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@classiccmp.org>
>Sent: May 10, 2018 7:29 AM
>To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk@classiccmp.org>
>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: 
>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
>

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