Drum vs. Core

Andy Holt andyh at andyh-rayleigh.freeserve.co.uk
Mon Jul 2 06:58:40 CDT 2007

 When I was at university (71-74), the college's mainframe
still used a drum from program overlays (probably really the virtual
memory backing storage, but possibly just dumping and restoring the
whole program between time slices. The machine was no slouch, it was
serving about a hundred terminals and running a couple of batch
streams as well (Maximop and George 2).

I take it that this was an ICL 1905E (probably at Swansea or QMC).
That processor benchmarks about the same as an IBM PC/AT (!)
[somehow I cannot see running 100 terminals off an AT]

Originally on the 1900 series the drum was considered a different peripheral
type (ie needed different programming) from an Exchangeable Disk (EDS) or
Fixed Disk (FDS), but, by the time we at City acquired Swansea's old
1905E and drum (and some EDS30s), new Executives appeared with
UDAS (Unified Direct Access) where the drum was programmed just the
same as a disk. These Execs, unlike the previous versions, were overlaid
(I think they were still, however, written in engineers assembler which used
numeric op-codes rather than the mnemonics of PLAN or GIN5).
Each overlay was (most of) a 128 word disk sector ... tight modular code
was rather forced! ... it also had to be position independent on a machine
with no real architectural support for such - and you had only minimal use
of the Datum/Limit registers in Exec mode.

The drum was, I think, 512K characters (128K 24-bit words). We put the
Exec overlays and the Maximop overlay file on the drum - I'm not sure
what else. It probably was too small for the Maximop swap file. George II
and the main compilers and consolidator (ICL's name for the link editor)
were probably also on the drum. George had little to gain fromsuch a
as it did not use overlays except that job wrap-up and start reloaded the
full code, some of which got overwritten during processing - British
systems of the era tended to use a "job description" at the start rather
the interspersed "control cards" typical of IBM.
We got rid of the drum at the start of our transition from ICL to
Honeywell -
it took up too much space in the machine room and the Level 66 was a
BIG machine (physically).

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