Date:	Fri, 9 Aug 1996 14:14:36 +0200 (DFT)
From:	"Garrido Freire, Fco. Javier ((R)JA.GAR. SOFT)" <garridof AT cs DOT us DOT es>
Reply-To:	"Garrido Freire, Francisco Javier" <Francisco-Javier DOT Garrido AT cs DOT us DOT es>
To:	Eli Zaretskii <eliz AT is DOT elta DOT co DOT il>
cc:	Orlando Andico <orly AT gibson DOT eee DOT upd DOT edu DOT ph>, djgpp AT delorie DOT com
Subject:	Was: Re: Multitasking in DOS
In-Reply-To:	<Pine.SUN.3.91.960627091350.9882L-100000@is>
Message-ID:	<Pine.A32.3.93.960809131932.11912C-100000@nervion.cs.us.es>
Organization:	Infoalum Mail Gateway <Infoalum AT cs DOT us DOT es>
MIME-Version:	1.0

On Thu, 27 Jun 1996, Eli Zaretskii wrote:

> 
> On Thu, 27 Jun 1996, Orlando Andico wrote:
> 
> > have done that. Your biggest problem is: the *&&%$%%^ non-reentrance of
> > DOG itself (although the BIOS is more robust..) which precludes a
> > multithreaded filesystem! what's "multitasking" without it?
> 
> Actually, that might not be as hard as it sounds, at least under native 
> DOS.  DOS has an internal data area known as SDA (Swappable Data Area) 
> which holds all the data structures that make it non-reentrant.  All you 
> need to do is to swap this area in and out on the task switch, and DOS 
> will never know it was re-entered.  The ``Undocumented DOS'' book has a 
> full chapter on this technique.
> 
> The catch, of course, is that it won't work with Win3.11 or Win95 with 
> 32-bit file access enabled, because then all file operations completely 
> bypass DOS, and I don't know what will such a multitasking app in a DOS 
> box do on these systems.  But under Win 3.1, or when 32-bit file access 
> is disabled, this will probably work on Windows too.
> 
I think so that it's not as hard as sounds. However, I don't
think SDA has to be necesarily swap in/out. In my demo I didn't
need swap anything at all. Well, perhaps if you don't want to have
a process waiting for 1/2 second until another's OPEN syscal ends,
maybe.

PS: After rereading Undoc DOS's chap 5..humm...This is something
to keep in mind, after all.

Thanks, Eli, for your moral support :)

PS2: I enclose my demo attached. Thanks, again, for your
comments.
-------------------------------------------------------------------
F. Javier Garrido F.   (R)JA.GAR. SOFT    Dpto. de Computacion e IA
MINIX User. (USENET comp.os.minix & SCS)  Facultad de Informatica
Email: Francisco-Javier DOT Garrido AT cs DOT us DOT es  y Estadistica de Sevilla
URL: http://www.cs.us.es/~garridof                       (SPAIN)
"The light that burns twice as bright, burns half as long ...
   ... and you have burned so very, very brightly, Roy."
					  TYRREL from Blade Runner

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