I am having an issue with a MASM32 program. I am trying to get this code to work:
.DATA
alfa dword 9
.CODE
start:
MOV ecx, alfa
MOV eax, 0
L1: ADD eax, ecx
DEC ecx
LOOP L1
CALL WRITEINT
exit
END start
I have found that the problem is only within the loop, everything else works fine. Can some please help me get the loop to work correctly, it does not return a value and doesn't exit. Is this supposed to happen? Thank you all in advance!
You are decreasing ecx twice. The dec instruction will decrease it, and the loop instrucction also decreases it.
Either use
dec ecx
jne L1
or use
loop L1
The reason why it doesn't exit is the following
dec ecx -> ecx = 8
loop l1 -> ecx = 7
dec ecx -> ecx = 6
loop l1 -> ecx = 5
dec ecx -> ecx = 4
loop l1 -> ecx = 3
dec ecx -> ecx = 2
loop l1 -> ecx = 1
And here comes the catch!
dec ecx -> ecx = 0 no condition
loop l1 -> ecx = 0xffffffff Loop will not break because it breaks on 0
dec ecx -> ecx = 0xfffffffe
loop l1 -> ecx = 0xfffffffd
...
... and so on. You get the idea. If you had used a debugger, you would have seen this after only a few iterations.
Actually ecx will reach 0 in this roundtrip, but it takes LONG time. :)