Path: utzoo!utgpu!water!watmath!clyde!att-cb!osu-cis!tut.cis.ohio-state.edu!husc6!uwvax!puff!avery
From: avery@puff.cs.wisc.edu (Aaron Avery)
Newsgroups: comp.sys.amiga
Subject: Re: 68030-based PC's
Message-ID: <1519@puff.cs.wisc.edu>
Date: 29 Mar 88 12:52:27 GMT
References: <1505@puff.cs.wisc.edu> <3540@cbmvax.UUCP>
Distribution: na
Organization: U of Wisconsin CS Dept
Lines: 27

In article <3540@cbmvax.UUCP>, daveh@cbmvax.UUCP (Dave Haynie) writes:
> The '020 usually ends up with a 4 clock memory cycle if you've got that MMU in
> there.  But the '030 also has a neat 1-clock burst mode; the CPU can't eat
> memory at that speed, but it can be used for cache fills and things like that.
> It does require some external circuitry and certainly some design cleverness,
> though the 2-clock memory cycle is going to require some design cleverness as
> well if you aren't willing to spend a small fortune for ridiculously fast
> memories (at the price of memory chips these days....).
> 
> Dave Haynie  "The B2000 Guy"     Commodore-Amiga  "The Crew That Never Rests"
>    {ihnp4|uunet|rutgers}!cbmvax!daveh      PLINK: D-DAVE H     BIX: hazy

True, the '030 has burst mode, and no, it doesn't really require any more
external circuitry then normal dynamic ram design stuff. But, that nibble
mode memory which supports said burst mode is not as cheap as "standard" DRAM.
And I say that if you're designing such a high-end product, you should go all
out. You'd want to have at least a 64k external cache running at a 2-clock
memory cycle, and nibble mode dynamic ram supporting the faster burst-mode
cache fill, to counteract the slower random access speed. You would need to
add some extra design cleverness to accomplish the compatibility between the
two memories, as the cache won't be able to write those 4 burst-mode bits
as fast as they're fetched, and probably some others. I believe that this
type of design running at 20 or 25MHz with a 68882 (32-bit interface, of 
course) would create a very impressive amiga system, don't you?

Aaron Avery (avery@puff.cs.wisc.edu)
	    ({seismo,caip,allegra,harvard,rutgers,ihnp4}!uwvax!puff!avery)