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From: phil@cs.mcgill.ca (Philip LOCONG)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Dual monitors
Message-ID: <1991Feb12.010248.7563@cs.mcgill.ca>
Date: 12 Feb 91 01:02:48 GMT
References: <166@cf_su20.cf_su10.Sbi.COM> <26810@uflorida.cis.ufl.EDU> <8130@davidsys.com>
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Organization: SOCS, McGill University, Montreal, Canada
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In article <8130@davidsys.com> douglass@davidsys.com writes:
>
>Also, I have done some testing of some systems I have available and
>found no difference in performance to my 16-bit VgaWonder with an
>IBM monochrome adapter in or out.
>Testing indicates (on *my* system, YMMV):
>16-bit VGA fastest (only text mode tested, of course)
>8-bit M[DG]A (or CGA) takes twice as long to write to as 16-bit VGA
>IBM EGA (8-bit) three times as long as 16-bit VGA 
>
>These results gathered on 6 and 8 MHz 80286 AT clones, 16 MHz Compaq 386,
>16 and 33 MHz 80386 clones.
>Test consisted of an assembly program that moved screenfuls of data
>from main memory to the screen 2000 times and counted timer ticks.
>
>Sorry to run on so long, but I just can't understand why an 8-bit
>card should slow down a 16-bit card!?!   At least it doesn't in my case.
>

As earlier posts suggested it, the ISA bus specifications force the bus
to run each 128k section of RAM entirely in 8-bit mode or entirely in
16-bit mode, that means the A-B section has to be either 8 or 16-bit.
E[V]GA uses A000-B000 and B800-C000 for RAM while the MDA uses
B000-B100. This means if you have both, the RAM will interface with
the bus in 8-bit mode. And then, there is also the fact that many
16-bit VGA cards (not all) have an internal 8-bit data path which
becomes the actual bottleneck wether you use 8 or 16-bit mode. On
the other hand, an original IBM monochrome adapter uses 4k of
static RAM and has a data rate of 1.8 M bytes/sec. This will give
the VGA cards of today a much better performance than MDA even when
run at 8-bit. Then there's also the BIOS in the C-D section...

Philippe Locong
phil@bart.cs.mcgill.ca