x86 32 Bit Assembly question

I am currently in the process of learning assembly and I was playing around with if statements. My current code is as follows.

write:
mov eax, 0x4     
sub esp, 4     开发者_如何学编程  
int 0x80         

main:
    ; The message has already been pushed to the stack
mov eax, 4
inc eax
cmp eax, 5
je write  

If I put the ret at the end of write: then I get a bus error 10 and if I do not I get an infinite loop resulting in a segmentation error. What should I do to make this work?

Use the call instruction instead of je to get into write. ret expects the return address to be on the stack, but it won't get pushed if you use a jump to get there! You're going to have to put esp back to whatever it was when you entered the function, too. Here's a best-guess example based on your code:

write:
  mov eax, 0x4
  sub esp, 4       
  int 0x80
  add esp, 4
  ret

main:  ; The message has already been pushed to the stack
  mov eax, 4
  inc eax
  cmp eax, 5
  jne dontwrite  ; skip calling 'write' if eax != 5
  call write
dontwrite:
       ; the rest of the program goes here

Try this instead. No need to call a procedure in your example.

main:  ; The message has already been pushed to the stack
  mov eax, 4
  inc eax
  cmp eax, 5
  jne dontwrite   ; Skip Write

  ; Write
  mov eax, 0x4
  sub esp, 4
  int 0x80

dontwrite:
       ; the rest of the program goes here