Stuck on writing a print function to convert and print binary to decimal with syscalls on NASM for x-CodePudding

I'm still new to assembly code on NASM for x86-64 Linux system and I'm trying to write a program which converts the number stored in rdi to decimal so it can be printed. I'm unsure how to write a proper function that will divide by 10 in a loop that uses the remainder as the digits for example: the number 165 stored in rdi will be divided 165/10 repeatedly and the remainder is 5, etc. and the digits output would look something to 0000000165. Any feedback would be appreciated! Here's my attempt:

section .data

BUFLEN:     equ                25
buf:     times BUFLEN db    0    

section .text

global _start
_start:

    mov rsi, 1
    mov rdi, 453265682
    call printnum  
    
    mov     rax,  60 
    mov     rdi,  0 
    syscall

printnum:

   mov r10, 10


  convert:
        
        mov rdx, 0
        mov rax, rdi
        div r10
        add r15, rdx
        cmp rax, 1
        jnle convert
        
        
    mov    rax,   1         ; = 1
    mov    rdi,   1             ; = 1
    mov    rsi,   buf                ; add of buffer to print
    mov    rdx,   BUFLEN             ; num of bytes to write
    syscall

    ret

CodePudding user response：

There are so many parts in your code that I want to change, but here is a minimal fix to just make your code work.

Below

mov r10, 10

add

mov r15, 9

Erase

add r15, rdx
cmp rax, 1
jnle convert

and change it to

mov rdi, rax
add rdx, '0'
mov [r15   buf], dl
dec r15
jns convert

This is the full printnum function with the fix.

printnum:
    mov r10, 10
    mov r15, 9

    convert:
        mov rdx, 0
        mov rax, rdi
        div r10
        mov rdi, rax
        add rdx, '0'
        mov [r15   buf], dl
        dec r15
        jns convert
        
    mov    rax,   1
    mov    rdi,   1
    mov    rsi,   buf
    mov    rdx,   BUFLEN
    syscall

    ret

It's up to you to think why this works and yours doesn't.