print_rax.asm

section .data
codes:
	db '0123456789ABCDEF'


section .text
global _start
_start:
	; number 1122... in hexadecimal format
	mov rax, 0x1122334455667788

	mov rdi, 1
	mov rdx, 1
	mov rcx, 64
	; Each 4 bits should be output as one hexadecimal digit
	; Use shift and bitwise AND to isolate them
	; the result is the offset in 'codes' array
.loop:
	push rax
	sub  rcx, 4
	
	; cl is a register, smallest part of rcx
	; rax -- eax -- ax -- ah + al
	; rcx -- ecx -- cx -- ch + cl
	sar rax, cl
	
	and rax, 0xf
	lea rsi, [codes + rax]
	mov rax, 1
	
	; syscall leaves rcx and r11 changed
	push rcx
	syscall
	pop rcx

	pop rax
	; test can be used for the fastest 'is it a zero?' check
	; see docs for 'test' command
	test rcx, rcx
	jnz .loop
	
	mov rax, 60 ; invoke 'exit' system call
	xor rdi, rdi
	syscall