Getting a string to print via bios interrupt

Question

I'm learning assembly and of course I'm experimenting with the classic 0x10 bios interrupt. The resources I've looked at show using lodsb to print a string, so to understand that opcode I'm trying to mimic it's behavior. This works fine with lodsb, but not with what I have. What am I doing wrong?:

start:
    mov ah, 0Eh ;for bios interrupt
    mov si, text_string ;set source index to begining of text_string

.repeat:
    ;I'm trying to emulate the behavior of lodsb to learn how it works:
    mov al, [si] ;put character at si in al register
    add si, 1 ;increment source index

    cmp al, 0 ;if the character is a zero (end of the string)
    je done ;end execution

    int 10h ;bios interrupt to put character on screen
    jmp .repeat

    text_string db 'Hello, World!', 0

done:
    ret

Answer 1

It's not usually a good idea to:

1. Assume that all registers are preserved across interrupt calls (or any calls, really); or
2. Not set up all the required registers for a given call.

On that first note, I would set ah to 0eh immediately before the int 10.

Int 10h/0eh requires the bh and bl be set to the page number and foreground color respectively. I would also do that immediately before the int 10 to ensure they're set correctly for the call.

---

As an aside, you need to ensure that the ds segment register is correct, with something like:

push cs
pop ds

since it looks like you're putting the string into your code segment. However, since the lodsb version apparently works, I assume you've got that covered.

And, from (admittedly faded) memory, 8086 has an inc si instruction that might save a small amount of space, not that it matters that much anymore but I was forged in a time when every byte counted :-)

Answer 2

First lodsb is a command that tells the computer to get a character in the code and increases the offset. The offset is determined by [SI], a register that can be easily set. Once SI is set lodsb gets the char and lodes it into al. From this point on its reading al and determing what to do. There is another question like this here.