Harvard Mark I

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The left end consisted of electromechanical computing components
The right end included data and program readers, and automatic typewriters
Closeup of input/output and control readers

The IBM Automatic Sequence Controlled Calculator (ASCC), called Mark I by Harvard University’s staff,[1] was a general purpose electromechanical computer that was used in the war effort during the last part of World War II.

One of the first programs to run on the Mark I was initiated on 29 March 1944[2] by John von Neumann, who worked on the Manhattan project at the time, and needed to determine whether implosion was a viable choice to detonate the atomic bomb that would be used a year later. The Mark I also computed and printed mathematical tables, which had been the initial goal of British inventor Charles Babbage for his "analytical engine".


The original concept was presented to IBM by Howard Aiken in November 1937.[3] After a feasibility study by IBM engineers, the company chairman Thomas Watson Sr. personally approved the project and its funding in February 1939.

Howard Aiken had started to look for a company to design and build his calculator in early 1937. After two rejections,[4] he was shown a demonstration set that Charles Babbage’s son had given to Harvard university 70 years earlier. This led him to study Babbage and to add references of the Analytical Engine to his proposal; the resulting machine “brought Babbage’s principles of the Analytical Engine almost to full realization, while adding important new features.”[5]

The ASCC was developed and built by IBM at their Endicott plant and shipped to Harvard in February 1944. It began computations for the U.S. Navy Bureau of Ships in May and was officially presented to the university on August 24, 1944.

Design and construction[edit]

The ASCC was built from switches, relays, rotating shafts, and clutches. It used 765,000 electromechanical components and hundreds of miles of wire, comprising a volume of 51 feet (16 m) in length, 8 feet (2.4 m) in height, and 2 feet (0.61 m) deep. It weighed about 10,000 pounds (4,500 kg). The basic calculating units had to be synchronized and powered mechanically, so they were run by a 50-foot (15 m) shaft driven by a 5 horsepower (3.7 kW) electric motor, which served as the main power source and system clock. From the IBM Archives:

The Automatic Sequence Controlled Calculator (Harvard Mark I) was the first operating machine that could execute long computations automatically. A project conceived by Harvard University’s Dr. Howard Aiken, the Mark I was built by IBM engineers in Endicott, N.Y. A steel frame 51 feet (16 m) long and eight feet high held the calculator, which consisted of an interlocking panel of small gears, counters, switches and control circuits, all only a few inches in depth. The ASCC used 500 miles (800 km) of wire with three million connections, 3,500 multipole relays with 35,000 contacts, 2,225 counters, 1,464 tenpole switches and tiers of 72 adding machines, each with 23 significant numbers. It was the industry’s largest electromechanical calculator.[6]

The enclosure for the Mark I was designed by futuristic American industrial designer Norman Bel Geddes. Aiken considered the elaborate case to be a waste of resources, since computing power was in high demand during the war and the funds ($50,000 or more according to Grace Hopper) could have been used to build additional computer equipment.[7]


The Mark I had 60 sets of 24 switches for manual data entry and could store 72 numbers, each 23 decimal digits long.[8] It could do 3 additions or subtractions in a second. A multiplication took 6 seconds, a division took 15.3 seconds, and a logarithm or a trigonometric function took over one minute.[9]

The Mark I read its instructions from a 24-channel punched paper tape. It executed the current instruction and then read in the next one. A separate tape could contain numbers for input, but the tape formats were not interchangeable. Instructions could not be executed from the storage registers. This separation of data and instructions is known as the Harvard architecture (although the exact nature of this separation that makes a machine Harvard, rather than Von Neumann, has been obscured with the passage of time; see Modified Harvard architecture).

At first, the Mark I had no conditional branch instruction. This meant that complex programs had to be physically lengthy. A program loop was accomplished by joining the end of the paper tape containing the program back to the beginning of the tape (literally creating a closed loop). Later modifications introduced program branching.[10] The first programmers of the Mark I were computing pioneers Richard Milton Bloch, Robert Campbell, and Grace Hopper.[11]

Instruction format[edit]

The 24 channels of the input tape were divided into three fields of eight channels. Each accumulator, each set of switches, and the registers associated with the input, output, and arithmetic units were assigned a unique identifying index number. These numbers were represented in binary on the control tape. The first field was the binary index of the result of the operation and the second, the source datum for the operation. The third field was a code for the operation to be performed.[8]

Contribution to the Manhattan project[edit]

In 1928 L.J. Comrie was the first to turn IBM “punched-card equipment to scientific use: computation of astronomical tables by the method of finite differences, as envisioned by Babbage 100 years earlier for his Difference Engine”.[12] Very soon after, IBM started to modify its tabulators to facilitate this kind of computation. One of these tabulators, built in 1931, was The Columbia Difference Tabulator[13]

John von Neumann had a team at Los Alamos that used “modified IBM punched-card machines”[14] to determine the effects of implosion. On 29 March 1944, he demanded to run certain problems regarding implosion on the Mark I. In early August 1944 he arrived with two mathematicians to write a simulation program to study the implosion of the first atomic bomb.[15]

The Los Alamos group completed its work in a much shorter time than the Cambridge group. However, the punched-card machine operation computed values to six decimal places, whereas the Mark I computed values to eighteen decimal places. Additionally, Mark I integrated the partial differential equation at a much smaller interval size [or smaller mesh] and so...achieved far greater precision.[14]

“Von Neumann joined the Manhattan Project in 1943, working on the immense number of calculations needed to build the atomic bomb. He showed that the implosion design, which would later be used in the Trinity and Fat Man bombs, was likely faster and more efficient than the gun design.”[16]

Aiken and IBM[edit]

Aiken published a press release announcing the Mark I listing himself as the sole “inventor”. James W. Bryce was the only IBM person mentioned, even though several IBM engineers including Clair Lake and Frank Hamilton had helped to build various elements. IBM chairman Thomas J. Watson was enraged, and only reluctantly attended the dedication ceremony on August 7, 1944.[17][18] Aiken, in turn, decided to build further machines without IBM’s help, and the ASCC came to be generally known as the "Harvard Mark I". IBM went on to build its Selective Sequence Electronic Calculator (SSEC) to both test new technology and provide more publicity for the company's own efforts.[17]


The Mark I was followed by the Harvard Mark II (1947 or 1948), Mark III/ADEC (September 1949), and Harvard Mark IV (1952) – all the work of Aiken. The Mark II was an improvement over the Mark I, although it still was based on electromechanical relays. The Mark III used mostly electronic componentsvacuum tubes and crystal diodes—but also included mechanical components: rotating magnetic drums for storage, plus relays for transferring data between drums. The Mark IV was all-electronic, replacing the remaining mechanical components with magnetic core memory. The Mark II and Mark III were delivered to the US Navy base at Dahlgren, Virginia. The Mark IV was built for the US Air Force, but it stayed at Harvard.

The Mark I was disassembled in 1959, but portions of it are displayed in the Science Center, as part of the Harvard Collection of Historical Scientific Instruments. Other sections of the original machine were transferred to IBM and to the Smithsonian Institution.[19]

See also[edit]


  1. ^ The machine’s name as actually displayed on the hardware itself is "Aiken-IBM Automatic Sequence Controlled Calculator Mark I". An early photograph (Wilkes 1956:16 figure 1-7) displays the name as "IBM Automatic Sequence Controlled Calculator".
  2. ^ Bernard Cohen, p.164 (2000)
  3. ^ Bernard Cohen, p. 53 (2000)
  4. ^ Bernard Cohen, p.39 (2000) It was first rejected by the Monroe Calculator Company and then by Harvard University.
  5. ^ "IBM’s ASCC introduction 2". Retrieved 14 December 2013. 
  6. ^ IBM Archives: FAQ / Products and Services
  7. ^ Computer Oral History Collection, 1969-1973, 1977 Grace Murray Hopper Interview, January 7, 1969, Archives Center, National Museum of American History [1]
  8. ^ a b Maurice Vincent Wilkes (1956). Automatic Digital Computers. New York: John Wiley & Sons. pp. 16–20. 
  9. ^ Campbell, 1999, p.43
  10. ^ Campbell, 1999, p.53
  11. ^ Wexelblat, Richard L. (Ed.) (1981). History of Programming Languages, p. 20. New York: Academic Press. ISBN 0-12-745040-8
  12. ^ "Columbia University Computing History: L.J. Comrie". Retrieved 15 December 2013. 
  13. ^ "The Columbia Difference Tabulator - 1931". Retrieved 15 December 2013. 
  14. ^ a b #AIKEN, Bernard Cohen p.166 (2000)
  15. ^ Bernard Cohen, p. 164 (2000)
  16. ^ "Atomic Heritage Foundation: John von Neumann". Retrieved 15 December 2013. 
  17. ^ a b Emerson W. Pugh (1995). Building IBM: Shaping an Industry and Its Technology. MIT Press. ISBN 978-0-262-16147-3. 
  18. ^ Martin Campbell-Kelly; William Aspray, (1996). Computer: A History of the Information Machine. Basic Books. p. 74. ISBN 0-465-02989-2. 
  19. ^ "Collection of Historical Scientific Instruments Mark I". Atlas Obscura. Retrieved 2016-05-24. 

External links[edit]