Harvard Mark II

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Harvard Mark II
Also known asAiken Relay Calculator
DeveloperHoward Aiken and Grace Hopper
Release date1947; 75 years ago (1947)
Mass25 short tons (23 t)
PredecessorHarvard Mark I
SuccessorHarvard Mark III

The Harvard Mark II, also known as the Aiken Relay Calculator,[1][2][3] was an electromechanical computer built under the direction of Howard Aiken at Harvard University, completed in 1947. It was financed by the United States Navy and used for ballistic calculations at Naval Proving Ground Dahlgren. Howard Aiken and Grace Hopper worked together to build and program the Mark II.


Logbook entry containing the "bug"

The contract to build the Mark II was signed with Harvard in February 1945, after the successful demonstration of the Mark I in 1944. It was completed and debugged in 1947, and delivered to the US Navy Proving Ground at Dahlgren, Virginia in March 1948,[4] becoming fully operational by the end of that year.[5]

The Mark II was constructed with high-speed electromagnetic relays instead of the electro-mechanical counters used in the Mark I, making it much faster than its predecessor. It weighed 25 short tons (23 t) and occupied over 4,000 square feet (370 m2) of floor space.[6][3][7] Its addition time was 0.125 seconds (8 Hz) and the multiplication time was 0.750 seconds. This was a factor of 2.6 faster for addition and a factor of 8 faster for multiplication compared to the Mark I. It was the second machine (after the Bell Labs Relay Calculator) to have floating-point hardware. A unique feature of the Mark II is that it had built-in hardware for several functions such as the reciprocal, square root, logarithm, exponential, and some trigonometric functions. These took between five and twelve seconds to execute. Additionally, the Mark II was actually composed of two sub-computers that could either work in tandem or operate on separate functions, to cross-check results and debug malfunctions.[1]

The Mark I and Mark II were not stored-program computers – they read instructions of the program one at a time from a tape and executed them. This separation of data and instructions is known as the Harvard architecture. The Mark II had a peculiar programming method that was devised to ensure that the contents of a register were available when needed. The tape containing the program could encode only eight instructions, so what a particular instruction code meant depended on when it was executed. Each second was divided up into several periods, and a coded instruction could mean different things in different periods. An addition could be started in any of eight periods in the second, a multiplication could be started in any of four periods of the second, and a transfer of data could be started in any of twelve periods of the second. Although this system worked, it made the programming complicated, and it reduced the efficiency of the machine somewhat.[8]

The Mark II is also known for being the computer with the first recorded instance of an actual bug (a moth) disrupting its operation. The insect was extracted from the machine's electronics and taped to the log book, with the note "first actual case of [a] bug being found", on September 9, 1947.[9][10]

See also[edit]


  1. ^ a b Rife 2006, p. 93.
  2. ^ Instruments & Control Systems. Chilton Company. 1957. p. 1302.
  3. ^ a b Harvard Alumni Bulletin. 1948. p. 554.
  4. ^ Darroch, Robert S. Harding & Don. "Technology, Invention, and Innovation Collections". amhistory.si.edu. SERIES 2: PHOTOGRAPHS OF MARK II, 1948: (43) 18 February 1948 Packing Mark II for move to Dahlgren, (44) 20 February 1948 Crates on truck at Harvard, ready to go to Dahlgren. Retrieved 2018-05-16.
  5. ^ Rife 2006, Search for "mark ii" Dahlgren 1948, pp. 95-96, 278.
  6. ^ Infantry Journal. United States Infantry Association. 1948. p. 57.
  7. ^ Science News. Science Service. 1948. p. 295.
  8. ^ Williams, 1985, pages 248-251
  9. ^ "September 9, 1947 First Instance of Actual Computer Bug Being Found". Retrieved 2021-03-16.
  10. ^ "Sep 9, 1947 CE: World's First Computer Bug". Retrieved 2021-03-16.

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