or instruction in nasm-CodePudding

I'm trying to learn nasm, following this tutorial , I have written this code

section .text
   global   _start

_start:
   mov   al,   1ah   ; 0001 1010
   mov   bl,   40h   ; 0100 0000

   or    al,   bl    ; 0101 1010 ==> 'Z'
   add   al,   byte  '0'  ; convert from decimal to ascii

   mov   [result], al 

   mov   eax,  4        ;syscall (write)
   mov   ebx,  1        ;file descirptor 
   mov   ecx,  result   ;message to write
   mov   edx,  1        ;message length
   int   0x80           ;call kernell
   jmp   outprog

outprog:
   mov   eax,  1
   int   0x80

segment .bss
   result   resb  1

the output of nasm -f elf hello.asm ; ld -m elf_i386 -s -o hello hello.o; ./hello is a weird character �% where it had to print 'z' am I missing soemthing?

CodePudding user response：

If the comment on or al, bl ; 0101 1010 ==> 'Z' already says that this is a character, then it's not clear why you still add something to it.

Your addition add al, byte '0' adds 48 to the ASCII code for 'Z':

  0101 1010    90  'Z'
  0011 0000    48  '0'
  ---------
  1000 1010   138  'è' in codepage 437

The addition of byte '0' is only needed to convert values in the range 0 to 9 into characters in the range "0" to "9".

it had to print 'z'

To convert the uppercase 'Z' into the lowercase 'z' that you seem to expect, the addition would need to be 32.

mov   al,   1ah   ; 0001 1010
mov   bl,   40h   ; 0100 0000

or    al,   bl    ; 0101 1010 ==> 'Z'
add   al,   32    ; 0111 1010 ==> 'z'