Question

I am working on a program – it should be simple – on a Linux OS using NASM and x86 Intel Assembly Syntax.

The problem I am having is that I cannot create a working loop for my program:

section .data
    hello:    db 'Loop started.', 0Ah   ;string tells the user of start
    sLength:  equ $-hello               ;length of string

    notDone:  db 'Loop not finished.', 0Ah ;string to tell user of continue
    nDLength: equ $-notDone                ;length of string

    done:     db 'The loop has finished', 0Ah ;string tells user of end
    dLength:  equ $-done                      ;length of string

section .text

    global _start:
_start:
    jmp welcome         ;jump to label "welcome"

    mov ecx, 0          ;number used for loop index
    jmp loop            ;jump to label "loop"

    jmp theend          ;jump to the last label

welcome:

    mov eax, 4
    mov ebx, 1
    mov ecx, hello
    mov edx, sLength
    int 80              ;prints out the string in "hello"

loop:
    push ecx            ;put ecx on the stack so its value isn't lost

    mov eax, 4
    mov ebx, 1
    mov ecx, notDone
    mov edx, nDLength
    int 80              ;prints out that the loop isn't finished

    pop ecx             ;restore value
    add ecx, 1          ;add one to ecx's value
    cmp ecx, 10
    jl loop             ;if the value is not ten or more, repeat

theend:

;loop for printing out the "done" string

I am getting the first string printed, one “Not done” and the last string printed; I am missing nine more “Not Done”s! Does anyone have any idea as to why I am losing my value for the ecx register?

Thank you.

Answer 1

_start:
    jmp welcome

This means all the code below the JMP is not executed, especially the mov ecx,0 (which should be xor ecx,ecx for a shorter instruction)

Don’t start with a jump, start with some code. A JMP is a jump, it’s not going back after you’ve jumped, it just continues the execution.

So after jumping to Welcome:, you go directly to Loop:, thus missing the ecx=0 code.

cmp ecx, 10
jl loop

ECX is not 0, it definitely is greater than 10h, so the loop is not taken.

Try this:

_start:
    mov eax, 4
    mov ebx, 1
    mov ecx, hello
    mov edx, sLength
    int 80              ;prints out the string in "hello"
    xor ecx,ecx         ;ecx = 0

loop:
    push ecx            ;save loop index
    mov eax, 4
    mov ebx, 1
    mov ecx, notDone
    mov edx, nDLength
    int 80              ;prints out that the loop isn't finished

    pop ecx             ;get loop index back in ECX
    add ecx, 1          ;add one to ecx's value
    cmp ecx, 10
    jl loop             ;if the value is not ten or more, repeat

theend: