print hex numbers in AT&T ASM

printhex.S

.global main

.set print_hex_msb4mask, 0xf000000000000000
.data
print_hex_chars:
    .ascii "0123456789abcdef"

.text
syscall4:
    movq  %rax, %rax
    movq  %rbx, %rdi
    movq  %rcx, %rsi
    syscall
    ret

print_stdout:
    enter $0, $0
    pushq %rbx
    pushq %rax
    movq $1, %rax
    movq $1, %rbx
    popq %rcx
    popq %rdx
    call syscall4
    leave
    ret

print_hex:
    enter $0, $0
    cmp $0, %rax
    jne .print_hex_skip
    jmp .print_hex_end
    .print_hex_skip:
        movq  %rax, %rbx
        movq  $print_hex_msb4mask, %rdx
        andq  %rdx, %rbx

        cmpq  $0, %rbx
        jne   .print_hex_skip_end
        shlq  $4, %rax
        jmp  .print_hex_skip
    .print_hex_skip_end:
    push %rax
    .print_hex_loop:
        popq  %rax
        cmpq  $0, %rax
        je    .print_hex_end
        pushq %rax

        movq  $print_hex_msb4mask, %rbx
        andq  %rbx, %rax
        shrq  $60, %rax
        addq  $print_hex_chars, %rax
        movq  $1, %rbx
        call print_stdout

        popq  %rax
        shlq  $4, %rax
        pushq %rax
        jmp .print_hex_loop
    .print_hex_end:
    pop  %rax
    xor  %rax, %rax
    leave
    ret

println:
    enter $0,$0
    pushq $10
    mov %rsp, %rax
    mov $1, %rbx
    call print_stdout
    leave
    ret

main:
    mov $0xBabeCafe, %rax
    call print_hex 
    call println
    mov $0xDeadBeef, %rax
    call print_hex
    call println

    mov    $60, %rax
    mov    $0, %rdi
    syscall