[linux/asm_x-86_64] Emulazione di 'cat'

; asmcat :: linux 'cat' asm-x_86-64 revisited
; $ nasm -f elf64 thisfile.asm
; $ ld -f elf_x86_64 thisfile.o -o filename

global _start

section .bss

  Buffer   resb 0x1000       ; 4096 byte

section .text

_start:

  push     rbp
  mov      rbp,rsp

  cmp      rsp,0x2           ; argc = 2
  jl       _exit

  mov      rax,0x2           ; open((argv[1]), 0, 0)
  mov      rdi,[rsp+0x18]
  mov      rsi,0
  mov      rdx,0
  syscall
  mov      rbx,rax           ; save fhandle

_input:

  mov      rax,0             ; read
  mov      rdi,rbx
  mov      rsi,Buffer
  mov      rdx,0x1000
  syscall

  cmp      rax,0
  jl       _exit
  jne      _output

  mov      rax, 0x3          ; close file
  mov      rdi, rbx
  syscall
  jmp      _exit

_output:

  mov      rdx, rax
  mov      rax,0x1           ; write
  mov      rdi,0x1
  mov      rsi,Buffer
  syscall
  cmp      rax, 0
  jle      _exit
  jmp      _input

_exit:

  mov rax,0x3c               ; exit
  syscall

vogy@eve:~$ for i in `seq 10000`; do cat /etc/init.d/cups >> asmcat_test; done
vogy@eve:~$ ls -sk | grep asmcat_test
31100 asmcat_test
vogy@eve:~$ time cat asmcat_test
.....
real    0m5.444s
user    0m0.002s
sys     0m1.621s
vogy@eve:~$  time ./asmcat asmcat_test
.....
real    0m4.858s
user    0m0.007s
sys     0m1.268s

.....
  Buffer   resb 0x1000          ; 4096 byte
.....
  mov      rdx,0x1000