Question

I am running a C program that calls an external assembly function. For academic purposes, I am trying to perform strcat. I pass the two strings to my assembly program as char * parameters. I push ebp to the stack and assign string1 and string2 to edx and ebx like so:

mov edx, [ebp+8]
mov ebx, [ebp+4]

Now the rest is as follows:

procStr1:
     cmp BYTE PTR [edx], 0
     jne readStr1
procStr2:
     cmp BYTE PTR [ebx], 0
     jne readStr2
     jmp bottom
readStr1:
    inc edx
    jmp procStr1
readStr2:
    mov BYTE PTR [edx], 'a'
    inc edx
    inc ebx
    jmp procStr2

bottom:
    inc edx
    mov BYTE PTR [edx], 0
    pop ebx
    pop edx
    pop ebp
    ret

I am simply testing to see if it works by adding a’s to the end of string1. If I enter ‘hi’ and ‘bye’ I expect to get hiaaa printed out by the C program (by printing out string1). Instead I get usually 13 a’s after string1, no matter how big string2 is. I would appreciate any input, it is really boggling my mind..

Answer 1

Did you do a:

push ebp
mov ebp, esp

at the top?

If so, your arguments are now found at:

mov edx, [ebp+8]
mov ebx, [ebp+Ch]  ; 0xC, not 4 -- C-language passes args right-to-left

Also,

bottom:
    inc edx       ; This inc should be removed -- edx already points one
                  ; byte beyond the ultimate copied byte.
    mov BYTE PTR [edx], 0