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Diff — Pilot ACE

Comparing rev 1 → rev 2

	{{infobox
	\| title = Pilot ACE
	\| designer = design by Alan Turing · built under James Wilkinson / NPL
	\| first appeared = 10 May 1950 (first program)
	\| platform = ~800 valves · 1 MHz · mercury delay lines
	\| memory = 128 → 352 × 32-bit words + 4096-word drum (1954)
	\| fate = shut down May 1955 — preserved at the Science Museum
	}}

	# Pilot ACE

	Pilot ACE — the pilot model of Alan Turing's Automatic Computing Engine — is the machine that Turing designed and bureaucracy nearly prevented. Turing produced the ACE specification at the National Physical Laboratory in 1945–46, a design detailed enough to be a working blueprint; NPL's delays so frustrated him that he left for Manchester in 1948 before construction finished. The smaller Pilot Model that eventually emerged, built under James H. Wilkinson with Donald Davies, Harry Huskey, and Mike Woodger, ran its first program on 10 May 1950 — making it one of Britain's first working stored-program computers, and the only one directly descended from Turing's hand.[[cite: Yates, D.M., Turing's Legacy: A history of computing at the NPL 1945–1995 (Science Museum, 1997), via the [Pilot ACE article](https://en.wikipedia.org/wiki/Pilot_ACE) — the design lineage, team, and first-run date]]

-	Turing's original ACE concept was, characteristically, ahead of the hardware around it — his design included a hardware stack and subroutine machinery that wouldn't become standard practice for years. The Pilot Model was the pragmatic cut-down that could actually be built, and even so it became the fastest of the early British machines: 1 MHz, approximately 800 vacuum tubes.[[cite: [Pilot ACE](https://en.wikipedia.org/wiki/Pilot_ACE) — the 1 MHz clock and valve count]]
+	Turing's original ACE concept was, characteristically, ahead of the hardware around it — his 1946 report specified a subroutine discipline built on BURY and UNBURY: a return-address stack kept in memory with its pointer in register TS31, a design that anticipated structured subroutine linkage by years. The Pilot Model was the pragmatic cut-down that could actually be built, and even so it became the fastest of the early British machines: 1 MHz, approximately 800 vacuum tubes.[[cite: [Pilot ACE](https://en.wikipedia.org/wiki/Pilot_ACE) — the 1 MHz clock and valve count]]

	# Arithmetic by software

	The Pilot ACE shipped with a memorable hole: no hardware multiply or divide. Multiplication started out as fixed-point software — which immediately proved unusable because numbers escaped range — so Wilkinson's team wrote floating-point routines instead. The experience made Wilkinson the era's leading expert on floating-point error analysis and produced his classic book on rounding errors — a career's worth of numerical-methods insight that started with a missing instruction.[[cite: [Pilot ACE](https://en.wikipedia.org/wiki/Pilot_ACE) — the missing multiply/divide, the software floating point, and Wilkinson's rounding-error book]]

	The hardware that did exist was finicky in the instructive way of delay-line machines: instruction timing depended on where the instruction physically sat in the mercury lines — an addition could take anywhere from 64 to 1024 microseconds depending on when the word came around. Main memory was 128 32-bit words, later 352, with a 4,096-word drum grafted on in 1954 — which is when Brian Munday's General Interpretive Programme arrived, a matrix-package system whose "bricks" read their own array bounds off the drum and could therefore run on any size of matrix without recompilation. Woodger's "block floating" storage scheme — one shared exponent per array — was invented for it.[[cite: [Pilot ACE software](https://en.wikipedia.org/wiki/Pilot_ACE#Software) — GIP, the bricks, and Woodger's block-floating scheme]]

	# From prototype to product line

	Though intended as a proof-of-concept, Pilot ACE was useful enough to go into service in late 1951 and work for four years — and useful enough that English Electric turned it into a product: the DEUCE, the commercial version sold from 1955. The Pilot machine itself was switched off in May 1955 and given to the Science Museum, where it remains — one of the few first-generation British machines you can still stand in front of.[[cite: [Pilot ACE](https://en.wikipedia.org/wiki/Pilot_ACE) — the DEUCE commercial line, May 1955 shutdown, and the Science Museum residency]]

	```fact
	Pilot ACE's instruction timing was position-dependent — a delay-line machine's geography is its performance model. Programmers scheduled code by placing hot instructions at the front of the mercury tubes, an optimization you performed with an address map, not a profiler.
	```

	# Where it sits

	Pilot ACE is the "what might have been" machine: a Turing original that arrived late, scaled down, and still managed to be Britain's fastest early computer, a commercial ancestor (via DEUCE), and the training ground where Wilkinson became the world's expert on floating-point error. Its designer never saw it run — the machine is the artifact, and it outlived him.

	## Check your understanding

	- [ ] Who designed the ACE, and who actually built the Pilot Model after the designer left?
	- [ ] What two arithmetic operations did Pilot ACE lack in hardware — and what did that absence produce?
	- [ ] Why was instruction timing variable on the Pilot ACE?
	- [ ] What commercial machine descended from Pilot ACE?

	```hide Show answers
	1 — Alan Turing designed the ACE at NPL (1945–46); James Wilkinson's team (Davies, Huskey, Woodger) built the Pilot after Turing left in 1948.
	2 — multiply and divide — done in software, which forced floating-point routines and turned Wilkinson into the era's authority on rounding errors.
	3 — it used mercury delay lines — an instruction's cost depended on where it physically sat in memory (64–1024 µs for an add).
	4 — the English Electric DEUCE — the commercial version sold from 1955.
	```

	[[Category:Hardware]]
	[[Category:Computing]]
	[[Category:History]]