Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!ucsd!ogicse!zephyr.ens.tek.com!gvgpsa!gold.gvg.tek.com!grege
From: grege@gold.gvg.tek.com (Greg Ebert)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Shadowing ROM and Memory speed
Message-ID: <1792@gold.gvg.tek.com>
Date: 12 Dec 90 17:14:14 GMT
References: <2147@meaddata.meaddata.com>
Organization: Grass Valley Group, Grass Valley, CA
Lines: 33


 gregs@meaddata.com (Greg Smith) writes:

>[...]
>I have 1Mb of memory and would like to find some software 
>to shadow my ROM and see if that helps.

Shadowing requires special hardware, as well as some customizations to
your BIOS, which will be supplied by the hardware manufacturer.

Here's how it works: Assuming shadowing is enabled, the system and video
BIOS code is copied into RAM which has the same physical address as the
BIOS. After copying, the RAM is write protected (Gee, I wonder why :-] )
and then all memory-accesses to the BIOS addresses are routed to shadow
RAM, instead of going out onto the system bus. This is where the speedup
occurs: Bus cycles are *required* to run at a minimum of 250nsec, whereas
a local-memory access is only a fraction of this (ie, < 100nsec).
>
>Can you 
>increase the performance by buying faster memory?

Generally, no. Memory cycles must be run in an integral number of wait
states, and the number of wait states is usually coded into state-machines
surrounding the processor. Before you flame me, I'm only talking about
memory which is coupled to the processor *without* the system bus.

----- Boycott redwood products ---------------------------- Recycle -----
                                                                 #####
{uunet!tektronix!gold!grege}     Register to vote, then        ##  |  ##
grege@gold.gvg.tek.com           vote responsibly             #    |    #
							      #   /|\   #
Support high oil prices, waste tax $$ on war, evade domestic   #/  |  \#
problems, and die young on foreign soil- Just say YES to Bush   #######