beginner boot program issues

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VexisMorlock

Joined: 18 Dec 2020
Posts: 2

VexisMorlock 18 Dec 2020, 11:03

Hey guys,

I am just getting into this and have been experimenting with writing boot programs and ran into an issue. I am just booting up this program into virtualbox and I am getting the wrong output sorry if this is a simple issue I am still new.

Code:


format binary as 'img'
jmp 0x07b5:START

START:
        mov ah, 0eh ;tele prompt
        mov             edx,say
writing:
        mov             al,[edx]
        int     10h ;print
        jmp             done

say:
        db      'B'

done:
        mov             al,'X'
        int             10h
        

        cli ; clear interrupt flag
        hlt ; halt execution

times 510-$ db 0
dw 0xAA55

When I run this I end up with "SX" instead of "BX". When I run a similar program in windows or Linux I get the correct ascii back. Is there something about running programs in real mode that I do not understand? Would love some help thanks in advance.

18 Dec 2020, 11:03

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20758
Location: In your JS exploiting you and your system

revolution 18 Dec 2020, 11:35

Your segment register DS hasn't been initialised.

Code:

mov al,[cs: edx]

18 Dec 2020, 11:35

VexisMorlock

Joined: 18 Dec 2020
Posts: 2

VexisMorlock 18 Dec 2020, 14:21

revolution wrote:

Your segment register DS hasn't been initialised.
Code:
mov al,[cs: edx]    

I am assuming that has to do with segmented registers after spending a bunch of time looking into it, and now hate myself. It may be because I don't really know how to implement this but I just ended up with only 'X' for the output. I did get my code to do what I wanted after spending a bunch of time looking at tuts online here. I changed the 2nd line to

Code:

org 0x7c00

and the 2nd to last line to:

Code:

times 510-($-$$) db 0

I'm going back to the books to try and understand this a bit better. Thank you for trying to help me. If you can recommend any resources for learning about segmentation it would be awesome. It's not super easy to find.

18 Dec 2020, 14:21

Mac2004

Joined: 15 Dec 2003
Posts: 314

Mac2004 17 Jul 2021, 04:38

I took a quick look at your code.

Firstly you are not setting a stack pointer at all.

Secondly you are not setting proper values for int 10h call.
(https://en.wikipedia.org/wiki/INT_10H), which says'
...
Teletype output AH=0Eh AL = Character, BH = Page Number, BL = Color (only in graphic mode)
...'

The character needs to be in AL, which you are doing.
BH register is not set by you and BH register is probably in a state where bios has left.

Regards
M2004

17 Jul 2021, 04:38

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