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

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

The enclosure for the Mark I was designed by futuristic American industrial designer Norman Bel Geddes. Aiken considered the elaborate casing 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.

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).

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, the second was the source datum for the operation and the third field was a code for the operation to be performed.

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. Aiken, in turn, decided to build further machines without IBM­s 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.

The Mark I was followed by the Harvard Mark II (1947 or 1948), Mark III/ADEC (September 1949), and Harvard Mark IV (1952) ň 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 components+vacuum 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.