods, like using a memory area to handle the counting. Use your imagination.

07 Oct 2011, 06:08

ManOfSteel

Joined: 02 Feb 2005
Posts: 1154

ManOfSteel 07 Oct 2011, 06:09

You should push eax after incrementing it and pop it back before comparing it to 10. You can't trust the API to save registers.

EDIT: geez there's an invasion in progress. Smile

07 Oct 2011, 06:09

bolzano_1989

Joined: 06 Oct 2011
Posts: 4

bolzano_1989 07 Oct 2011, 07:12

Thank you ManOfSteel, Madis731, goldenspider for your clear explanation and your fix Smile

07 Oct 2011, 07:12

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