Path: utzoo!attcan!uunet!cs.utexas.edu!sun-barr!decwrl!elroy.jpl.nasa.gov!ucla-cs!uci-ics!zardoz!tgate!ka3ovk!teemc!cfctech!pte!kiy
From: kiy@pte.UUCP (Kevin Young)
Newsgroups: comp.sys.ibm.pc
Subject: Re: BIOS Shadowing / Memory usage???
Summary: Less wait states
Keywords: bios shadow memory
Message-ID: <1633@pte.UUCP>
Date: 25 Jul 89 13:13:13 GMT
References: <6541@athertn.Atherton.COM> <45900251@uxe.cso.uiuc.edu> <1491@lzfme.att.com> <6687@athertn.Atherton.COM> <1288@crdgw1.crd.ge.co
Reply-To: kiy@pte.UUCP (Kevin Young)
Distribution: na
Organization: Precise Technology & Electronics, Warren, MI
Lines: 32

In article <1288@crdgw1.crd.ge.com> stenstro@algol.crd.ge.com (Ross Stenstrom) writes:
>So, what IS shadowing anyway...
>Ross Stenstrom                                          
>    stenstro@algol.crd.ge.com (no m!)       uunet!crdgw1.ge.com!stenstrom

Ross,
Shadowing is a technique that increases the effective memory
access speed of software sitting in ROM/EPROM by moving it to
DRAM and then executing it there.  EPROM technology is inherently
slower than DRAM technology and typically requires 2 to 3 wait
states per access depending on system clock speed whereas DRAM
takes 0 to 1 wait states.  The shadow RAM itself sits at the same
memory location as the BIOS EPROM/ROM and is switched by hardware
on the CPU card.  The initialization procedure is to first copy
the contents of the EPROM to low memory, switch the select bit to
enable the shadow DRAM, and then move the copy in low memory back
into the shadow DRAM.  Finally, the write-enable to the shadow DRAM
is disabled, preventing errant programs from erasing its contents.

Shadowing is relatively easy to implement in a PC when using 1MB
memory chips because the memory needed for shadowing is already
available.  It is simply hidden behind the BIOS chips by the address
decoder. 

Hope this helps!
Kevin

-- 
>|< Kevin I Young					uunet!edsews!pte!kiy
    Precise Technology & Electronics, Inc.
    Custom Automated Test and Measurement Equipment for Industry
    Old Saying: "If you can't do it in real time, then don't do it at all" - me