can't compare user input with number, nasm elf64

Question

I swear I\'ve read more than 20 pages today, from NASM\'s manual to Universities\' guides to Wikipedia to everything in between but I just can\'t wrap my head around this, I wro

Answer 1

You need to compare characters, not integer values. Change:

cmp rcx,0

to

cmp byte [rcx],'0'

Ditto for the comparison with 1.

Answer 2

Assuming you're trying to run this on 64 bit AMD64 linux, you need to make sure you use the right way of passing parameters to the kernel (the calling convention).

For a 64 bit app on linux/x64, it is as follows:

• rax - system call id
• rdi - argument 1
• rsi - argument 2
• rdx - argument 3
• r10 - argument 4
• arg 5 and 6 are in r9 and r8
• rcx and r11 values are not preserved across syscall, but all other registers will not be clobbered.

In addition, Paul R's answer is also correct. The ASCII representation of '0' is not decimal zero. Another gotcha is that the stdin operates in buffered mode by default, so you won't actually get any data until you press enter. The code below reads two bytes into the tmp buffer (pointed to by rsi) and compares the first with ascii-zero. The second byte is a newline character that we're not particularly interested in.

; constants
section .data
  lblZero:      db    'Not zero, try again', 0xa;
  tmp:          db    0,0;
; code

section .text
BITS 64
  global _start
_start: 
  mov rax, 0   ; sys_read(int fd, void *ptr, int count)
  mov rdi, 0   ; 0 = stdin 
  mov rsi, tmp ; @ tmp
  mov rdx, 2   ; 2 bytes (one for our number, another to store newline)
  syscall
  cmp byte[rsi], '0' ; is input '0' (or decimal 48) ? 
  je  done
  mov rax, 1            ; sys_write(int fd, void *ptr, int count)
  mov rdi, 1            ; 1 = stdout
  mov rsi, lblZero      ; @lblZero
  mov rdx, 20           ; 20 bytes
  syscall
  jmp _start

done:   
  mov rax, 60 ; sys_exit
  mov rdi, 0
  syscall