Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!iuvax!cica!ctrsol!ginosko!uunet!portal!cup.portal.com!Devin_E_Ben-Hur
From: Devin_E_Ben-Hur@cup.portal.com
Newsgroups: comp.sys.ibm.pc
Subject: Re: Memory in Compaq 386
Message-ID: <21803@cup.portal.com>
Date: 3 Sep 89 00:38:25 GMT
References: <1538@draken.nada.kth.se> <6019@pt.cs.cmu.edu>
Organization: The Portal System (TM)
Lines: 27

> In article <1538@draken.nada.kth.se> d88-eli@nada.kth.se (Erik Liljencrantz) 
wr
> ites:
> }like them to be). CEMM is a memory hog. It consumes 33Kb conventional memory
> }and gives me 256Kb EMS memory. It also degrades the performance of the
> }CPU (from 33Mhz to 31Mhz according to Landmark).
> 
> Not surprising, since you have to turn on Virtual-86 mode to be able to 
> emulate EMS on the 386, which means that *every* memory access has to go 
> through the on-chip memory managment unit, which it doesn't have to do in rea
l
> mode.
> 
> -- 
> {backbone}!cs.cmu.edu!ralf   ARPA: RALF@CS.CMU.EDU   FIDO: Ralf Brown 1:129/4
6


Not quite.  In virtual mode, every segment-register load takes extra cycles
for memory management, but normal memory accesses occur at the same speed
as real mode.  Far calls, interrupts, POP <seg>, far returns, MOV <seg>, and
LxS instructions run much slower in virtual mode than real mode, but most
other instruction timings are unaffected.  This means stupid large-model
C code gets hurt far worse than small model or good asm programs.

Devin_Ben-Hur@Cup.Portal.Com
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