Path: utzoo!attcan!uunet!yale!husc6!cfa!ward
From: ward@cfa.harvard.EDU (Steve Ward)
Newsgroups: comp.arch
Subject: Re: History of personal computing (LONG
Summary: oops, the devil made me do it, DRAM's
Message-ID: <1077@cfa.cfa.harvard.EDU>
Date: 10 Aug 88 22:23:17 GMT
References: <5946@venera.isi.edu> <46500024@uxe.cso.uiuc.edu> <1075@cfa.cfa.harvard.EDU>
Organization: Harvard-Smithsonian Ctr. for Astrophysics
Lines: 41


 
Well, DRAM's used to work that way 15 years ago!

Seriously, for some reason (the devil made me do it) I was thinking of
those old, very dynamic, CCD-style memory devices.  The absence of a
clock line on current DRAM's should make one suspicous as to its true
nature.

Present DRAM's utilize a static X-Y array of capacitors read via sense
(comparator) amps.  There is one capacitor for one bit.  The capacitor
charge is renewed during the read cycle.  (isn't this a logical shift
register of length 1? :-) )  The read cycle is clearly the place and
time to do this since one has to know what to refresh.

Back to the original topic, magnetic cores used a destructive readout
method that inherently destroyed in absolute terms the stored information.
This may be splitting hairs, but I don't think that this is the case
with reading a capacitor charge via a comparator.  Allright, maybe
in practical terms the charge is emaciated to the point of unreliability
without immediate refresh.  Though I had the wrong device, my point
was that the readout process wasn't inherently destructive in the
way it is for magnetic cores.  I still have my flame retardent undies
on.  :-)

Here is one write-in response:

-> Your understanding of DRAM operation is wrong, not even close.  No, DRAMs never
-> use shift registers for storage.  You probably confused DRAMs with bubble
-> memories or CCD RAMS.  DRAMs are similar to SRAMs in that a 2D array of memory
-> cells is addressed using x-y decoding.  DRAMs store the data bits in capacitors
-> instead of the latches used by SRAMs.  Reading the capacitor state is
-> destructive because most of the charge flows out to the sense line.  Get your
-> facts straight before you pose as an authority.

I don't recall posing as anything, let alone an AUTHORITY.  Does this mean
most people just BELIEVE whatever I write?  Hmm, posibilities......
:-)                                                                    :-)

Steven Ward
ward@cfa