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The IBM Automatic Sequence Controlled Calculator (ASCC), called the Mark I by Harvard University^[1], was the first large-scale automatic digital computer in the USA. It is considered by some to be the first universal calculator.

The electromechanical ASCC was devised by Howard H. Aiken, created at IBM, shipped to Harvard in February 1944, and formally delivered there on August 7, 1944. The main advantage of the Mark I was that it was fully automatic—it didn't need any human intervention once it started. It was the first fully automatic computer to be completed. It was also very reliable, much more so than early electronic computers. It is considered to be "the beginning of the era of the modern computer"^[2] and "the real dawn of the computer age"^[3].

Contents 1 Design and Construction 2 Operation 3 Instruction Format 4 Aiken and IBM 5 Successors 6 Myth on origins of "bugs" 7 Comparison with other early computers 8 Notes 9 See also 10 References 11 External links

The building elements of the ASCC were switches, relays, rotating shafts, and clutches. It was built using 765,000 components and hundreds of miles of wire, amounting to a size of 51 feet (16 m) in length, eight feet (2.4 m) in height, and two feet (~61 cm) deep. It had a weight of 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's Dr. Howard Aiken, the Mark I was built by IBM engineers in Endicott, N.Y. A steel frame 51 feet 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 ASSC used 500 miles 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^[4].

The Mark I had 60 sets of 24 switches for manual data entry and could store 72 numbers, each 23 decimal digits long.^[5] 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 and executed the current instruction and then read in the next one. It 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). This separation of data and instructions is known as the Harvard architecture. The first programmers of the Mark I were Richard Milton Block, Robert Campbell, and computing pioneer Grace Hopper, respectively.^[6]

The 24 channels of the input tape were divided into 3 fields of 8 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.^[7]

This section does not cite any references or sources. Please improve this section by adding citations to reliable sources. Unverifiable material may be challenged and removed. (October 2007)

At the dedication ceremony, Aiken failed to mention the involvement of IBM in designing and building the computer. IBM was not pleased with this, and parted ways with Aiken. IBM named the computer the ASCC but Harvard and Aiken renamed it the Mark I. IBM went on to build the SSEC.

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, but it also used electromechanical relays. The Mark III used some electronic components and the Mark IV was all-electronic, using solid state components. The Mark III and Mark IV used magnetic drum memory and the Mark IV also had magnetic core memory. The Mark II and Mark III went 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 eventually disassembled, although portions of it remain at Harvard in the Science Center.

A story told by Grace Hopper about staff finding the first computer bug, a moth crushed in a relay of the Mark I, was widely misinterpreted as the origin of the term bug in the sense of a technical problem. See Computer bug - Etymology.

Defining characteristics of five early digital computers

Computer	First operation	Place	Decimal/Binary	Electronic	Programmable	Turing complete
Zuse Z3	May 1941	Germany	binary	No	By punched film stock	Yes (1998)
Atanasoff–Berry Computer	Summer 1941	USA	binary	Yes	No	No
Colossus	December 1943 / January 1944	UK	binary	Yes	Partially, by rewiring	No
Harvard Mark I – IBM ASCC	1944	USA	decimal	No	By punched paper tape	Yes (1998)
ENIAC	1944	USA	decimal	Yes	Partially, by rewiring	Yes
	1948	USA	decimal	Yes	By Function Table ROM	Yes

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. ^ Stoll, E. L. (1983), "Mark I", in Ralston, Anthony & Reilly, Edwin D., Encyclopedia of computer science and engineering (2nd ed.), New York: Van Nostrand Reinhold Company Inc., pp. 916-917, ISBN 0-442-24496-7
3. ^ Pylyshyn, Zenon W. (1970), Perspectives on the computer revolution, Englewood Cliffs,N.J.: Prentice-Hall, Inc., page 5.
4. ^ IBM Archives: FAQ / Products and Services
5. ^ Wilkes 1956:16-17
6. ^ Wexelblat, Richard L. (Ed.) (1981). History of Programming Languages, p. 20. New York: Academic Press. ISBN 0-12-745040-8
7. ^ Wilkes 1956:17-18

• History of computing hardware
• Harvard Mark II
• Harvard Mark III
• Harvard Mark IV
• Howard Aiken
• Other early computers:
  • Zuse Z3 (Germany)
  • Manchester Mark I (UK)
  • Atanasoff–Berry Computer (USA)
  • ENIAC (USA)
  • Colossus (UK)
  • IBM Selective Sequence Electronic Calculator (USA)

• Cruz, Frank da (Aug 2004). The IBM Automatic Sequence Controlled Calculator. Columbia University Computing History. Retrieved on October 2006.
• Campbell-Kelly, Martin; Aspray, William (1996). Computer: A History of the Information Machine. Basic Books. ISBN 0-465-02989-2. 
• Wilkes, M. V. (1956). Automatic Digital Computers. New York: John Wiley & Sons, 305 pages. QA76.W5 1956. 

• IBM Archive: IBM ASCC Reference Room
• ASCC operational manual (PDF)