Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!rutgers!apple!sun-barr!newstop!texsun!pollux!attctc!chasm
From: chasm@attctc.DALLAS.TX.US (Charles Marslett)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Is Bios shadowing and multiple 16-bit cards a problem?
Summary: Usually BIOS shadowing helps...
Keywords: BIOS shadowing 16-bit cards
Message-ID: <8635@attctc.DALLAS.TX.US>
Date: 13 Jul 89 13:32:53 GMT
References: <15099@ut-emx.UUCP>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 46

In article <15099@ut-emx.UUCP>, allred@ut-emx.UUCP (Kevin L. Allred) writes:
> I recall reading some where about confilicts between the BIOS's of two
> 16-bit cards when using RAM shadowing.  Since I'm getting a system the
> supports BIOS shadowing, and I may get a 16 bit VGA card and a 16 bit
> HD controller, I want to make sure I won't end up causing myself
> grief.  Can anyone tell me whether there might be a problem or not?

The shadowing of the BIOS actually helps if it is done right, because the
real problem arises when a 16-bit access is made to the BIOS of one card
(with an 8-bit BIOS interface) and the other card jumps on the bus and
tells the motherboard hardware that the addressed card is a 16-bit card
(this is a lie, understandable when you look at the bus timing to do real
16-bit transfers at bus speeds greater than 6 MHz, but still a lie).

If the shadowing is done by copying one byte at a time on boot up, all cards
should be compatible with each other (except for the really broken ones ;^).
If shadowning is done by copying one word at a time (oops!), it may make no
difference, or it may exacerbate the problem.

To avoid the problem entirely, just make sure that all the cards you use have
either an 8-bit BIOS or a 16-bit BIOS, then it doesn't matter if the addresses
get confused (when the 16-bit transfer line is yanked, that is).  Here you
may have a problem though -- Tseng based VGAs look like they have 16-bit
BIOSes, but they don't.  STB RapidMeg EMS cards can run in 8-bit, 16-bit,
or dynamic modes (depending on the mother board implementation and other
cards resident in the C0000-DFFFF area).  Video 7 VRAM cards usually have
16-bit BIOS access, but on Everex boxes they run "fast" 8-bit accesses
and occasionally crash the floppy (figure that one out!).  Some 16-bit
disk controllers use a BIOS, some don't, etc.  The best bet is to make sure you can
return anything you buy, and have a viable 8-bit and 16-bit version of
each.  Mix and match (you may strike it lucky, or the BIOS shadowing code
may be done right) and it will work the first time!

Charles Marslett
> -- 
> 
> 	Kevin Allred
> 	allred@emx.cc.utexas.edu
> 	allred@ut-emx.UUCP


===========================================================================
Charles Marslett
STB Systems, Inc.  <== Apply all standard disclaimers
Wordmark Systems   <== No disclaimers required -- that's just me
chasm@killer.dallas.tx.us