The IBM Automatic Sequence Controlled Calculator (ASCC), called Mark I by Harvard University’s staff, 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 by John von Neumann. At that time, von Neumann was working on the Manhattan Project, 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 Mark I was disassembled in 1959, but portions of it were displayed in the Science Center as part of the Harvard Collection of Historical Scientific Instruments until being moved to the new Science and Engineering Complex in Allston in July 2021. Other sections of the original machine were transferred to IBM and the Smithsonian Institution.
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.
More information: IBM
Howard Aiken had started to look for a company to design and build his calculator in early 1937.
After two rejections, 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.
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 7, 1944.
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 23 m3–16 m in length, 2.4 m in height, and 0.61 m deep. It weighed about 4.3 t. The basic calculating units had to be synchronized and powered mechanically, so they were operated by a 15 m drive shaft coupled to a 3.7 kW electric motor, which served as the main power source and system clock.
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 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.
The main sequence mechanism was unidirectional. This meant that complex programs had to be physically lengthy. A program loop was accomplished by loop unrolling or by joining the end of the paper tape containing the program back to the beginning of the tape. At first, conditional branching in the Mark I was performed manually. Later modifications in 1946 introduced automatic program branching by subroutine call.
The first programmers of the Mark I were computing pioneers Richard Milton Bloch, Robert Campbell, and Grace Hopper. There was also a small technical team whose purpose was to actually operate the machine, some of whom were IBM employees before being required to join the Navy to work on the machine. This technical team was not informed of the purpose of their work while at Harvard.
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.
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. 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.
John von Neumann had a team at Los
Alamos that used modified IBM punched-card machines to determine the
effects of implosion. In March 1944, he proposed to run certain problems
regarding implosion on the Mark I, and in 1944 he arrived with two
mathematicians to write a simulation program to study the implosion of
the first atomic bomb.
More information: Harvard University
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 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.
More information: History of Information
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