Mark I (and IBM Again)

As a doctoral student at Harvard, Aiken began to investigate the possibility of building a large-scale, programmable, automatic computing device and managed to arouse interest in his project, particularly from Thomas Watson, Sr., head of International Business Machines (IBM). In 1939, IBM agreed to underwrite the building of Aiken’s first calculator, the Automatic Sequence Controlled Calculator, which became known as the Harvard Mark I.

Like Babbage, Aiken aimed for a general-purpose programmable machine rather than an assembly of special-purpose arithmetic units. Unlike Babbage, Aiken had access to a variety of tested, reliable components, including card punches, readers, and electric typewriters from IBM and the mechanical electromagnetic relays used for automatic switching in the telephone industry. His machine used decimal numbers rather than the binary numbers of the majority of later computers. Sixty registers held whatever constant data numbers were needed to solve a particular problem. The operator turned a rotary dial to enter each digit of each number. Variable data and program instructions were entered via punched paper tape. Calculations had to be broken down into specific instructions similar to those in later low-level programming languages. The results (usually tables of mathematical function values) could be printed by an electric typewriter or output on punched cards. Huge (about 8 feet [2.4 m] high by 51 feet [15.5 m] long), slow, but reliable, the Mark I worked on a variety of problems during World War II, ranging from equations used in lens design and radar to the designing of the implosive core of an atomic bomb.

Compared to later computers such as the ENIAC and UNIVAC, the sequential calculator, as its name suggests, could only perform operations in the order specified.

ENIAC

The Electronic Numerical Integrator and Computer (ENIAC) was developed by John W. Mauchly and John Presper Eckert, Jr., at the University of Pennsylvania. The machine had been financed by the U.S. army during the Second World War as a calculator for ballistic tables. With Mauchly providing theoretical design work and J. Presper Eckert heading the engineering effort, ENIAC was completed too late to influence the outcome of the war.

By today's standards for electronic computers ENIAC was a grotesque monster. Its thirty separate units, plus power supply and forced-air cooling, weighed over thirty tons. Its 19, 000 vacuum tubes, 1, 500 relays, and hundreds of thousands of resistors, capacitors, and inductors consumed almost 200 kilowatts of electrical power.

But ENIAC was the prototype from which most other modern computers evolved. It embodied almost all the components and concepts of today's high-speed, electronic digital computers. Its designers conceived what has now become standard circuitry such as the gate, buffer and used a modified Eccles-Jordan flip-flop as a logical, high-speed storage-and-control device.

ENIAC could discriminate the sign of a number, compare quantities for equality, add, subtract, multiply, divide, and extract square roots. ENIAC stored a maximum of twenty 10-digit decimal numbers. Its accumulators combined the functions of an adding machine and storage unit. No central memory unit existed, per se. Storage was localized within the functioning units of the computer.

The primary aim of the designers was to achieve speed by making ENIAC as all-electronic as possible. The only mechanical elements in the final product were actually external to the calculator itself. These were an IBM card reader for input, a card punch for output, and the 1, 500 associated relays.

ENIAC, however, did not have any programming interface. For each new program, cables had to be plugged in the right devices, adaptors used on the right connections, dials and switches set for the right values etc. Thus, it need not surprise that the planning of a computation, not only the translation of the mathematics into a general scheme but also the realisation of the scheme as a combination of cables and switch settings, could take weeks and had to be done with utmost care.

ENIAC was frustrating to use because it wouldn’t run for more than a few minutes without blowing a tube, which caused the system to stop working. Every time a new problem had to be solved, the staff had to enter the new instructions by rewiring the entire machine. The solution was the stored program concept, an idea that occurred to just about everyone working with electronic computers after World War II.

Rather than the program being set up by wiring or simply read sequentially from tape or cards, the program instructions would be stored in memory just like any other data. Besides allowing a computer to fetch instructions at electronic rather than mechanical speeds, storing programs in memory meant that one part of a program could refer to another part during operation, allowing for such mechanisms as branching, looping, the running of subroutines, and even the ability of a program to modify its own instructions.

Notes:

Enigma machine was an electro-mechanical rotor cipher machine used in the 20-th century for enciphering and deciphering secret messages. It was invented by the German engineer Arthur Scherbius at the end of World War I.

J. Presper Eckert and John Mauchly designed ENIAC, the first general-purpose electronic digital computer, as well as EDVAC, BINAC and UNIVAC I.

Eccles-Jordan flip-flop - триггер (триггерная система) - класс электронных устройств, обладающих способностью длительно находиться в одном из двух устойчивых состояний и чередовать их под воздействием внешних сигналов. Используются, в основном, в вычислительной технике для организации регистров, счетчиков, процессоров, ОЗУ.