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From: grege@gold.GVG.TEK.COM (Greg Ebert)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: 16-bit VGA in an 8-bit slot
Message-ID: <1330@gold.GVG.TEK.COM>
Date: 21 Aug 90 18:58:23 GMT
References: <8960@uhccux.uhcc.Hawaii.Edu> <12306@netcom.UUCP>
Distribution: usa
Organization: Grass Valley Group, Grass Valley, CA
Lines: 15


An AT system will run an 8-bit I/O or memory cycle if -IOCS16 or -MCS16 is
not asserted. When you plug a 16-bit card into an 8-bit slot, the video card
can't yank the -MCS16 line, so the system 'thinks' it's an 8-bit card. Now
the issue is whether or not the card freaks-out.

I accidentally plugged an AST VGA card into an 8-bit slot, but it worked
just fine. In fact, it's designed to do just that.

If you're looking for optimum video performance, you need a system which can
'shadow' the video BIOS into CPU-card-resident RAM. That eliminates wait
states during code-fetches from video BIOS. The bottleneck is the AT bus,
which must run at 8 Mhz. Even a zero wait-state cycle on the AT bus will
result in multiple wait states on, say, a 20 Mhz 386 [not to mention a 33 Mhz
486 :-O ].