June 19, 1963 Revised March 24, 1964

FP6000 PROJECT MEMORANDUM #22

The following specification has been drawn up by Product Planning in
conjunction with Engineering. Revisions have been underlined.

W. R. Whittall
Product Planning
Computer Systems.

DRUM CONTROL SPECIFICATION

1. There will be one size of drum which can be fitted with from 1 to 6 groups of 64
heads. Each track contains 400 words so the amount of storage is modular in
units of 25,600 words from 25,600 up to 153,600 words per drum. It will be
possible to prevent writing on one or more blocks of 12,800 words (each block
starting at a multiple of 12,800) by appropriate setting of 12 manual switches
in the drum unit

2. The time for one revolution is about 34 ms.

3. The rate of transfer is about 1 word every 180 us.

4. One parity bit per 24 bit word will be recorded on the drum and checked when reading.

5. The drum system will consist of a number of drum control units each of which will be
capable of controlling up to 16 unsynchronized drums either directly or via an exchange.

6. The exchange will consist of a number of exchange sub-units, one per control, which
will allow the drum to be controlled by any of the controls connected to it. An operation
will be accepted by a control even if the drum is busy with another control. The operatic
will be executed when the drum becomes free. It will be possible to connect controls
in two different "Basic Computers" to the same drum.

7. Each control will be connected as a "fast, word peripheral" with one word of buffer
register.

8. Transfers will consist of from 1 to 32,768 words as specified by a fifteen bit count
sent to the control by special register, with zero interpreted as 32,768. The nine
least significant bits of this count will also be set in the control word.

9. Transfers may start on one drum and continue on another.

10. There will be one control word (in store) and one special register associated with
each control. The first special register write operation will be used to specify
reading or writing and the number of words to be transferred and second will specify
the drum address of the first word of the data transfer. A drum address will consist
of a 6-bit binary word address, a 13-bit binary block address and a 5-bit binary
drum number. There will be 25 words in each block with provision for increasing
this number (up to a maximum of 64) to take advantage of improvements in technology.
There will be 16 blocks per track so that the number of clocks on a drum will depend
on the number of heads fitted - up to a maximum of 6,144 blocks i.e. 153,600 words.
 
The special register bits are allocated as follows:

First special register write operation:-

Second special register write operation:-

Special register read operation:-

NOTE:- There must be no special register read operations between the two special register
write operations.

11. The available words on any drum will be numbered upwards from zero. Provision
will be made for automatically selecting word 0 on the next drum when a transfer
reaches the last available word on any given drum.

12. A START bit will be accepted by a control even if the selected drum (or exchange)
is busy with another control. The transfer will proceed when the drum becomes
available.

13. The control will be designed so that continuation of a transfer may be
postponed for one drum revolution at any point in the transfer if a hesitation
request is not answered in time, so that drum transfers never become overdue.

14. The control will be designed to minimize the total cost of the control plus
any special equipment needed to rewrite and timing tracks as a maintenance
procedure on site.

15. The control will be designed to minimize tne changes necessary to accomodate
any forseeable drum development such as increased number of words per track
or increased number of tracks per drum.

W. R. Whittall