EDSAC Replica Project: Recreating the Historic Automatic Digital Computer

1. Electronic Delay Storage Automatic Calculator The EDSAC Replica Project Andrew Herbert, with thanks to Chris Burton, July 2012

2. The EDSAC Replica Project The Proposition Project Organisation Feasibility Studies Costs and Timescale

3. The Proposition An enquiry in 2010 by Hermann Hauser, well-known Cambridge technology entrepreneur: “Would it be feasible to build a replica of the famous EDSAC?” Assume the goal is to replicate the machine as it was in May 1949 when it ran its first program Let it be a tangible tribute to Maurice Wilkes, though he was somewhat sceptical about the proposal!

4. Edsac Firsts • The first machine to provide a “computing service” • Conservatively designed, highly reliable • Mathematicians, scientists, engineers at Cambridge University) took turns to use it as a personal computer • Contributed to Cambridge scientific advances in astronomy, X-ray crystallography and many other fields • The biggest single leap in computing power ever • 1,500x speed of the mechanical calculators it replaced • The first machine to read in symbolic programs (as opposed to patching, hand keying etc) • Hardware “initial instructions” embodied a relocating assembler to read in user’s program and library routines from paper tape.

5. Overall Organisation EDSAC Replica Limited A charitable trust Sponsors + University of Cambridge + BCS Fundraising Ownership Legal Management Board CCS + TNMoC+ Project Manager Overall operations The Replica Project Manager + volunteers Day to day operations

6. Key Facts for Programmers • Two registers: accumulator and multiply • 512 words of memory • 35 bit memory: two 17 bit half words plus “sandwich digit” • Fixed point arithmetic • Paper tape input • Teleprinter output • Initial instructions embody simple assembler

7. Order Code F (5) n (10) L - • A n a += [n] • S n a -= [n] • H n m := [n] • V n a += m*[n] • N n a -= m*[n] • T n n := a; a := 0 • U n n := a • C n a += m&[n] • R 2n-2 a := a >> n • L 2n-2 a := a >> n • E n jmp if a<0 • G n jmp if a≥0 • I n n:=input • O n output:=[n] • F n check • X no op • Y round a • Z stop

8. EDSAC Architecture Automatic Digital Computers, M.V. Wilkes, 1956

9. Mercury Delay Line Memory Maurice Wilkes with a battery of 16 storage tanks Each tank holds 16 x 36 bit words as a train of acoustic pulses Computer has to synchronize with the memory

10. Serial Computing Most of EDSAC is serial Process one bit of a word at the time Reduces number of components needed From Edsac Report

11. Decoding and Coincidence Have to go parallelto decode functionnumber and memoryaddress Automatic Digital Computers, M.V. Wilkes, 1956

12. Building the Replica

13. Authenticity • We don’t have a complete blueprint, so we aim to... • be consistent with photographs and contemporary records • use period components and circuits whenavailable • use camouflaged modern components otherwise • adhere to EDSAC architectural principles(i.e., serial processing) when designing

14. Feasibility Studies • Documents & knowledge acquisition • Physical design • Logic design & simulation • Electronic design & experiments • Acquisition of parts • Areas of work not started • Skills required

15. Documents & Knowledge Acquisition • Original technical description & diagrams from Cambridge Computer Laboratory archives • Original photographs & published papers • Recollections of pioneers • All collected in project Dropbox • EDSAC ran for 10 years so need to understand the evolution of the machine. (our target 6th May 1949)

16. Physical Design • Scanning and measuring from photos • 12 racks, 120 chassis (“panels”) • An original chassis exists to measure • The above chassis has been drawn up and a sample made • We don’t know how many types of chassis there were, or where they were placed in the racks

17. Logic Design & Simulation • Need to know how EDSAC works in detail • Incomplete & inconsistent diagrams • Evidence of much re-design during commissioning • Need to extrapolate undocumented areas of logic • Simulation essential to give confidence before committing to building anything

18. Typical Logical Diagram From Edsac Report

19. Typical Timing Diagram From Edsac Report

20. Logic Simulation • Bill Purvis has written a simulator for whole logic - can run a program, very slowly. • Several areas such as reader and printer modelled as ‘black boxes’

21. Electronic design • Electronic design is incomplete and lots of redesigning went on during commissioning • AC-coupled circuits - unfamiliar! • AND-gate uses 3 pentodes and 3 diodes • Main components: flip-flop, inverter, short delay, pulse amplifier • Experiment shows stage delay is very short • Requires many lumped-constant delays

22. Typical Circuit Diagram From Edsac Report

23. Mapping Logic to Circuits to Chassis • Use photos to try to guess what each chassis does • Physical location of more than half the logic is now understood - the easy bits! • Some partial clues from logic diagrams

24. Mapping Logic to Chassis

25. Memory Tanks Maurice Wilkes with a battery of 16 storage tanks, each 16 x 36 bit words The 5 ft steel tubes contain mercury as the acoustic delay medium

26. Replica Memory Tanks • Risky and costly to use mercury, except perhaps in one example tank • Precision engineering required: tubes and end plates – aligned to within 0.001” end-to-end • Will use nickel delay lines as a reasonable alternative • Use semiconductor shift registers to get off the ground quickly

27. Acquisition of Parts • Many, but not all, valves are available and already to hand • B9G valveholders will be problematic • Authentic ‘period’ resistors and capacitors may be difficult to find and too unreliable to use • Lumped-constant delay lines need to be made, lots of coils to wind

28. Areas not yet looked at • HT power supply - +250v at say 15 amp • Negative power supplies • Electrical hazard of open circuit wiring • The ‘three oscilloscope unit’ • Tape reader • Teleprinter

29. Skills mix needed • Understand logic and map to electronic circuits • Map electronic circuits to individual chassis • Wiring up 120 chassis - 3000 valves – 60,000 solder joints! • Ability to track down lots of components • Delicate manipulative skills for delay lines • Some circuit design capability for replica store

30. Costs and Timescale • Preliminary estimates indicate cost in the region of £250,000 • With adequate availability of volunteers to do the construction, it could take 3-4 years

31. Current Status • In addition to design research reported here… • Charity registered and bank account opened • Initial donations to fund first year • Detailed planning started • Initial milestones – pulses, counting, storing

32. Work in Progress • Demonstrate EDSAC Pulses: Clock Pulse Generator and Digit Pulse Generator chassis operation • Demonstrate Counting: Clock Pulse Generator + Half Adder + Short Tank • Demonstrate Store Cycles: Address Decoding + Store Regeneration + Long Tank

33. Electronic Delay Storage Automatic Calculator The EDSAC Replica Project