gistfile1.S

.equ EXIT, 1
.equ READ, 3
.equ OPEN, 5
.equ BRK, 45

.equ BUF_SZ, 4096

.section .data
filename:
  .ascii "my_file\0"

.section .bss
.lcomm buffer, BUF_SZ

.section .text
.globl _start
_start:
  movq $buffer + BUF_SZ, %r12 # save bss end address to r12

  movq $OPEN, %rax
  movq $filename, %rbx
  movq $0, %rcx
  movq $0666, %rdx
  int $0x80

  movq %rax, %r13 # save fd to r13

  read_loop:
    movq $READ, %rax
    movq %r13, %rbx
    movq $buffer, %rcx
    movq $BUF_SZ, %rdx
    int $0x80

    cmpq $0, %rax # end of file
    je read_loop_end

    movq %r12, %rbp # copy old end to rbp
    addq %rax, %r12 # add read sz to r12

    movq $BRK, %rax # allocate to new end
    movq %r12, %rbx
    int $0x80

    cmpq %rax, %r12 # if rax != r12, alloc failed
    jne error_exit

    # copy buf to newly allocated space
    # rcx has the buffer address
    copy_loop:
      cmpq %r12, %rbp
      je end_copy
      movb (%rbp), %al
      movb %al, (%rbp)
      incq %rcx
      incq %rbp
      jmp copy_loop

    end_copy:

    jmp read_loop

  read_loop_end: # ok exit
    movq $EXIT, %rax
    movq $0, %rbx
    int $0x80

  error_exit: # error exit
    movq $EXIT, %rax
    movq $1, %rbx
    int $0x80