From: Jay Jaeger <cube1_at_home.com>
Date: Tue Apr 18 19:31:01 2000

At 11:09 AM 4/18/00 -0400, allisonp_at_world.std.com wrote:
> > Hi,
> > I think I heard somewhere that magnetic drums are somewhat like core
> memory,
> > i.e. they retain their contents when shut off... I believe I read this in
>Actually they are cylindrical disks with multiple heads in parallel so
>that you can write a word or read one rather than serializing the bits.
>Drums were used on PDP-8s, PDP-10s and likely others for small storage or
>more commonly swapping out contents of memory. Think of this if you can
>read or write a word in 1/1800(or 2400rpm) or a block of core eaqually as
>fast swapping is viable to expensive and hard to get memory.

I think it is helpful to distinguish between two different devices.

One is a head-per-track disk, sometimes called a Fixed Head disk.
DEC had several of those devices on their machines. The one I am most familiar
with is the RC64 on my PDP-11, but there were also RF series drives for older
machines. The generally had a disk platter, like a normal disk, but unlike
a normal
disk they had one head dedicated for each track on the disk -- no seeks!

A drum, on the other hand, was cylindrical (hence the name). (The first audio
records were cylindrical too, incidentally). Some drums were
also head-per-track devices, but not all. Some had a head that moved from
place to
place along the drum. Univac called each such location a "position".

There is no inherent reason why a drum would not necessarily serialize the
data --
one could construct them either way, depending on the requirements.

I am familiar with a couple of cases where the data was most definitely
serialized: the
IBM 650 and the Monrobot desk-sized computer.

Jay R. Jaeger The Computer Collection
cube1_at_home.com visit http://members.home.net/thecomputercollection
Received on Tue Apr 18 2000 - 19:31:01 BST

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