Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!texbell!attctc!chasm
From: chasm@attctc.Dallas.TX.US (Charles Marslett)
Newsgroups: comp.sys.ibm.pc
Subject: Re: What are the differences between 8-bit and 16-bit VGA cards?
Summary: I agree, but I don't :^)
Message-ID: <10403@attctc.Dallas.TX.US>
Date: 29 Nov 89 15:12:52 GMT
References: <6980@portia.Stanford.EDU> <340030@hpsgpa.HP.COM> <7025@portia.Stanford.EDU>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 51

In article <7025@portia.Stanford.EDU>, dhinds@portia.Stanford.EDU (David Hinds) writes:
<> In article <340030@hpsgpa.HP.COM>, plim@hpsgpa.HP.COM (Peter Lim) writes:
<> > That doesn't sound right. As far as I know, the 16-bit access thing
<> > is done totally in hardware. When a 16-bit card is plugged into an
<> > AT-bus, it tells the AT bus system that it it 16-bit and the system
<> > will read/write in 16-bit...
<> 
<>     For the ATI VGA Wonder card, the bus width seems to be at least partially
<> under software control.  There is an option in the 'VSETUP' program to enable
<> or disable 16-bit ROM accesses.  The card is actually supposed to work in
<> either 16-bit or 8-bit slots; in the 8-bit slot, the extra pins just hang in
<> . . .  Aside from the
<> ROM accesses, it is not obvious that any software would use 16-bit accesses
<> to video memory, or what they would actually do.  In the true VGA graphics
<> modes, at least, almost all video memory accesses are indirect, and data is
<> churned through a series of arithmetic units on the VGA controller before
<> being put in video memory.  These steps map a 32-bit internal VGA memory bus
<> onto an external 8-bit bus, using various VGA registers to control different
<> kinds of rotating, masking, and logical operations.  It would not be trivial
<> to redesign these units to map to a 16-bit bus.  For one thing, lots of VGA
<> graphics control registers would need to be 16 bits long to be useful.  There
<> is no obvious way to make these extensions transparant and compatible with
<> existing VGA software.  And certainly, all existing VGA graphics software 
<> uses 8-bit accesses.

Actually, I agree so long as we are refering to 16-color modes.  The VGA (and
SuperVGA) 256-color graphics modes and the text modes cannot take advantage of
the latches and internal EGA/VGA logic box, so they can run (and in the case of
the text modes, are almost certainly programmed in the BIOS to run) in 16-bit
mode.  So standard software CAN benefit from 16-bit video memory accesses --
it just has to be in modes 0-3, 7 or 19 (though sometimes modes 4-6 benefit, the
controller chip is a factor here).

<> . . .  Proper
<> VGA software should never do direct accesses to video memory, so as far as I
<> can see, a wider bus won't do anything.  I would like to be wrong, though (?).

Of course, someone has to access video memory directly (I've written drivers
that don't, and the screen remains remarkably bland when I run them ;^).

I would say that the only real benefit 16-bit video access provides shows up
when you run 256-color software (especially in the high resolution modes).  Text
is fast enough already, and I never use the CGA modes (almost never?).  But
a 2:1 speedup is nice when a screen clear takes a significant fraction of a
second.

<> - David Hinds
<>   dhinds@portia.stanford.edu

Charles Marslett
STB Systems, Inc.  <-- apply all standard disclaimers