I trying to learn assembly language, so I tried to write a program for sorting a number of integers using bubble-sort.

# Sort an array using bubble-sort algorithm

    .text
    .file   "main.s"
    .globl  _start

_start:
    pushq   %rbp
    movq    %rsp, %rbp
    subq    $28, %rsp

    movl    $0, -4(%rbp)
    
    movl    $23, -8(%rbp)
    movl    $31, -12(%rbp)
    movl    $12, -16(%rbp)
    movl    $43, -20(%rbp)
    movl    $10, -24(%rbp)
    movl    $18, -28(%rbp)
    
    leaq    -28(%rbp), %rdi

    callq   bubble_sort

    movl    -28(%rbp), %eax
    callq   to_string
    movq    %rcx, %rdx
    callq   print_message

    addq    $28, %rsp
    popq    %rbp

    # terminate program
    movq    $1, %rax
    xorq    %rdi, %rdi
    syscall

    ret

#------------------------------------------
.LBL0:
    .ascii  "Array before sorting:n"
.LBL1:
    .ascii  "After sorting array:n"

#------------------------------------------
    .globl  print_message
print_message:
    pushq   %rdi

    movq    $4, %rax
    movq    $1, %rdi
    syscall

    popq    %rdi

    retq

#------------------------------------------
    .globl  bubble_sort
bubble_sort:
    pushq   %rbp
    movq    %rsp, %rbp

    movl    $0, -4(%rbp)

    movq    %rdi, -8(%rbp)  # Save first element addr

    movl    $0, -16(%rbp)
loop_0: cmpl    $6, -16(%rbp)
    jge out_0

    movl    $0, -24(%rbp)   # Not swapped yet...
    movl    $0, %ecx
    movq    -8(%rbp), %rax  # Restore first element addr
loop_1: cmpl    $5, %ecx
    jge out1_1

    movl    (%rax), %r8d
    movl    4(%rax), %ebx
    cmpl    (%rax), %ebx    # Compare for swapping
    jle out1_0

swap:   movl    (%rax), %ebx
    movl    %ebx, -28(%rbp)
    movl    4(%rax), %ebx
    movl    %ebx, (%rax)
    movl    -28(%rbp), %ebx
    movl    %ebx, 4(%rax)
    movl    $1, -24(%rbp)   # Swapped!
    
out1_0: addq    $4, %rax

    incl    %ecx
    jmp loop_1
out1_1:
    movl    -24(%rbp), %ebx
    cmpl    $0, %ebx    # check swap flag
    jne out_0

    movl    -16(%rbp), %ebx
    incl    %ebx
    movl    %ebx, -16(%rbp)
    jmp loop_0
out_0:

    popq    %rbp
    
    retq

#------------------------------------------
    .globl  to_string
to_string:
    pushq   %rbp
    movq    %rsp, %rbp
    subq    $16, %rsp

    movl    $10, -12(%rbp)
    leaq    -5(%rbp), %rsi
    xorl    %ecx, %ecx

loop1:  xorl    %edx, %edx
    divl    -12(%rbp)
    orl $48, %edx   // Convert to ASCII value
    movb    %dl, (%rsi)
    cmpl    $0, %eax
    je  out1
    leaq    -1(%rsi), %rsi
    incl    %ecx
    jne loop1


out1:
    movb    $0, -4(%rbp)
    
    addq    $16, %rsp
    popq    %rbp
    addl    $3, %ecx
    movl    %ecx, %edx

    retq

I think problem comes from the I access the array elements. but I don’t know any way to solve this.

Note: the print_message routine use syscall to write an string to stdout.
syscall 4 in freebsd similar to syscall 1 in linux.