Path: utzoo!utgpu!cunews!bnrgate!bwdls58!mlord
From: mlord@bwdls58.bnr.ca (Mark Lord)
Newsgroups: comp.sys.ibm.pc.misc
Subject: Re: Is DOS 5.0 really so slooow!
Message-ID: <7153@bwdls58.bnr.ca>
Date: 25 Jun 91 19:50:40 GMT
References: <1991Jun20.223605.18977@IRO.UMontreal.CA> <1991Jun23.034521.7819@galileo.cc.rochester.edu> <8043@spdcc.SPDCC.COM> <91174.112235DSB100@psuvm.psu.edu>
Sender: mlord@bwdls58.bnr.ca
Reply-To: mlord@bwdls58.bnr.ca (Mark Lord)
Organization: Bell-Northern Research, Ottawa, Canada
Lines: 49

In article <91174.112235DSB100@psuvm.psu.edu> DSB100@psuvm.psu.edu (David Barr) writes:
<In article <8043@spdcc.SPDCC.COM>, jin@spdcc.COM (Jerry Natowitz) says:
<
<>In article <1991Jun23.034521.7819@galileo.cc.rochester.edu>
<>gwni@troi.cc.rochester.edu (G. Wayne Nichols) writes:
<>>My experience indicates that any use of HIMEM.SYS will dramatically
<>>slow down the Landmark rating, if not your actual system.
<
<>I have played around with various configurations and found that EMM386 seems
<>to be the culprit - my apps are runjning in protected mode when I use it for
<>upper memeory access.  I don't even have to do any devicehighs or loadhighs,
<>just having emm386 noems is enough.  Maybe I'll try removing himem for laughs.
<
<Yes, if you think about it, it is obvious.  EMS memory is _paged_ memory.
<That means when an application requests memory that is not in the 64K page
<frame, the page is swapped out, and another is brought in.  This is a
<ludicrous system for managing memory, but for some stupid DOS programs,
<this is required, even on a 386.  XMS is faster than EMS because it is
<a direct access system.  If an application needs a memory value in XMS,
<it merely addresses it and reads it.  I never use EMS unless I want to
<slow my system down.  =)

Excuse me while I roll in laughter!

Dave does have a good point in that applications which use EMS may indeed
run slower than those which do not, but not always.. XMS applications spend
a lot of time copying data between the lower 1Mb and the extended memory
above 1Mb.  EMS applications merely ask the EMS manager to flip some control
bits to remap 16K pages into the lower 1Mb.  On a system with real EMS hardware,
or on a 386 system with real remapping capability, this operation does not
require that large amounts of memory be copied back and forth.. just the page
bits get written to (notice that disk caching programs such as PC-KWIK 
recommend using EMS instead of XMS for the cache if you like to run your
serial ports at high speeds).  

However, this point is irrelevant.  Having EMS present on the system will 
normally not affect applications which do not use it!!  In the case above,
using EMM386 to "simulate" EMS is the culprit, not EMS or EMS applications.
EMM386 places the 386/486 CPU into Virtual-8086 mode, which generally slows
things down slightly, and specifically slows interrupts down more than slightly.

This is more likely the real cause of the slight performance hit.
-- 
MLORD@BNR.CA  Ottawa, Ontario *** Personal views only ***
begin 644 NOTSHARE.COM ; Free DOS 4.xx utility - use instead of SHARE.EXE
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