Null pointer assignment when using BSS

[mElo]

hi!
There's a problem that has been sticking to me for one week now, and I can't
figure out what's the cause: I'm coding a program that uses the BIOS EDD
interrupts (Fn41-4E) to access my hard drive (for now it just reads the
partition table).

There's a module coded in assembly (NASM0.98) and another coded in C (under
DOS, with BC++4). I link this little mess, with TLINK v7.1...
Here's the problem: when I execute the program it aborts with a "Null
pointer assignment" message.

It found that this problem comes from the declaration of variables in the
_BSS segment (in the assembly source), because when I move the declarations
in the _DATA segment it works well !

;For example this doesn't work
SEGMENT _BSS CLASS=BSS
cylinders resw 1
heads resw 1
sectors resw 1
lba resd 1

SEGMENT _DATA CLASS=DATA
drive dw 80h

Here's the link where I uploaded the source:
http://www.guetali.fr/home/nirvana8/melo/files/diskio006.zip

That's all,
forgive my english, it may be bad sometimes (frensh people!),
have fun coding,
bye...
mElo
EDIT, my old old friend... :-)

[david lindauer]

do you have your data and BSS segments in the same group? If not you may be
accessing an area of memory you don't think you are accessing.

David

[mElo]

david lindauer <dlin...@notifier-is.net> wrote in message news:<3CDC1775...@notifier-is.net>...

> do you have your data and BSS segments in the same group? If not you may be
> accessing an area of memory you don't think you are accessing.

In fact, yes they are in the same group, I checked it in the MAP
file. It may be because I declare my CODE, DATA and BSS segment with
the same name as the OBJ files produced by the C compiler (BCC++4),
what is to say: _TEXT, _DATA, _BSS.

Anyway as you and Geezer noticed me, I should GROUP my segments in the
assembly files, like the compiler does: CGROUP and DGROUP.

Geezer wrote to me the solution:
>Here is your CC.BAT file:
> nasm -f obj diskio.asm
> nasm -f obj timer.asm
> bcc -3 -c -mt -v main.c partable.c
>
>-mt is for TINY memory model
>
>Here is your LK.BAT file:
> tlink /Tde /c /C /Lc:\tc\lib /s /v c0s diskio timer partable >main,t,,cs.lib
>
>c0s is for SMALL memory model
>
>You must change one of these!
>
>Also, maybe you need a GROUP statement in the .ASM files
>
>--
>geezer@ | http://www.execpc.com/~geezer
>execpc.com | http://www.execpc.com/~geezer/osd

Thanks
bye
mElo

> mElo wrote:
>
> > hi!
> ...