或 nasm 指令

Question

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 nasm -f elf hello.asm; ld -m elf_i386 -s -o hello hello.o; ./hello 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?

Answer 1

If the comment on or al, bl; 0101 1010 ==> 'Z' 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'