Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!ucsd!swrinde!cs.utexas.edu!uunet!zephyr.ens.tek.com!tekcrl!tekgvs!toma
From: toma@tekgvs.LABS.TEK.COM (Tom Almy)
Newsgroups: comp.sys.ibm.pc
Subject: Re: What are the differences between 8-bit and 16-bit VGA cards?
Keywords: VGA
Message-ID: <6419@tekgvs.LABS.TEK.COM>
Date: 28 Nov 89 17:02:53 GMT
References: <6980@portia.Stanford.EDU>
Reply-To: toma@tekgvs.LABS.TEK.COM (Tom Almy)
Distribution: usa
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 30

In article <6980@portia.Stanford.EDU> dhinds@portia.Stanford.EDU (David Hinds) writes:
>
>    I have an ATI VGA Wonder card, and I'm confused about what the 16-bit
>bus actually does for me. 

Well it can be a sure win for character modes since characters are actually
represented as 16 bit entities (8 bits of f/b color and 8 bits for the
character). My NNANSI.SYS driver takes advantage of that. And if the card
happens to be an 8 bitter then the bus hardware conveniently does the two
8 bit accesses for me (but slower, of course).

Yes, the benefit is fuzzier for 16 color graphic modes.  Use of the 
graphics controller is limited to 8 bit operations, but for applications
that want to read and write to each individual bit plane directly 16
bit operation is possible and works faster. I worked on an application
for which graphic operations were done in memory to a "shadow" display, then
the information was copied from the shadow display to the real display
memory (in regions that had changed). Not only did using a 16 bit card
help, but it turned out using the shadow display offered a several times
performance improvment compared to similar programs that didn't use the
shadow display -- on an 80386 you can access system RAM over 10 times
faster than display memory, therefore *avoid* all display memory accesses.
In fact, the display memory is treated write-only, even when XORing pixels
to the screen!

Tom Almy
toma@tekgvs.labs.tek.com
Standard Disclaimers Apply 
(And even if I could, I'd never want credit for any of these turkey 
 display controller designs!)