Date:	Thu, 4 Mar 1999 20:42:08 +0100 (EET)
From:	Wojciech Zabolotny <wzab AT ise DOT pw DOT edu DOT pl>
X-Sender:	wzab AT wzab DOT nasz DOT dom
To:	Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
cc:	djgpp AT delorie DOT com
Subject:	Re: HELP! Assembly language dual mode interrupt handler for djgpp program
In-Reply-To:	<Pine.SUN.3.91.990301171051.24475E-100000@is>
Message-ID:	<Pine.LNX.3.96.990304202240.490A-100000@wzab.nasz.dom>
MIME-Version:	1.0
Reply-To:	djgpp AT delorie DOT com

On Mon, 1 Mar 1999, Eli Zaretskii wrote:
> 
> > The interrupts rate is so high (up to 20 kHZ), that I have to 
> > install both real mode and protected mode handlers.
> 
> Are you sure?  Did you try to get away with a protected-mode handler 
> alone?  (See section 18.11 of the FAQ for some hints.)  The dual RM/PM 
> setup is a lot of work (see below), so you want to be sure you actually 
> need it before you embark on that journey...
> 
Well, I think I'll try to use just the protected mode handler.

However I still have a problem.
At the begining of my handler's code I have to set the DS register
(I'd like to avoid using the "go32" wrappers, because they used unlocked
memory), so I should start with something like this (in NASM):

	[SECTION .text]
apps_cs		resw	1
irq_handler	push	eax
		mov	ax,[cs:apps_cs]
		mov	ds,ax
	
However I don't know how to fill the "apps_cs" variable. It has to 
be located in the code segment (because at the begining of the interrupt
CS is the only segment register with known contents), but I can not write
to the code segment :-(.
I can create an alias selector with write access but it is a "dirty"
solution.
Is there any documented variable located in code segment which holds the 
DS for my application?
Looking into libc sources I have found "___djgpp_ds_alias",
"___djgpp_our_DS" and ___djgpp_app_DS", but none of them is documented...
Particularly the "___djgpp_app_DS" seems to be very promissing. Is it safe
to use it?

			Thanks for any hints
			Wojtek Zabolotny
			wzab AT ise DOT pw DOT edu DOT pl

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