global _start ; entry point label
section .data
  str: db "Please give me a string: " ; \n = 0xA
  strSize: equ $ - str

  stdin: equ 0  ; POSIX
  stdout: equ 1 ; POSIX

section .bss
  strBuffer: resb 64    ; reserve 64 bytes
  strBufferSize: equ 64 ; same as above

section .text
  ; Syscall clobbered registers
  ; rax rdi rsi rdx r10 r8 r9
_start:
  mov rax, 1
  mov rdi, stdout  ; fd to write to
  mov rsi, str     ; pointer
  mov rdx, strSize ; size
  syscall

  xor rax, rax ; 0
  mov rdi, stdin     ; fd to read from
  mov rsi, strBuffer ; where to store it
  mov rdx, strBufferSize ; maximum number of bytes
  syscall

  mov rbx, rax ; store the read bytes

  ; Calculate the halfway point of read bytes
  dec rax ; subtract the read newline from it
  xor rdx, rdx ; clear high bits dividend
  ; rax is already set
  mov rdi, 2  ; set the divisor
	div rdi ; rax = rax / rdi
  
  ; Alter the middle byte
  mov rdx, strBuffer
  add rdx, rax
  mov byte [rdx], '_'

  mov rax, 1
  mov rdi, stdout  ; fd to write to
  mov rsi, strBuffer     ; pointer
  mov rdx, rbx ; size
  syscall

  mov rax, 60
  xor rdi, rdi
  syscall