cat.asm

default rel

section .bss
buffer:   resb 64

section .text
global start

start:
  cmp qword [rsp], 1
  je stdin

  mov rax, 0x2000005  ; open()
  mov rdi, [16 + rsp] ; argv[1]
  mov rsi, 0x0        ; O_RDONLY
  syscall

  jc term
  mov r12, rax        ; save file descriptor in r12

loop_start:
  mov rdi, r12
  mov rax, 0x2000003  ; read()
  mov rsi, buffer
  mov rdx, 64
  syscall

  mov r13, rax        ; save how many bytes were read

  mov rax, 0x2000004  ; write()
  mov rdi, 1          ; stdout
  mov rsi, buffer
  mov rdx, r13
  syscall

  cmp r13, 0          ; if we read 0 bytes, exit
  jne loop_start

term:
  mov rax, 0x2000001  ; exit()
  mov rdi, 0          ; return 0
  syscall

stdin:
  mov r12, 0
  jmp loop_start