Question

I’m trying to make a program using NASM that takes input from command line arguments. Since string length is not provided, I’m trying to make a function to compute my own. Here is my attempt, which takes a pointer to a string in the ebx register, and returns the length of the string in ecx:

len:
    push ebx
    mov ecx,0
    dec ebx
    count:
        inc ecx
        inc ebx
        cmp ebx,0
        jnz count
    dec ecx
    pop ebx
    ret

My method is to go through the string, character by character, and check if it’s null. If it’s not, I increment ecx and go to the next character. I believe the problem is that cmp ebx,0 is incorrect for what I’m trying to do. How would I properly go about checking whether the character is null? Also, are there other things that I could be doing better?

Answer 1

You are comparing the value in ebx with 0 which is not what you want. The value in ebx is the address of a character in memory so it should be dereferenced like this:

cmp byte[ebx], 0

Also, the last push ebx should be pop ebx.