The Harvard Mark II, also known as Aiken Relay Calculator, was an electromechanical computer built under the direction of Howard Aiken and was finished in 1947. It was financed by the United States Navy. Howard Aiken and Grace Hopper worked together to program and build the Mark II.

The Mark II was constructed with high-speed electromagnetic relays [电磁继电器] instead of electro-mechanical counters used in the Mark I, making it much faster than its predecessor. It weighted 25 short tons (23 t). 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 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.

The Mark II ran some realistic test programs in 1947. It was delivered to the US Navy Proving Ground at Dahlgren, Virginia in 1948 and became operational by the end of that year.

Grace Brewster Murray Hopper (December 9, 1906 – January 1, 1992) was an American computer scientist and United States Navy rear admiral (lower half). One of the first programmers of the Harvard Mark I computer, she was a pioneer of computer programming who invented one of the first linkers. Hopper was the first to devise the theory of machine-independent programming languages, and the FLOW-MATIC programming language she created using this theory was later extended to create COBOL, an early high-level programming language still in use today.

Effective November 8, 1985, the rank of commodore was renamed rear admiral (lower half) and Hopper became one of the Navy's few female admirals. 海军准将。

Prior to joining the Navy, Hopper earned a Ph.D. in mathematics from Yale University and was a professor of mathematics at Vassar College. Hopper attempted to enlist in the Navy during World War II but was rejected because she was 34 years old. She instead joined the Navy Reserves. Hopper began her computing career in 1944 when she worked on the Harvard Mark I team led by Howard H. Aiken.

In the 1970s, Hopper advocated for the Defense Department to replace large, centralized systems with networks of small, distributed computers. Any user on any computer node could access common databases located on the network. She developed the implementation of standards for testing computer systems and components, most significantly for early programming languages such as FORTRAN and COBOL. The Navy tests for conformance to these standards led to significant convergence among the programming language dialects of the major computer vendors. In the 1980s, these tests (and their official administration) were assumed by the National Bureau of Standards (NBS), known today as the National Institute of Standards and Technology (NIST).

Anecdotes

• Throughout much of her later career, Hopper was much in demand as a speaker at various computer-related events. She was well known for her lively and irreverent speaking style, as well as a rich treasury of early war stories. She also received the nickname "Grandma COBOL".
• While she was working on a Mark II Computer at Harvard University in 1947, her associates discovered a moth that was stuck in a relay; the moth impeded the operation of the relay. While neither Hopper nor her crew mentioned the phrase "debugging" in their logs, the case was held as an instance of literal "debugging." For many years, the term bug had been in use in engineering. The remains of the moth can be found in the group's log book at the Smithsonian Institution's National Museum of American History in Washington, D.C.
• Grace Hopper is famous for her nanoseconds visual aid. People (such as generals and admirals) used to ask her why satellite communication took so long. She started handing out pieces of wire that were just under one foot long (11.8 inches (30 cm))—the distance that light travels in one nanosecond. She gave these pieces of wire the metonym "nanoseconds." She was careful to tell her audience that the length of her nanoseconds was actually the maximum speed the signals would travel in a vacuum, and that signals would travel more slowly through the actual wires that were her teaching aids. Later she used the same pieces of wire to illustrate why computers had to be small to be fast. At many of her talks and visits, she handed out "nanoseconds" to everyone in the audience, contrasting them with a coil of wire 984 feet (300 meters) long, representing a microsecond. Later, while giving these lectures while working for DEC, she passed out packets of pepper, calling the individual grains of ground pepper picoseconds.
• Jay Elliot described Grace Hopper as appearing to be "'all Navy', but when you reach inside, you find a 'Pirate' dying to be released".

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