AArch64 Looping first 50 even numbers.

Hi everyone, I am writing a blog about a practice exercise for assembly language for AArch64. Recently, I was studying about the assembly language commands and registers for both x86_64 and arch64 systems. I completed several labs, which gave me good understanding about the registers and commands for assembler for both systems. Here is the link to them.

I decided to some practice exercises to improve the understanding of registers and to memorize those commands. I decided to make a program which will loop through first 50 numbers and print the even numbers only. 

Here is the source code of the program:

.text
.globl _start
min = 0
max = 51                         /* loop exits when the index hits this number (loop condition is i<max) */

_start:

        mov     x19, min
        mov     x21, 10

loop:

        /*Start of loop*/

        adr     x25, msg        /*setting the address of string*/

        udiv    x20, x19, x21   /*unsigned divide  - calculating first digit*/
        msub    x22, x21, x20, x19  /* calulating reminder - second digit */

        cmp     x19, x21
        b.lt DoubleDigits

        add     x20, x20, '0'
        strb    w20, [x25, 0]   /*writing first digit*/

DoubleDigits:
        add     x22, x22, '0'
        strb    w22, [x25, 1]   /*writing second digit*/


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

        mov     x8, 64
        svc     0

        /*End of loop*/

        add     x19, x19, 2
        cmp     x19, max
        b.lt    loop

        mov     x0, 0           /* status -> 0 */
        mov     x8, 93          /* exit is syscall #93 */
        svc     0               /* invoke syscall */

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

In this program, I started by defining some macros for minimum and maximum value of the loop. Then I assigned min value to the register which will work as iterator for the loop. In the beginning of the loop I performed udiv and msub to find the first and second digit of the number(I performed these on iterator register - x19 ) Then I checked if first digit is 0 and skipped printing it. For printing the values I used svc to invoke syscall for print which is  value 64. After printing both of the digits I checked that if iterator is less than max value, If it is less then loop should repeat or if value is higher than loop should break.

Then, I successfully compiled the program and it ran perfectly. Here is the output of the program: