64-bit Assembly Language

In this blog post, I will demonstrate how to create a loop that prints messages along with loop index values, displaying digits from 0 to 30 using both AArch64 and x86_64 architectures.

AArch64

.text
.globl _start

min = 0
max = 31

_start:
        mov     x19, min
        mov     x17, 10

loop:
        mov     x0, 1          
        adr     x1, msg       
        mov     x2, len        

        mov     x18, x19
        udiv    x9, x18, x17
        add     x13, x9, 0x30
        msub    x10, x9, x17, x18
        add     x14, x10, 0x30
        adr     x15, msg
        strb    w13, [x15, 7]

        strb    w14, [x15, 8]
        mov     x8, 64         
        svc     0            

        add     x19, x19, 1
        cmp     x19, max
        b.ne    loop

.data
msg:    .ascii      "Loop :    \n"
len=    . - msg

Result

x86_64

.globl  _start

min = 0
max = 31

_start:
        movq     $min, %r15                        
        movq     $0x30, %r12                     

loop:
        movq     $min, %rdx                        
        movq     %r15, %rax                      
        movq     $10, %r10                       
        div      %r10                            
        movq     %rax, %r14                      
        movq     %rdx, %r13                      

        add      $0x30, %r14                     
        add      $0x30, %r13                     
        mov      %r13b, msg+8                    

        cmp      %r12, %r14
        mov      %r14b, msg+7                    

        movq     $len, %rdx                      
        movq     $msg, %rsi                      
        movq     $1, %rdi                              
        movq     $1, %rax                               
        syscall

        inc     %r15                           
        cmp     $max, %r15                           
        jne     loop                           

        movq     $min, %rdi                                
        movq     $60, %rax                       
        syscall

.section .data
        msg:    .ascii   "Loop :   \n"
        len = . - msg

Result

AArch64 vs x86_64
I find the instruction set of AArch64 to be more straightforward and user-friendly, as I don't have to prefix every number with a '$' sign. Additionally, achieving the same task in x86_64 requires much longer lines of code compared to AArch64, which can be a bit time-consuming.

Experience in general
The debugging process was challenging for me, as a solid grasp of memory addresses, registers, and stacks is essential to uncover what's happening in the code. Furthermore, my lack of familiarity with the syntax of each architecture added an extra layer of difficulty to this project.

Another challenge for me was to remove the leading zero when dealing with single digits. For example, the desired output should be 0, 1, 2, ... 29, 30 instead of 00, 01, 02, ... 29, 30. Unfortunately, I couldn't figure out how to suppress the leading zero in this lab.