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From: ronc@fai.UUCP (Ronald O. Christian)
Newsgroups: net.arch
Subject: Re: VERY LARGE main memories
Message-ID: <334@fai.UUCP>
Date: Wed, 3-Sep-86 14:29:32 EDT
Article-I.D.: fai.334
Posted: Wed Sep  3 14:29:32 1986
Date-Received: Thu, 4-Sep-86 22:18:40 EDT
References: <2017@sdcsvax.UUCP> <884@gilbbs.UUCP> <145@mn-at1.UUCP>
Reply-To: ronc@fai.UUCP (Ronald O. Christian)
Organization: Fujitsu America, Inc.
Lines: 34
Keywords: memory size

In article <145@mn-at1.UUCP> alan@mn-at1.UUCP (Alan Klietz) writes:
>In article <884@gilbbs.UUCP>, mc68020@gilbbs.UUCP (Thomas J Keller) writes:
>> 
>> [...]  The primary
>> impediment seems to be the delays caused by propagation delays in the
>> decoding trees.   Anyone care to enlight me (us)?
>> 
>
>If you use DRAMs you have access times on the order of 50-200ns.  That is
>enough time for fast ECL-type logic to do plenty of decoding.

I wonder about this.  I had a design problem awhile back involving 45ns
ram used as a lookup table, where I couldn't decode the address fast enough.
I found some ECL gates that would do the decoding faster, but then found to
my chagrin that you lost so much time in the conversion from ECL back to TTL
that the overall response ended up being slower.  I've had the same problem
in gate arrays, where the propagation delay through the on-chip conversion
to ECL, the array itself, and the conversion back to TTL was greater than
the prop delay through a straight TTL array of similar complexity.  If
you're going ECL for decoding, I think you really need to have ECL memories
to gain anything.

Then there's GaAs...  So fast you can spend a lot of time converting
to a different logic family.  I like GaAs.  Expensive, though.


				Ron
-- 
--
		Ronald O. Christian (Fujitsu America Inc., San Jose, Calif.)
		seismo!amdahl!fai!ronc  -or-   ihnp4!pesnta!fai!ronc

Oliver's law of assumed responsibility:
	"If you are seen fixing it, you will be blamed for breaking it."