I am pretty new to assembly and currently trying to write a programm that executes a caesar encryption with a user specified key and user specified content. The key and the content have to be a-z or A-Z and to verify that, I build a simple algorithm checking if the ascii value is in the corresponding range. The problem is that even when given normal characters a-z or A-Z, it still returns false.

This is a minimal reproducible example of my algorithm. Even though a is given to the algorithm, it still returns error. I have no idea why this is the case, I have checked the ascii table and everything.

section .text
  global _start

_start:
cmp byte [char], 123
jg _error
cmp byte [char], 97
jg _success
cmp byte [char], 90
jg _error
cmp byte [char], 65
jg _success
jmp _error

_success:
  mov rax, 1
  mov rdi, 1
  mov rsi, success
  mov rdx, len_success
  syscall
  xor rbx, rbx
  jmp _exit

_error:
  mov rax, 1
  mov rdi, 2
  mov rsi, error
  mov rdx, len_error
  syscall
  mov rbx, 1
  jmp _exit

_exit:
  mov rax, 60
  mov rdi, rbx
  syscall

section _data
  char db 'a'
  error db "Error"
  len_error equ $ - error
  success db "Success"
  len_success equ $ - success