.section .data
buf:
  .byte 0       // input/output character buffer

.section .text
.global _start
_start:
  mov x19, #0   // value
  mov x20, #0   // count

loop:
  mov x8, #63   // read
  mov x0, #0    // stdin
  adr x1, buf
  mov x2, #1
  svc 0

  cmp x0, #0    // count of bytes read; 0 means EOF
  b.ne 1f

exit:
  mov x8, #93   // exit
  mov x0, #0    // error code (no error)
  svc 0

1:
  adr x1, buf
  ldurb w9, [x1]

  cmp w9, #92   // backslash; start of comment
  b.ne 2f

comment:
  mov x8, #63   // read
  mov x0, #0    // stdin
  adr x1, buf
  mov x2, #1
  svc 0

  cmp x0, #0
  b.eq exit

  adr x1, buf
  ldurb w9, [x1]
  cmp w9, '\n'
  b.ne comment
  b loop

2:
  cmp w9, '0'
  b.lt 3f
  cmp w9, '9'
  b.gt 3f

  sub w9, w9, '0'
  b 4f

3:
  cmp w9, 'a'
  b.lt 5f
  cmp w9, 'f'
  b.gt 5f

  sub w9, w9, ('a' - 10)
  b 4f

4:
  lsl x19, x19, #4
  add x19, x19, w9, uxtb
  add x20, x20, #1
  b loop

5:
  cmp w9, ' '
  b.ne loop

  tst x20, #1
  cinc x20, x20, ne
  lsr x20, x20, #1

6:
  cmp x20, #0
  b.eq loop

  mov x8, #64   // write
  mov x0, #1    // stdout
  adr x1, buf
  sturb w19, [x1]
  mov x2, #1
  svc 0

  lsr x19, x19, #8

  sub x20, x20, #1
  b 6b