The Manchester Mark 11
S. H. Lavington
Upon arrival at Manchester in December 1946, Williams and Kilburn set about perfecting a digital store, at first using the commercially available type CV1131 12-inch diameter cathode ray tubes [Kilburn, 1948; Williams and Kilburn, 1949]. The principle of a two-state electrostatic store can be visualised from the following simple experiment. Start with a focussed CRT beam and turn the beam current on (thus producing a charged "dot") and off again repeatedly. Negative voltage pulses will be induced by capacitive coupling in a pick-up plate placed close to the. outer surface of the CRT screen. Now move the beam whilst it is on so as to write a "dash" on the screen, then move the beam back whilst the current is off, and then switch on the current again. This time a positive voltage pulse is induced. With dots and dashes representing logical 0's and l's, readable as negative and positive voltage signals, a binary storage system is available. Other representations such as a "focus/defocus" system were also used. Now although the electrostatic charge leaks away in about 0.2 seconds, automatic refreshing (re-writing) of the information in less than 0.2 seconds is a simple matter electronically. (cf a modern MOS solid-state store.) Since the refresh rate is rapid, long term drifts in electrode supply voltage, etc are not critical and a robust store can be made from standard components. In contrast, the mercury acoustic delay-line stores chosen by other workers had to be constructed to close physical tolerences. The biggest advantage of the CRT store was that it allowed random access whereas other contemporary systems were sequential.
By the Autum of 1947 the Manchester group had successfully stored 2048 digits for a period of hours [Kilburn, 1948] and the way was clear for the construction of a prototype computer "to subject the system to the most searching tests possible" [Williams et al., 1951]. Kilburn took the initiative with the logical design. The "baby machine," as it was called, had a specification which may be expressed in modern terminology as follows:
Serial binary arithmetic using two's complement integers
Single-address format order code
Main store: 32 words, extendable to 8192 words, random access
Computing speed: 1.2 milliseconds per instruction.
The machine first ran a program in June 1948 [Williams and Kilburn, 1948] and as far as can be ascertained it was therefore the world's first stored-program computer. A complete diagram of the prototype Mark 1 is given in Williams et al.  and Fig. 1 is a simplified version showing the main flow of information. The Williams Tube which implemented the control register was also used to hold the present instruction (P1) itself subsequent to its being read out of main store. Either the value of control or the value of this P1 could be fed from the "control" Williams Tube to
1Excerpted from S. H. Lavington, A History of Manchester
Computers, NCC Publications, Manchester, England, 1975, pp. 7-10. Editor's
note: Further discussion of the Mark 1 can be found in the above publication.
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