how fast were drum memories?

[Dave Wade]

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/gethomas/manchester_drums.html

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