computer science

Harvard Mark II

Harvard Mark II is a computer science topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Harvard Mark II rather than just read about it. In short: The Harvard Mark II, also known as the Aiken Relay Calculator, was an electromechanical computer built under the direction of Howard Aiken at Harvard University, completed in 1947. It was financed by the United States Navy and used for ballistic calculations at Naval Proving Ground Dahlgren.

Key takeaways

Harvard Mark II belongs to computer science; place it in that map before memorising details.
Learn the definition first, then one example that makes the definition concrete.
Connect Harvard Mark II to a quantity you can measure, compute or draw — that is where exam questions come from.
Reproduce the core statement of Harvard Mark II from memory before moving on to harder problems.

Reference excerpt

The Harvard Mark II, also known as the Aiken Relay Calculator, was an electromechanical computer built under the direction of Howard Aiken at Harvard University, completed in 1947. It was financed by the United States Navy and used for ballistic calculations at Naval Proving Ground Dahlgren. Computer pioneers Edmund Berkeley and Grace Hopper worked together under Aiken to build and program the Mark II.

Overview

The contract to build the Mark II was signed with Harvard in February 1945, after the successful demonstration of the Mark I in 1944. It was completed and debugged in September 1947, and delivered to the US Navy Proving Ground at Dahlgren, Virginia in March 1948, becoming fully operational by the end of that year. The Mark II was constructed with high-speed electromagnetic relays instead of the electro-mechanical counters used in the Mark I, making it much faster than its predecessor. It weighed 25 short tons (23 t) and occupied over 4,000 square feet (370 m2) of floor space. Its addition time was 0.125 seconds (8 Hz) and the multiplication time was 0.750 seconds. This was a factor of 2.6 faster for addition and a factor of 8 faster for multiplication compared to the Mark I. It was the second machine (after the Bell Labs Relay Calculator) to have floating-point hardware. A unique feature of the Mark II is that it had built-in hardware for several functions such as the reciprocal, square root, logarithm, exponential, and some trigonometric functions. These took between five and twelve seconds to execute. Additionally, the Mark II was actually composed of two sub-computers that could either work in tandem or operate on separate functions, to cross-check results and debug malfunctions. The Mark I and Mark II were not stored-program computers – they read instructions of the program one at a time from a tape and executed them. The Mark II had a peculiar programming method that was devised to ensure that the contents of a register were available when needed. The tape containing the program could encode only eight instructions, so what a particular instruction code meant depended on when it was executed. Each second was divided up into several periods, and a coded instruction could mean different things in different periods. An addition could be started in any of eight periods in the second, a multiplication could be started in any of four periods of the second, and a transfer of data could be started in any of twelve periods of the second. Although this system worked, it made the programming complicated, and it reduced the efficiency of the machine somewhat. The Mark II is also known for being the computer with the first recorded instance of an actual bug (a moth) disrupting its operation. The insect was extracted from the machine's electronics and taped to the log book, with the note "first actual case of [a] bug being found", on September 9, 1947.

See also Harvard Mark I Harvard Mark III Harvard Mark IV Bug (engineering)#History

References

Further reading Staff of the Computation Laboratory (1949). Description of a Relay Calculator. Annals of the Computation Laboratory of Harvard University, Vol. XXIV. Harvard University Press, Cambridge, Mass. Rife, James P. (2006). The sound of freedom: Naval Weapons Technology at Dahlgren, Virginia 1918–2006. Government Printing Office. p. 93. ISBN 978-0160872488. Williams, Michael R. (1997). A History of Computing Technology. IEEE Computer Society Press. ISBN 0-8186-7739-2.

External links Photographs related to the Mark II from the Grace Hopper Collection at the Smithsonian

Worked examples

Example 1 — a first encounter with Harvard Mark II

Start with the simplest possible case. Write down what Harvard Mark II claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In computer science, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Harvard Mark II before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Harvard Mark II ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Harvard Mark II

In research: Harvard Mark II appears in computer science research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry: Engineering practice reuses Harvard Mark II in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom: Harvard Mark II is common in secondary-school and first-year university syllabi. It links to neighbouring topics 1940s computers, Computer-related introductions in 1947, Electro-mechanical computers, so understanding it makes those chapters shorter.
In everyday life: Look for Harvard Mark II outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.

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How to study Harvard Mark II in 20 minutes

Read the reference excerpt below once, without taking notes.
Close the page and write down what Harvard Mark II means in your own words.
Compare your version with the excerpt and mark what you missed.
Work through the three examples above with pen and paper.
Explain Harvard Mark II out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Harvard Mark II in simple terms?

Why does Harvard Mark II matter?

Because it connects several computer science ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Harvard Mark II?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Harvard Mark II.