Date:	Wed, 5 Mar 1997 07:43:21 +0200 (IST)
From:	Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
To:	Christoph Kukulies <kuku AT gilberto DOT physik DOT rwth-aachen DOT de>
cc:	djgpp AT delorie DOT com
Subject:	Re: dpmi_int - any caveats?
In-Reply-To:	<199703031534.QAA12308@gil.physik.rwth-aachen.de>
Message-ID:	<Pine.SUN.3.91.970305073149.14421A-100000@is>
MIME-Version:	1.0

On Mon, 3 Mar 1997, Christoph Kukulies wrote:

> > On Mon, 3 Mar 1997, Christoph Kukulies wrote:
> > 
> > > I read djgppfaq.txt chapter 18.2 and as a result I thought I
> > > had it working but the combo of my program and the driver
> > > seems to be very unstable. I cannot invoke the program twice.
[snip]
> #define X(x)  *((char *)__tb) = x&0xff;*((char *)__tb+1)=(x>>8)&0xff;\
>         *((char *)__tb+2) = (x>>16)&0xff;*((char *)__tb+3)=(x>>24)&0xff;
>
>       printf(" %08lx %08lx %08lx\n",
> 	   *((unsigned long *)__tb+0),
> 	   *((unsigned long *)__tb+1),
> 	   *((unsigned long *)__tb+2));

The above two snippets from your program (and their likes) are the source
of your trouble.  You can't treat `__tb' as if it were an address of a C
variable, because it is outside the usual DJGPP address space.  (The FAQ
explains in section 18.4 what does ``DJGPP address space'' mean.) Whereas
a C variable or buffer are referenced using the DS selector (or SS, if
they are on the stack), `__tb' needs a special selector that can access
the conventional memory.  That's what functions like `_farpeekl' and
`_farpokew' are for; you should use them to put data to and get data from
the transfer buffer.  (`dosmemput' and `dosmemget' are other possible ways
of doing this, but in your case farptr functions are more appropriate,
since you only move words or double-words.)

For example, here's the definition of X() that should work (I didn't 
actually test it with your program, though):

	#define X(x) _farpokel(_dos_ds, __tb, (long)(x))

(`_dos_ds' is that special selector that should be used to access 
conventional memory addresses.)

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