flat assembler - Loop

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dieboy

Joined: 30 Dec 2004
Posts: 41
Location: Poland, Elblag

dieboy 15 Jul 2005, 17:02

Hi!
I'm gonna create a loop.

Code:

mov cx, 10
abc:
            invoke MessageBox, 0, 0, 0, 0
            loop abc

This code works if I compiled it under dos, but when under windows - i have a loop, that never end! What's going on?

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15 Jul 2005, 17:02

r22

Joined: 27 Dec 2004
Posts: 805

r22 15 Jul 2005, 17:17

TWO POSSIBLE ISSUES
1)The API function your calling changes the value of ECX.
2)You are only setting CX to 10 the upper half of ECX could still have a value in it (in which case the loop would seem to never end).

FIX

Code:

mov ecx,10
abc:
push ecx
invoke MessageBox, 0, 0, 0, 0
pop ecx
loop abc

OR
replace ecx with a register that isn't changed by API function like ebp, ebx, esi, edi.

15 Jul 2005, 17:17

T&K(r)

Joined: 10 Jul 2005
Posts: 18
Location: Poland - > Krakow

T&K(r) 15 Jul 2005, 17:24

I Tested this and i have the same problem even if i changeing registers (toECX) or pushing and poping.

The best will be if you write something like that :

Code:

mov cx, 10
abc:
            invoke MessageBox, 0, 0, 0, 0
            dec cx
            jcxz abc_end
            jmp abc
abc_end:

This is working - i checked it.

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15 Jul 2005, 17:24

dieboy

Joined: 30 Dec 2004
Posts: 41
Location: Poland, Elblag

dieboy 15 Jul 2005, 17:52

r22 wrote:

Code:

mov ecx,10
abc:
push ecx
invoke MessageBox, 0, 0, 0, 0
pop ecx
loop abc

I tested it, too.. And, unfortunetly, i have to remove loop from my code. Thanks for help.

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15 Jul 2005, 17:52

r22

Joined: 27 Dec 2004
Posts: 805

r22 15 Jul 2005, 22:16

--------------
mov ecx,10
.abc:
push ecx
push 0
push 0
push 0
push 0
call [MessageBox]
pop ecx
loop .abc ;dec ECX; JNZ .abc
-------------------
Works on my box (winxp sp2).
Maybe because your label isn't a local one (shouldnt affect it but who knows).

All the LOOP opcode does is
DEC ecx
JNZ labal
In most cases the above runs faster.

When you call an API like MessageBox, it doesn't save the states of EAX, ECX or EDX so you have to do it manually or use a register that IS preserved over API calls like EBX,EBP,ESI,orEDI.

15 Jul 2005, 22:16

Reverend

Joined: 24 Aug 2004
Posts: 408
Location: Poland

Reverend 15 Jul 2005, 23:18

T&K(r) wrote:

The best will be if you write something like that :

Code:

mov cx, 10
abc:
            invoke MessageBox, 0, 0, 0, 0
            dec cx
            jcxz abc_end
            jmp abc
abc_end:

This is working - i checked it.

Why do you use 'jcxz' opcode? After 'dec ecx' if ecx was 1 a ZF flag is signed (as ecx will have value 0), so you can use just 'jz'. And in this case even, you can omit defining another label 'abc_end', because you can use 'jnz' and jump back. Even though the loop is bad, because MessageBox changes ecx, and you do not preserve it anyway. I'll suggest this:

Code:

mov ecx, 10
abc:
            push ecx
            invoke MessageBox, 0, 0, 0, 0
            pop ecx
            dec ecx
            jnz abc

P.S: 'dec ecx\ jz XXXXXX' does the same as 'loop XXXXXX'. Loop is smaller in bytes but also slower

15 Jul 2005, 23:18

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