Subject: Video subsystem...

From: Patrick Mackinlay <[email-masked]>
Date: Fri, 25 Jun 1993 22:47:52 +0800 (WST)

>[...comments from Drew about video bandwidth deleted...]
>
>This is a good point; this certainly points to using VRAM.  At present there
>are far too many unknowns about the video section to really address this
>issue.  Someone needs to do some research as to what cheap video stuff
>there is.  Using the S3 chip (as some one suggested) may be a good idea,

When I asked about S3 chips, I was told that they were all SMT with no
plans to do PGA or other packages. The other suggestion was to use the
6845 and the 3730's video stuff to do a very simple framebuffer solution.
As Drew points out, this would seem to suck up a lot of the available
memory bandwidth. There are a couple of other suggestions that might be
worth looking at. Firstly, to do the dumb framebuffer option, but use
VRAM. This would require a (relatively simple) bit of logic to clock
dots out of the VRAM at the right rate, and I don't think the 6845 is
capable of doing this.

The other option that I think has some merit is to use the TMS34010.
This chip used to be pretty expensive, but I think it's dropped a lot
since the last time I looked. The advantage with this chip is not so
much speed, but simplification of the overall design. Depending on
the price of the actual device, I think it could be one way to make
the video section pretty cheap. I've been told by some other guys
that it's quite easy to deal with as far as integration with the main
CPU is concerned (that's what it's designed for, after all!). I have
a few concerns that it might mean putting another ROM on the board,
but that might be cheaper and easier than other solutions (I think
it's got dynamic bus sizing stuff on-board in any case).

Any comments on this? Anyone have pricing info on the 34010?

BTW: There's a possibility the ET4000 or other VGA chips come in
non-SMT versions, but I'd personally prefer to stay away from them.
As far as I can see, the best way to slow a video system down is
to stuff it through a VGA chip...

Pat -- "There's only one thing left to do Mama, I got to ding a ding dang
	my dang a long ling long" (Jesus Built My Hotrod -- Ministry)
GCS d* -p+ c++ l++ m--- s+/- !g w- t- r

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From: Stephen Siu-ming Wong <[email-masked]>
Date: Sat, 26 Jun 93 14:41:46 HKT

Hi Risciers,

>it's got dynamic bus sizing stuff on-board in any case).

I strongly opt for TMS34010/020 solution, as they are CPU and any
one of them will greatly enhance the performance of X.  There
shouldn't be an extra ROM, every thing should be boot strap from
the main CPU, just add some extra object code file on the main
CPU EPROM, and let the main CPU to initial load the TMS CPU.  The
initial TMS code shouldn't be very complicated, it should handle
the most basic text mode only, and let linux (or whatever OS) to
put some still advanced (and large) code on the TMS CPU.

Moreover, VRAM solution should give the best performance (Video
wise and system wise).  2-4MB should be enough for TMS code +
cache (font/pixel/etc) + frame buffer (assume 1024x768x256).

I don't mind if opt for less cost, choose TMS34010, although it is
not the best performer, but it is really cheap.  Of course, it will
be the best to be able to swap in a TMS34020 by users.

>to stuff it through a VGA chip...

Please, DON"T consider ET4000 chip or TVGA or whatever VGA chips, they
are brain damage.  They are designed for the limited ISA bus and Intel
segment architecture!  If we want on-board video, why should we deal
with such inferior technology!  I want a real (and fast) video system!

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From: [email-masked] (Jeremy Fitzhardinge)
Date: Mon, 28 Jun 1993 13:00:38 +1000 (EST)

Patrick Mackinlay bubbles:

Just a comment:  My preference for onboard video would be a simple frame
buffer using vram.  It would be nice if we could get the chip described
in the "the interesting dec paper" (the thesis was that smart video chips
are an anachonism, and are effectivly useless when tied to a fast risc
chip -- mainly because they are always a generation of speed behind
the cpu, and the cpu can saturate the memory bandwidth anyway).

In any case, it is much more important to have the video memory directly
and completely accessable by the CPU than have limited access and have a
"smart" chip.

> Any comments on this? Anyone have pricing info on the 34010?

The TMS34010 is *slow*.  It only has a 16 bit memory data bus, which has
a max transfer of about 6MB/s at 40MHz.  This is because it is a very
CISCy chip, with a not particularly fast memory interface (partly because
all addressess are bit addresses, and the hardware is more complex).
The 34020 is 32bit and probably much faster, but I think it is an
unnecessary complication.  It would make the video inaccessable from
the cpu (unless you want shared memory, ugh), making it much harder
to program.  Neil almost certainly has his own views on this :-).

Neither chip would require another rom on the board.