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KF5JRV > TECH 14.07.16 12:22l 119 Lines 6639 Bytes #999 (0) @ WW
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Subj: IBM Automatic Sequence Controlled Calculator
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The IBM Automatic Sequence Controlled Calculator (ASCC), called Mark I by
Harvard University's staff, was a general purpose electro-mechanical
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, 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, 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 24, 1944.
One of the first programs to run on the Mark I was initiated on 29 March 1944 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 was Charles Babbage's initial goal for his
analytical engine.
The Mark I was officially retired, after 15 years of service, in 1959.
Design and construction
The ASCC was built from switches, relays, rotating shafts, and clutches. It
used 765,000 components and hundreds of miles of wire, comprising a volume of
51 feet (16 m) in length, eight feet (2.4 m) in height, and two feet (~61 cm)
deep. It weighed about 10,000 pounds (4500 kg). The basic calculating units
had to be synchronized mechanically, so they were run by a 50-foot (~15.5 m)
shaft driven by a five-horsepower (4 kW) electric motor. 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\rquote 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.
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.
Contribution to the Manhattan project
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.
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.
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.
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.
Operation
The Mark I had 60 sets of 24 switches for manual data entry and could store 72
numbers, each 23 decimal digits long. It could do three additions or
subtractions in a second. A multiplication took six seconds, a division took
15.3 seconds, and a logarithm or a trigonometric function took over one
minute.
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 long. A loop was accomplished by joining
the end of the paper tape containing the program back to the beginning of the
tape (literally creating a loop). Later modifications introduced program
branching. The first programmers of the Mark I were computing pioneers
Richard Milton Bloch, Robert Campbell, and Grace Hopper.
Instruction format
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.
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