how fast were drum memories?

Bill Degnan billdegnan at
Thu May 10 09:48:08 CDT 2018

There are two places I'd check.  The manual for the Royal McBee LGP-30 and
the book Computer Structures: Readings and Examples by G Bell et al.

On Thu, May 10, 2018 at 10:37 AM, Grif via cctalk <cctalk at>

> I wonder how the late generation paging disks (fixed head per track) like
> DG used in the 80's compared?
> -----Original Message-----
> >From: Paul Koning via cctalk <cctalk at>
> >Sent: May 10, 2018 7:29 AM
> >To: "General Discussion: On-Topic and Off-Topic Posts" <
> cctalk at>
> >Subject: how fast were drum memories?
> >
> >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:
> (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
> >

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