how fast were drum memories?

ANDY HOLT andy.holt at
Thu May 10 12:14:55 CDT 2018

>>>> from  "Dave Wade via cctalk" <cctalk at>
To: "General Discussion: On-Topic and Off-Topic Posts" <cctalk at>
Sent: Thursday, 10 May, 2018 5:53:38 PM
Subject: RE: how fast were drum memories?

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.

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.


for the spec on the 1962/3/4 drums for the ICT/ICL 1900 series
When we (City University) acquired our 1905E system (think it was ex Swansea uni.) it came with a 1964 drum (0.5M 24-bit words) made, I think, by Bryant.
Took up a lot of floor space and was probably the slowest of our peripherals to come up to "ready" from power on*

we "let it go" when we needed to install more equipment in the computer room - don't know what happened to it.

* the big CDC at ULCC had a huge drum† - but it had its problems - any power-off, even for a second or so, meant that it needed 
a full hour to be operational again.
† or, perhaps, a special fixed disk.


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