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From: erik@westworld.esd.sgi.com (Erik Fortune)
Newsgroups: comp.sys.ibm.pc.hardware
Subject: Re: Mentor & Trident VGA cards - which one is the best ?
Message-ID: <1991Apr11.035905.23838@odin.corp.sgi.com>
Date: 11 Apr 91 03:59:05 GMT
References: <1991Apr3.071713.6290@fel.tno.nl> <3725@d75.UUCP> <1991Apr10.100931.27295@Informatik.TU-Muenchen.DE> <1991Apr10.230915.18440@odin.corp.sgi.com>
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Reply-To: erik@westworld.esd.sgi.com (Erik Fortune)
Organization: Silicon Graphics Inc.
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Re: linear addressing mode on the ET4000
Aha.  Found it!

Reprinted without permission from:

Programmer's Guide to the EGA and VGA cards, Second Edition.
by Richard E. Ferraro
Published by Addison-Wesley
p. 932

----- beginning of excerpt

20.2.3 Linear or Segmented Addressing

The VGA can be configured to access memory in a linear or in a
segmented fashion through the SLS field.   The linear addressing
mode forces all display memory to be continuous.   This is a
much more efficient way to utilize the ET4000.   If the SLS Field 
is 1, this linear mapping scheme is enabled.   If 0, the
standard VGA segmented memory mapping scheme is enabled.

In extended DOS or OS/2 applications, it is possible to place
the VGA into high memory where there is room t place one Megabyte
of display memory into a contiguous address space.   This allows
the ET4000 to ignore the bank selection registers.   There are
two advantages to this approach.   The first ensures faster
programs; the second ensures continuous high-speed operation
from a hardware perspective.

The software advantage requires and extended operating system
and access to the memory through the 80286, 80386 or 80486
processors.   The software advantage becomes pronounced when 32-bit
instructions can handle the one megabyte without segment registers.

The hardware advantage is primarily aimed at video frame-grabbers
where the hardware has a continuous stream of data coming at
the ET4000 display memory, and the ET4000 cannot afford to
perform bank switching.

---- end of excerpt

One advantage of the ET4000 I forgot to mention before is
the ability to use VRAM (which allows for much faster access
to the frame buffer).   Of course, this advantage is purely
theoretical unless the VGA card you buy is engineered for
VRAM and it's extremely unlikely that any of the inexpensive
ET4000 based clones would be.   Does the prodesigner II use
vram?  Does anyone know of any (ET4000 based) vga cards that do?

On the topic of vram-based vga adapters -- does anyone know
anything about the video-7 VRAM-II?   I claims to support 1 meg
using a video-7 proprietary chip (at least the ASIC is labelled
"headland technologies").

I double checked the trident info.  Old mode is 128k segments
(with ugly segment registers), and new mode is 64k segments
(with a single less ugly segment register).   You get into
old mode by writing to the hardware version register (a read-only
register!) and toggle modes by reading the version register.
How bizarre.

I also forgot about the inverted page bit in one of the trident
segment registers.   Also bizarre and apparently pointless.

All in all, I think the ET4000 just feels "cleaner" to me
than the T8900.

-- Erik