Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!usc!samsung!uunet!systech!dan
From: dan@systech.bjorn.COM (Dan Gill)
Newsgroups: comp.arch
Subject: Re: Memory speed, why so slow?
Summary: don't mess with technology
Keywords: RAM,cache,memory
Message-ID: <902@systech.bjorn.COM>
Date: 10 May 91 15:57:27 GMT
References: <9245@idunno.Princeton.EDU>
Organization: Systech Corp., San Diego, CA
Lines: 53

In article <9245@idunno.Princeton.EDU>, ssr@stokes.Princeton.EDU (Steve S. Roy) writes:
> I have a question for those out there who are in the know about
> dynamic RAM.
> 
> Is it my imagination or is it true that while the storage capacity of
> dynamic RAM chips has increased by orders of magnitude the speed has
> not?  Every workstation (or PC for that matter) depends heavily on a
> cache of some kind, and for many applications the limiting speed is
> not the cpu but the main memory.

Well, the saga of DRAM's has been going on for sometime.  DRAM's have actually
decreased in speed by a large amount.  There was a day when I was using
4Kx1 Dynamic memories that ran at 450ns.  I still have some just for 
historical sake :-).   You might be glad that you don't have to be using
core memory cards that get you 16K or maybe 64k on a board the size of a
9U VME board.

There has always been a paradox between Sram and Dram.  Sram is faster.  Stated
fact.  You can get sub-10 ns Sram's today.  They are expensive and run HOT, 
but you can get them, but the capacity is not to terrific.  It takes several
transistors to make a cell of storage in an Sram.  Maybe you'll see
256K or so per part.  It is just not efficient to make mass storage out of
Sram's.  Well, it was in the 6502 days.

Dram's on the other hand are high capacity parts, and lower pin counts
than much smaller capacity Sram's.  Dram's are able to mux the address bus
from 20 pins to 10 pins, thus making them smaller,  but the fastest Dram's
are around 60 ns.  These are 1Mb and 4Mb parts.  Dram's are just an array
of capacitors.  Not a huge array of transistors.  You can cram a bunch
more caps into a phonebooth than transistors.  But Dram's need to be
refreshed, after all caps don't remember forever :-(.

Dram vendors have been able to increase the capacity of them because of the
mere fact that more stuff can be crammed into a package.  The fact that speed
has been increased has also been a fallout of the same thing.  

No, 60ns is not fast enough.  Vendors have come up with nibble and page-mode
operations to speed Dram's up to sub 20ns, but this assumes either sequential
accesses or small ranges of addresses.

The world is just one big trade-off.  Either you can have speed or you can
have capacity.  What workstation guys do then is to meet in the middle by
making huge banks of Dram at a relatively low speed (say 80-100ns) and stick
a fairly large cache beside it at a blazing speed (say 15 ns) and rely
on one to help the other.

Maybe one day the Dram will run at 20 ns sustained, but then the 100mhz sparc
will be out and, well here we go again 8-).
-- 
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"On second thought, let us not go to Camelot.  It is a silly place"
Dan Gill                                          uunet!systech!dan
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