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From: hammond@petrus.UUCP (Rich A. Hammond)
Newsgroups: net.arch
Subject: Re: VERY LARGE main memories
Message-ID: <292@petrus.UUCP>
Date: Fri, 29-Aug-86 08:05:07 EDT
Article-I.D.: petrus.292
Posted: Fri Aug 29 08:05:07 1986
Date-Received: Sat, 30-Aug-86 03:25:48 EDT
References: <1130@bu-cs.bu-cs.BU.EDU>
Organization: Bell Communications Research, Inc
Lines: 31

> 	Barry Shein, Boston University writes
> ... consider:
> 
> A one MIP machine zeroing memory in a loop:
> 
> 		CLRL	R1
> 	LOOP:
> 		CLRL	(R1)+
> 		CMPL	R1,HIMEM
> 		BNE	LOOP
> 
> would (theoretical machine) take 3 * (1G/1M) or 3000 seconds or
> a little less than one hour to complete. It's hard to believe
> such a machine could make -effective- use of that much memory.

That calculation left out the word size (or were you talking about 1GW
versus 1Gb?)  Thus, for a 32 bit 1 MIPS machine, clearing 1 Gb of memory
would take (3 / 1M) * (1  Gb / 4) = 750 seconds or 12.5 minutes.

Of course, for general purpose computation, Barry's point is correct,
you can't use huge amounts of memory.

However, data base machines benefit greatly from the reduced overhead
if all data is available in memory, even if they don't touch much of it
at any time.

Also, our Convex C-1, when doing vector operations, can touch 8 bytes
every 100 ns, so to zero a 1Gb space  takes (1 Gb / 8) /10M = 12.5 seconds.

Cheers,
Rich Hammond	Bellcore