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From: NiTiN on 24 Jul 2008 06:06
Hi!

I've got some really simple code that boots the system and displays a
string. I've looked at a couple of examples and I'm aware of the load
to 0x7c00. My jump references are fine but when I try to access
variables, I get nulls instead of the actual data.

Here's the source code:
bits 16
org 0x7c00

jmp start

helloMsg db 'A'

start:
mov al, [helloMsg] ;copy the A
cmp al, 65
jne wrongChar

call printChar ;print the A
jmp end

wrongChar:
mov al, 66 ;print B if wrong character
call printChar

end:
jmp $

printChar:
mov ah, 0eh
;mov al, 65
mov bh, 0h
mov bl, 7h
int 10h
ret

;510 bytes plus the 2 byte signature
zero_padding: times 510 - ($ - $$) db 0

;boot signature
boot_sig: dw 0xaa55


The output I get on running this code is 'B' when it should be 'A'. I
must be missing something.

My setup is VMWare running on Windows, and an NASM compiler.

Any help is appreciated.

Regards,
Nitin
From: Frank Kotler on 24 Jul 2008 08:19
NiTiN wrote:
> Hi!
>
> I've got some really simple code that boots the system and displays a
> string. I've looked at a couple of examples and I'm aware of the load
> to 0x7c00. My jump references are fine but when I try to access
> variables, I get nulls instead of the actual data.
>
> Here's the source code:
> bits 16
> org 0x7c00
>
> jmp start
>
> helloMsg db 'A'
>
> start:

xor ax, ax
mov ds, ax

> mov al, [helloMsg] ;copy the A
> cmp al, 65
> jne wrongChar
>
> call printChar ;print the A
> jmp end
>
> wrongChar:
> mov al, 66 ;print B if wrong character
> call printChar
>
> end:
> jmp $
>
> printChar:
> mov ah, 0eh
> ;mov al, 65
> mov bh, 0h
> mov bl, 7h
> int 10h
> ret
>
> ;510 bytes plus the 2 byte signature
> zero_padding: times 510 - ($ - $$) db 0
>
> ;boot signature
> boot_sig: dw 0xaa55
>
>
> The output I get on running this code is 'B' when it should be 'A'. I
> must be missing something.

ASSume nothing, except that your code is loaded at 7C00h, and the boot
drive is in dl. Might want to think about where your stack is, too...

Best,
Frank


From: Wolfgang Kern on 24 Jul 2008 09:22

"NiTiN" asked:

> I've got some really simple code that boots the system and displays a
> string. I've looked at a couple of examples and I'm aware of the load
> to 0x7c00. My jump references are fine but when I try to access
> variables, I get nulls instead of the actual data.

I'd try a colon

helloMsg: db 'A'
__
wolfgang
_________________________
> Here's the source code:
> bits 16
> org 0x7c00
>
> jmp start
>
> helloMsg db 'A'
>
> start:
> mov al, [helloMsg] ;copy the A
> cmp al, 65
> jne wrongChar
>
> call printChar ;print the A
> jmp end
>
> wrongChar:
> mov al, 66 ;print B if wrong character
> call printChar
>
> end:
> jmp $
>
> printChar:
> mov ah, 0eh
> ;mov al, 65
> mov bh, 0h
> mov bl, 7h
> int 10h
> ret
>
> ;510 bytes plus the 2 byte signature
> zero_padding: times 510 - ($ - $$) db 0
>
> ;boot signature
> boot_sig: dw 0xaa55
>
>
> The output I get on running this code is 'B' when it should be 'A'. I
> must be missing something.
>
> My setup is VMWare running on Windows, and an NASM compiler.
>
> Any help is appreciated.
>
> Regards,
> Nitin


From: NiTiN on 24 Jul 2008 10:48
On Jul 24, 4:19 pm, Frank Kotler <fbkot...(a)verizon.net> wrote:

-- SNIP --

> > . My jump references are fine but when I try to access
> > variables, I get nulls instead of the actual data.

> xor ax, ax
> mov ds, ax

> ASSume nothing, except that your code is loaded at 7C00h, and the boot
> drive is in dl. Might want to think about where your stack is, too...
>
> Best,
> Frank

OMG, you were right!!! I set the ds, as you've suggested with a:
mov ax, cs
mov ds, ax
...and it worked!
From: NiTiN on 24 Jul 2008 10:55
On Jul 24, 5:22 pm, "Wolfgang Kern" <nowh...(a)never.at> wrote:
> I'd try a colon
>
>  helloMsg: db 'A'
> __
> wolfgang

Thanks for the suggestion. I still got the 0 instead of 65. I then
tried setting 'ds', as Frank suggested and that worked for me. Thanks
anyway!

Just to share something that I read from the NASM documentation to
figure out their syntax (I did some work with TASM about half a decade
ago), it mentions that variable declarations and labels are treated
the same in NASM and you can simply specify the variable name without
using the 'offset' keyword.
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