Change only one letter in string

Question

I have 2 string and one letter.

selectedWords BYTE "BICYCLE"
guessWords BYTE "-------"
inputLetter BYTE 'C'

Base on this answers, I write code who compere if selectedWords have letter C and If this is the case he need to change string guessWords:

guessWords "--C-C--"

But from some strange reason I get all other possibilities, just not correct one. Some suggestions on how to solve this problem.

Answer 1

First, forget the so called string instructions (scas, comps, movs). Second, you need a fixed pointer (dispkacement) with an index, e.g [esi+ebx]. Have you considered that WriteString needs a null-terminated string?

INCLUDE Irvine32.inc

.DATA
selectedWords BYTE "BICYCLE"
guessWords BYTE SIZEOF selectedWords DUP ('-'), 0   ; With null-termination for WriteString
inputLetter BYTE 'C'

.CODE
main PROC

    mov esi, offset selectedWords       ; Source
    mov edi, offset guessWords          ; Destination
    mov ecx, LENGTHOF selectedWords     ; Number of bytes to check
    mov al, inputLetter                 ; Search for that character
    xor ebx, ebx                        ; Index EBX = 0

    ride_hard_loop:
    cmp [esi+ebx], al                   ; Compare memory/register
    jne @F                              ; Skip next line if no match
    mov [edi+ebx], al                   ; Hang 'em lower
    @@:
    inc ebx                             ; Increment pointer
    dec ecx                             ; Decrement counter
    jne ride_hard_loop                  ; Jump if ECX != 0

    mov edx, edi
    call WriteString                    ; Irvine32: Write a null-terminated string pointed to by EDX
    exit                                ; Irvine32: ExitProcess

main ENDP

END main