Alan Turing's ACE
Written by Historian   
Monday, 28 May 2012
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Alan Turing's ACE
Building the Ace
ACE and Deuce

The ACE test assembly, as it was called, was never finished. Work started in 1947 and after a few months Turing left to take a year’s sabbatical at Kings College Cambridge.  Wilkinson took over control of the project and Turing never returned.




In late 1947 Darwin decided that the mathematicians didn't know enough engineering to build the ACE. It was decided that the NPL Radio Division should become involved and the morale of the Mathematics Division collapsed.

At first the Radio Division wasn't sure it wanted to be involved in the very odd design that Turing had invented but in August 1947 it was announced that a "pilot" ACE would be built. The Pilot ACE was essentially an extension of the test machine but this time it was really going to be built. The machine ran its first program in the May of 1950.



Pilot ACE

The Pilot ACE was a very strange machine by today's standards mainly because it was heavily influenced by the type of hardware available and the need to be fast and efficient. The memory was based on acoustic delay lines. Bit streams were stored by converting them into sound pulses which circulated in a mercury delay line. The modern equivalent of such a memory device would be a shift register clocked to keep a bit pattern circulating.



Turing's sketch of a delay line

Other computers of the time used delay line storage but Turing had decided to try to make the best possible use of crude technology. The main store of the machine used ten delay lines each holding 32 words of 32 bits. There were also six temporary stores implemented as short delay lines each capable of holding a 32-bit number.

The operations of the Pilot ACE allowed the programmer to specify move operations from one delay line to another. This was achieved by waiting for the number to come round and then "gating" it into the data flow of another delay line. Because it was arranged so that the numbers emerged from the delay lines at the same moment you could only move the nth number in a delay line to become the nth number in another delay line.

If you wanted to change the order of numbers in the long delay lines you had to first transfer the number to a short delay line and then wait for the position in the destination to come round.

This made programming more like juggling.

If you also look at Turing's sketches for the logic of the machine you will be surprised to discover that it doesn't look like a modern arrangement of And, Or and Not. In fact the gates used look more like artificial neurons of the McCullock-Pitts type. In other words, the logic looks altogether more biologically inspired than the Boolean logic we tend to use today.



 Turing's Rotator (Carpenter & Doran, The other Turing machine, 1977)



Turing's Logic Gates in Modern Form
(Source: The Other Turing Machine, The Computer Journal, Vol 20 (3) pp. 269-279, 1977)






Last Updated ( Tuesday, 29 May 2012 )

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