Question

I have a problem, that i cant figure out:

In assembly language, write a function that receives a pointer to an array of integers and the size of this array, and changes the array by reversing its elements without copying the array to stack. Use the dedicated instructions and registers to work with arrays (esi, edi; lodsb, stosb, cld, std).

Example: 1 2 3 4 5 -> 5 4 3 2 1

Anyone have any suggestions?

Answer 1

Reversing an array with lodsb and stosb requires cld and std for every element (because one of the pointers needs to increment and the other needs to decrement), or alternatively, you can just forget cld and std and just cancel the incorrect increment (or decrement) of the other pointer by subtracting 2 (or adding 2) to it after each element.

Anyway, using lodsb and stosb in this case makes things unnecessarily complicated in my opinion. I’d use something like this:

    mov esi,start_address
    mov edi,end_address

    mov ecx,edi
    sub ecx,esi

x1: test ecx,ecx
    jz @ready

    mov al,[esi]
    xchg al,[edi]
    mov [esi],al
    inc esi
    dec edi
    dec ecx
    jmp x1

@ready: