print_hex.S

 // void print_hex(unsigned char num);
 
 .data
buff:
  .ascii "0x00\n"
  
  .global print_hex
  .text
print_hex:
  mov %rdi,%rax
  and $0x0F,%rax
  add $0x30,%rax
  cmp $0x3A,%rax
  jl 1f
  add $0x7,%rax
1:
  mov %al,buff+3(%rip)

  mov %rdi,%rax
  shr $0x4,%rax
  and $0x0F,%rax
  add $0x30,%rax
  cmp $0x3A,%rax
  jl 1f
  add $0x7,%rax
1:
  mov %al,buff+2(%rip)

  mov $0x1,%rdi
  lea buff(%rip),%rsi
  mov $0x5,%rdx
  mov $0x1,%rax

  syscall

  ret