10 May
2018
10 May
'18
9:53 a.m.
I don't think early drums were terribly fast, but this wasn't a problem because
often they were on serial machines, and the data had to sync with the clock speed of the
machine.
I know that the Manchester Mk1 which evolved from the baby had a drum added. The design of
the drum used changed as the machine evolved. There is some info on this evolution here.
http://curation.cs.manchester.ac.uk/computer50/www.computer50.org/mark1/get…
Its interesting to note that the size of the drum was decreased to around 6" as
suggested by others.
The Ferranti Pegasus also had a drum for main storage and delay lines for
"registers".
This was a physically large drum with a capacity of 5120 40 it words.
The Ferranti Pegasus the clock speed was 333Khz and this was derived from clock tracks
written on the drum avoiding any sync problems.
However in order to achieve this transfer rate the designers built the tracks in pairs
with alternate bits coming from different tracks..
The large diameter of the drum gave problems getting consistent flying height for the
heads, which resulted in large changes in signal level.
There are notes on this here:-
http://www.cs.man.ac.uk/CCS/res/res07.htm#h
and here
http://www.computerconservationsociety.org/resurrection/res42.htm#f
If any one is interested I have more pictures of the Pegasus..
Dave
-----Original Message-----
From: cctalk <cctalk-bounces at classiccmp.org> On Behalf Of Ed Sharpe via
cctalk
Sent: 10 May 2018 16:51
To: paulkoning at comcast.net; cctalk at classiccmp.org; cctalk at classiccmp.org
Subject: Re: how fast were drum memories?
SOME OTHER DRUM USE AS I REMEMBER IT......
FOR STORAGE HP 2000A TIMESHARE
SYSTEM USED AS DRUM AS SYSTEMS WERE UPGRADED AND DISCS
ADDED FOR 2000C ETC THRU F SOME KEPT
THEIR DRUM AS SWAPPING MEDIA. AS THE HEARD PER
TRACK WAS FASTER THAN MOVING
HEAD FOR USER SPACE AREA SWAPPING
---- AMD -------AND SOME OF THE GE-PAC PROCESS CONTROL
STUFF USED VERMONT RESEARCH DRUMS... ED#
In a message dated 5/10/2018 7:29:19 AM US Mountain Standard Time,
cctalk at classiccmp.org writes:
Drums were used as main memory in a number of early computers, and as
secondary memory for a while longer. I wonder how fast real ones (actually
constructed) managed to be.
What prompted this question is reading an interesting document:
https://ir.cwi.nl/pub/9603 (in Dutch), "Principles of electronic calculating
machines, course notes February 1948" by Prof. A. van Wijngaarden at the
Mathematical Center (now CWI) in Amsterdam. It's quite a fascinating short
introduction into computing technology of that era. (One comment in the
intro: "The field is new. At the moment, the Eniac is the only working
machine..." -- probably not quite accurate given some classified machines,
but not too far wrong.)
The section on main memory describes a bunch of different technoly
possibilities, one of them drum memory. He writes that a drum of 8 cm
diameter (a bit over 3 inches) and "a couple of decimeters height" could hold
maybe 100k bits, with a track pitch of "a few millimeters". So far so good. He
goes on to suggest that such a drum might spin at 1000 revolutions per
second, i.e., 60,000 rpm. That seems amazingly high. I could see it being
physically possible for a drum of only 40 mm radius, but it sure doesn't sound
easy. It's a good goal to strive for given that the logic, even in the days of
vacuum tubes, can run at cycle times of just a couple of microseconds. As
one more way to speed things up he suggests having multiple rows of
read/write heads, where the addressed word would be picked up by
whichever head sees it soonest. 10 rows and 60k rpm would give you 50
microseconds average access time which "even for a parallel computer would
be a very attractive number". (Pages 17-18)
I'm wondering what the reality of fast drum memories looked like, and
whether anyone came even close to these numbers. Also, am I right in
thinking they are at least in principle achievable? I know I could run the stress
numbers, but haven't done so.
paul