Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!utastro!bigtex!james
From: james@bigtex.cactus.org (James Van Artsdalen)
Newsgroups: comp.sys.ibm.pc.misc
Subject: Re: Need info: Slow I/O in protected mode
Keywords: Protected mode, QEMM, I/O
Message-ID: <48840@bigtex.cactus.org>
Date: 17 Oct 90 08:58:10 GMT
References: <4925@tuminfo1.lan.informatik.tu-muenchen.dbp.de>
Reply-To: james@bigtex.cactus.org (James Van Artsdalen)
Distribution: comp
Organization: Institute of Applied Cosmology, Austin TX
Lines: 37

In <4925@tuminfo1.lan.informatik.tu-muenchen.dbp.de>,
	lenz@charly.bl.physik.tu-muenchen.de (Udo Lenz) wrote:

>  I'm developing a time critical application for an interface card,
> from which a lot of data is read via assembly 'in ax,dx' commands.
> When benchmarking the I/O routines on a 20 Mhz 386, I've found out,
> that the 'in ax,dx' command takes roughly a microsecond longer if I
> have installed Quarterdesks QEMM memory manager. [...]

> The assembly reference manual says though, that the timing of the
> 'in ax,dx' command in protected mode is as fast as in real mode,
> except if encurring a protection fault.

That's not how I interpret the Intel book.  It indicates to me 27
clocks in virtual 8086 mode.

> My questions are:

> What is the reason for the delay in the 'in ax,dx' command when using
> QEMM?

Lots of microcode, and I presume a fetch from the I/O permission bitmap.

> Am I really in protected mode when installing QEMM as a device driver
> in config.sys?

Yes.

> If in protected mode, why do my old assembly routines, which do segment
> register arithmetic and other dirty things, still run?

Because you're in virtual 8086 mode.  Read the Intel book: virtual
8086 mode can be made to act like real mode, but it's still virtual
mode timing.
-- 
James R. Van Artsdalen          james@bigtex.cactus.org   "Live Free or Die"
Dell Computer Co    9505 Arboretum Blvd Austin TX 78759         512-338-8789