NASM Coding: Basics - Program Examples

4. ; A program that takes an input (integer) from the user, and checks if it is even or odd.
   ; Output example:
   ; The number you entered is: 
   ; 8
   ; and it is even.
   
   section .data
       even_msg db "and it is even.", 10
       even_len equ $ - even_msg
   
       odd_msg db "and it is odd.", 10
       odd_len equ $ - odd_msg
   
       newline db 10
       msg db "The number you entered is: ", 10
       msg_len equ $ - msg
   
   section .bss
       buffer resb 16        ; Create char buffer [16]
   
   section .text
       global _start
   
   _start:
       ; --- read input ---
       mov rax, 0            ; syscall: read
       mov rdi, 0            ; stdin
       mov rsi, buffer       ; store input here
       mov rdx, 16           ; max bytes
       syscall
   
           ; --- Printing the entered number ---
       mov rax, 1
       mov rdi, 1
       mov rsi, msg
       mov rdx, msg_len
       syscall
   
       mov rax, 1
       mov rdi, 1
       mov rsi, buffer
       mov rdx, 16
       syscall
   
       mov rax, 1
       mov rdi, 1
       mov rsi, newline
       mov rdx, 1
       syscall
   
       ; --- convert ASCII to integer ---
       mov rsi, buffer       ; pointer to input
       xor rax, rax          ; result = 0
   
   convert_loop:
       movzx rbx, byte [rsi] ; load current char
   
       cmp bl, 10            ; newline?
       je conversion_done
       cmp bl, 0             ; null terminator?
       je conversion_done
   
       sub rbx, '0'          ; ASCII -> digit (e.g., '5' -> 5)
   
       imul rax, 10          ; result *= 10
       add rax, rbx          ; result += digit
   
       inc rsi               ; move to next char
       jmp convert_loop
   
   conversion_done:
   
       ; --- check even/odd ---
       test rax, 1           ; check lowest bit
       ; If the lowest bit it 1, the number is odd
       ; If the lowest bit it 0, the number is even
       jz is_even            ; if zero, then even
   
   is_odd:
       mov rax, 1
       mov rdi, 1
       mov rsi, odd_msg
       mov rdx, odd_len
       syscall
       jmp exit
   
   is_even:
       mov rax, 1
       mov rdi, 1
       mov rsi, even_msg
       mov rdx, even_len
       syscall
   
   exit:
       mov rax, 60
       mov rdi, 0
       syscall