Communications - July 2012

turing’s brilliant
innovation in the
aCe was to eliminate
delay-line waiting
time by what was
later called optimum
programming.

ACM’s
interactions
magazine explores
critical relationships
between experiences, people,
and technology, showcasing
emerging innovations and industry

American tradition of solving one’s
problems by means of much equip-
ment rather than by thought. I should
imagine that to put his code (which is
advertised as ‘reduced to the simplest
possible form’) into effect would re-
quire a very much more complex con-
trol circuit than is proposed in our full-
size machine.”
Wilkes took the view that build-
ing the EDSAC to a conventional de-
sign would be enough of a challenge.
Moreover, the EDSAC would be several
thousand times faster than existing
techniques and users would be best
served by having a conventional ma-
chine sooner rather than a novel de-
sign later. There was merit in both of
their positions.

aCe Design

The ACE was designed to fully exploit the potential of a mercury delay-line memory. The delay line was a spin-off from echo-cancellation devices in radar, and in 1946 it was the only proven memory technology. When used in the main memory of a computer, the delay line consisted of a five-foot mercury-filled steel tube. Digital data was converted to acoustic energy at one end of the tube and converted back to electrical signals at the other end. By feeding the output back into the input, digital data could be stored indefinitely. A five-foot tube had an acoustic delay of about one millisecond and the typical pulse duration of a computer was one microsecond; hence the delay line could store about 1,000 bits of information (usually 1,024 bits, of course). This was enough to store several instructions or numbers. Apart from its high cost and unreliability, the mercury delay line memory also had the problem of latency. That is, when an instruction or number was needed, the machine had to wait until it emerged from the delay line—an average time of half a millisecond. For this reason most delay-line based computers spent more time waiting for instructions and numbers than processing them.