Question

I’m working in Visual Studio 2010 and here’s a little bit of code I’m having trouble with:

void main()
{
int x;
char c1[100]="cd";
char c2[100]="abcdef";
_asm {
    lea edx,c2
    lea ecx,c1
    push edx
    push ecx
    call subsir
    add esp, 8   
    mov x, eax
}
}

char subsir (char *c1, char *c2) 
{
_asm {
mov ecx, c1
mov edx, c2
mov edi, 0
mov esi, 0
start: cmp [edx + edi], 0
je end
mov esi, 0
mov eax, [edx + edi]
cmp eax, [ecx + esi]
  .....

This is the first part of a program that’s meant to compare 2 words.

The problem I am having is that when I do

 mov eax, [edx + edi]

eax receives an adress and not the actual value.
I tested this by doing

 cmp eax, 97

which should activate a JE instruction (97 being the ASCII code for ‘a’) But it doesn’t. If I do

      cmp [edx + edi], 97

it does activate the JE jump.

I would be very grateful if you could tell me what am I doing wrong and how can I put the actual valuea in EAX instead of an adress.

Answer 1

No, it is actually reading the memory at [edx + edi]. Problem is that it is reading 32-bits at a time. Which is not okay, you are comparing string content. That requires comparing one byte at a time. Fix:

start: cmp byte ptr [edx + edi], 0
je end
mov esi, 0
mov al, [edx + edi]
cmp al, [ecx + esi]