random character in displaying string inside custom bootloader in QEMU

0
votes

I'm following a tutorial that introduce you in the magic world of bootloader.

The easiest example, print a character, works. Displaying a string gives me some problem: it displays random characters. It should display 12 characters, starting at the location inside si register

Here's the Nasm code (build command: nasm.exe bootloader.asm -f bin -o bootloader.bin)

[bits 16]
[org 0]
start:
    mov al, 68
    mov ah, 0x0E
    mov bh, 0x00
    mov bl, 0x07

    mov si, helloWorld
    call printString

    jmp $

printString:
    mov dx, 0
._loop:
    mov al, [si]
    int 0x10
    inc si

    inc dx
    cmp dx, 12
    jl ._loop

    ret


helloWorld:
    db 'AAAAAAAAAA'

times 510 - ($ - $$) db 0
dw 0xAA55

Then I create the .img file with dd.exe if=bootloader.bin of=bootloader.img count=1 bs=512

It boots correctly in QEMU (qemu-system-i386.exe) (well, it loads, because my bootloader still not boot) (maybe it's a problem of QEMU -difficoult-)

Here's the screenshot Ta-dah!

What's the problem in my code?

2
assemblynasmqemubootloader

2 Answers

1
votes

You should use [org 0x7c00] since that is where your bootloader will be loaded. I suggest you read more about boot sequence from this OSDev Wiki article.

0
votes

Yes you need ORG 0x7C00, so the addressing starts at 7C00h. You also need to set up the DS (data segment) register before accessing data on [SI] as there is no guarantee all BIOSES will have set it to 0.

Just do: MOV DX,CS MOV DS,DX at your start: label

After you have set up the DS you can access any data structures there.

For reference here are the first instructions of a Microsoft boot sector (after the jmp):

XOR CX,CX
MOV SS,CX
MOV SP,7BF4h
MOV ES,CX
MOV DS,CX