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Re: MIPS R3000 hardware design specification revision 1

To: drew@nag.cs.Colorado.EDU (Drew Eckhardt)
Subject: Re: MIPS R3000 hardware design specification revision 1
From: caret@pyramid.com (Neil Russell)
Date: Wed, 23 Jun 93 15:02:45 PDT
Cc: riscy@pyramid.com
In-reply-to: <199306231347.AA00538@nag.cs.Colorado.EDU>; from "Drew Eckhardt" at Jun 23, 93 7:47 am
Drew Eckhardt writes:

> Assuming that "average" users will be using something like 1024x768x256
> at 60hz, you'll be DMAing  45M a second.  Some people have problems at 
> < 72hz, which is 54M bytes a second.  Up the resolution to 1280x1024x256
> and you're looking at 90M/sec.
> 
> With 70ns DRAM, assuming that we are 2-way interleaved we can't get more
> than about 100M/sec.  So, we'd loose between 50 and 90 percent of our 
> memory bandwidth, our biggest performance bottleneck, if we put main
> memory on the same bus.

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,
if we can get it in a J-lead or PGA package.  (See comments on surface
mount in the spec).

> Can the IDT 79R3730 handle DRAM on one of the other busses?  

Yes, but what I expect that your thinking is to put DRAM on another
bus to releive contention on the main bus.  That is not possible.
We should use VRAM for that.

> How expensive is VRAM?  Since it's inherently dual ported, you get 
> a lot more CPU->video memory bandwidth than you do when you're trying
> to share DRAM with video serializers that want 45-90M/sec, making 
> bitblits (ie, text drawing, the moving of windows, etc). 

I think its 3-4 times the cost of DRAM; once again I'm guessing...

-- 
Neil Russell            (The wizard from OZ)
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