loop-gas.s

.text
.globl    _start

start = 0                       /* starting value for the loop index; note that this is a symbol (constant), not a variable */
max = 31                        /* loop exits when the index hits this number (loop condition is i<max) */

_start:
    mov     $start,%r15         /* loop index */

loop:
    /* ... body of the loop ... do something useful here ... */

    /* Printing .msg */
    movq    $len,%rdx   /*Length of data in address pointed by msg*/
    movq    $msg,%rsi   /*Message data on msg*/
    movq    $1,%rdi     /*1 for stdout*/
    movq    $1,%rax     /*sys_write instruction*/
    syscall

    /*dividing counter */
    movq    $0,%rdx     /*rdx must be zero before using div*/
    movq    %r15,%rax   /*moving counter to rax*/
    movq    $10,%r12
    div     %r12        /* counter/10 -> int in rax; remainder in rdx*/
    movq    %rax,%r13   /* save int in r13 */
    movq    %rdx,%r14   /* save remainder in r14 */


    /*leading digit*/
    movq    $len2,%rdx  /*Length of 1*/
    movq    $digit,%rsi /*digit msg(space)*/
    cmp     $0,%r13     /* 0 - r13 */
    je      lsd         /* jump to lsd if last comparison is zero */

    movb    %r13b,(%rsi)/* move quotient to rsi */
    add     $48,(%rsi)  /* ascii conversion */
    movq    $1,%rdi     /*stdout */
    movq    $1,%rax     /*sys_write */
    syscall

lsd:
    /*following digit*/
    movq    $len2,%rdx  /*Length*/
    movq    $digit,%rsi /*Message data on digit (blank space)*/
    movb    %r14b,(%rsi)/*Moves loop counter to address pointed by rsi (digit)*/
    add     $48,(%rsi)  /*Adds 48d. The first loop value is zero*/
    movq    $1,%rdi     /*stdout*/
    movq    $1,%rax     /*sys_write*/
    syscall

    /* Printing newline character*/
    movq    $len1,%rdx
    movq    $nl,%rsi
    movq    $1,%rdi
    movq    $1,%rax
    syscall
    
     /* loop condition */
    inc     %r15                /* increment index */
    cmp     $max,%r15           /* max - r15 */
    jl      loop                /* if negative flag = 1, continue. Else, call loop */
                                /* the 2 expression above mean 'if (max > r15){loop()};*/
    /*exit*/
    mov     $0,%rdi             /* exit status */
    mov     $60,%rax            /* syscall sys_exit */
    syscall

.section .data

msg:    .ascii "Loop: "
        len = . - msg

nl:     .ascii "\n"
        len1 = . - nl

digit:  .ascii " "
        len2 = . - digit