Trouble finding the length in the string

Question

I found the code here, on Stack Overflow to get the length of the string. Here's the code:

HELLO_MSG: db 'Hello, world!', 0
mov ebx, HELLO_MSG
call print_string

print_string:
 call str_len
 mov ecx, eax ; puts the length of the called string into ecx register
 cdq          ; clear the direction flag; not sure if this is needed
 mov ah, 0x0e ; scrolling teleype BIOS routine

 printing:
    mov al, byte bl
    int 0x10
    inc ebx
    loop printing

 ret
str_len:
 mov eax,ebx

 l00p:
    cmp byte[eax], 0
    jz lpend
    inc eax
    jmp l00p
 lpend:
    sub eax, ebx ; puts the length of the string into eax register

    ret

The problem is that when function calls str_len, it loops only 3 times, sometimes it does 2 times. And when it prints, it doesn't print the actual string, but some random letters. Does anybody know why it doesn't work? I'm trying to run it in the boot sector, so I don't really have any debugging tools.

Answer 1

There are more problems in your code:

1. You should jump over string constant on the beginning

   mov ebx, HELLO_MSG
   call print_string
   jmp $
   
   HELLO_MSG: db 'Hello, world!', 0
   
   print_string:
   ...
   

2. cdq instruction is not for clearing DF. cdq is used to convert doubleword in eax into sign-extended edx:eax value. You should cld instead. But you're right, when you don't use any string instructions (cmpsb, movsb...), DF usually isn't important.

3. I haven't setup base and stack pointers yet, because I'm not really using them within these printing strings.

   Yes, but you use call instruction, which pushes return address on stack. Usually, BIOS "allocates" small space for stack before running bootloader, so it shouldn't affect anything.

4. This line

    mov al, byte bl
   

   will set AL equal to BL, which is not the thing you want. You should use indirect byte addressing with ebx.

   mov al, byte [ebx]
   int 0x10
   inc ebx
   ...