strrev.asm

; File: strrev.asm
; A subroutine called from C programs.
; Parameters: string A
; Result: String is reversed and returned.


    SECTION .text
    global strrev
_strrev: nop
strrev:
    push    ebp
    mov ebp, esp

    ; registers ebx,esi, and edi must be saved if used
    push ebx
    push edi

    xor esi, esi    
    xor eax, eax
    mov ecx, [ebp+8]    ; load the start of the array into ecx
    jecxz   end     ; jump if [ecx] is zero

mainLoop:
    add eax, 1      ; icn eax would work as well
    add     ecx, 1      
    mov dl, [ecx]  ; load ecx
    cmp dl, 0      ; compare with 0
    je  reverseLoop     ; if ecx is zero, we're done
    jmp mainLoop        ; if ecx isn't zero, keep looping


reverseLoop:
    mov ecx, [ebp + 8]  ; reload the start of the array into ecx
    mov esi, ecx    ; esi points to start of array
    add ecx, eax
    mov edi, ecx    
    dec edi ;edi points to end of array
    shr eax, 1  ;eax is the count
    jz  end ; if string is 0 or 1 chars long, jump to end

reverseLoop_1:

    mov cl, [esi]   ;load initial array
    mov bl, [edi]   ;load end of array
    mov [esi], bl   ;swap
    mov [edi], cl
    inc esi
    dec edi
    dec eax     ;loop
    jnz reverseLoop_1   

end:
    pop edi     ; restore registers
    pop ebx
    mov esp, ebp    ; take down stack frame
    pop ebp
    ret