ENIAC, страница 2

The gate performed the switching or logical "and" function. It consisted of a single pentode which had a control voltage applied to its suppressor grid. Its function was similar to that of a single pole switch in that it "opened" (passed a pulse pattern) when the suppressor grid was positive and "closed" when the suppressor grid was negative.

The buffer contained two or more tubes connected through a common load resistor to form a circuit with the logical properties of the word "or." The grids of the tubes were normally biased at the cut-off point so that a positive input to any tube in the combination produced a negative output.

The flip-flop circuit contained two triodes so connected that only one would conduct at a given time. The bi-stable device had two inputs and two outputs. In the set, or normal position, one side of the output was positive, the other negative. In the reset, or abnormal position, these polarities were reversed. Logically, the flip-flop performed the functions of memory and that of a double-pole, double-throw switch. The state of each flip-flop was indicated by a neon lamp on the front panel of the computer units.

A group of ten flip-flops, (0-9), interconnected to count digit pulses, formed a decade ring counter which was capable of adding and storing numbers. The ring counter possessed the following characteristics: (1) At any one time only one flip-flop could be in the reset state; (2) A pulse to the counter input reset the initial flip-flop in the chain; (3) The circuit could be cleared so that a specific flip-flop was in the reset position while the others remained set.

Each flip-flop of a counter was termed a stage, and reception of a pulse at the input side advanced the counter by one stage. Information was recirculated through the counter; i.e., the last stage was coupled to the first. A variation of the basic counter circuit, the PM counter, controlled the sign of a number in the accumulator. Ten decade ring counters, one per decimal place, plus one PM counter, formed the basic arithmetic and storage unit of ENIAC--the accumulator. The decade ring counters were equipped with ten transmission circuits so that when any ring passed the nine positions, a pulse was passed to the next ring in the series. Input pulses reaching the accumulator added to or subtracted from its contents.

The accumulator was an essential element in all of ENIAC's arithmetic operations. Addition required two accumulators--one transferring its contents to the other. Subtraction, accomplished by a complement-and-add process, also used two accumulators. In normal multiplication, four accumulators stored the multiplier and multiplicand and accumulated the partial products. In division they shifted the remainder and stored the numerator, denominator, and quotient. The function table utilized the accumulators for storage of the argument and accumulation of the function value.

A synchronous system, ENIAC operated under the control of pulses from a cycling unit. The pulses were emitted at 10-microsecond intervals. The overall timing cycle or repetition rate was 200 microseconds, one addition time. Pulses were transmitted to all units continuously and simultaneously, and each computer operation took an integral number of addition times. For checking and trouble-shooting purposes, the cycling unit circuitry included provisions for operation in a one-addition or one-pulse-at-a-time mode.

The ENIAC was not originally designed as an internally programmed computer. The program was set up manually by varying switches and cable connections. However, means for altering the program and repeating its iterative steps were built into the master programmer. Digit trays, long racks of coaxial cables, carried the data from one functioning unit to another. Program trays, similarly, transferred instructions; i.e., programs. In purely repetitive calculations the basic computing sequence was set by hand. The master programmer automatically controlled repetition and changed the sequence as required.

The master programmer contained ten 6-stage counters--each routing incoming program pulses over a field of six output channels. The position of the counters was controlled by either the number of pulses which had been supplied to the output channels or by the number of pulses received at a special input terminal. In this fashion, the number of sequences could be fixed in advance or made contingent on the results of a computation.

Each functioning unit of ENIAC was equipped with local program-control circuits. These circuits contained switches which were set for the function required. When the local program circuit was stimulated by a program pulse, the unit performed the desired operation. After it finished, a program- completion pulse was emitted, via the program tray coaxial line, to the next unit in the operational sequence.

In addition to its cycling unit, twenty accumulators, and master programmer, ENIAC included an initiating unit, a high-speed multiplier, a divider, a square-root unit, and three portable function tables.

The initiating unit turned ENIAC on and off, cleared it, and initiated computation.

The high-speed multiplier did its work in much the same fashion as a human would. It contained a built-in multiplication table capable of multiplying up to 9 times 9. Multiplication of the multiplicand by each digit of the multiplier took one addition time. The left- and right-hand figures of each product of a digit of the multiplicand and the multiplier were accumulated separately to form two partial products, which, when combined, formed the final product. The multiplication process for two 10-digit numbers took 2.6 milliseconds.

The divider and square-root unit worked by repeated subtraction and addition, a time-consuming procedure which took an average of 25 milliseconds for a 10-digit number. The divisor was subtracted from the dividend, and the sign of the partial remainder was tested after each step. When the sign became negative, the remainder was shifted up-scale and the divisor was added until the sum became positive. An accumulator serving as a quotient register kept a count of the number of additions and subtractions for the successive decimal places. Extraction of a square root was a similar process.

The principal purpose of the function tables, which actually were banks of switch-controlled resistor matrices, was the storage of the arbitrary functions called for by the problem. The switches selected one of 12 digits and 2 signs for each of the 104 values of an independent variable that were stored in each table. The functional similarity between modern computers and the ENIAC is rather astounding, although the ENIAC was designed almost two decades ago.

The ENIAC was formally dedicated at the Moore School of Electrical Engineering of the University of Pennsylvania on February 15, 1946, and it was accepted by the U.S. Army Ordnance Corps in July, four years after the original suggestion by Dr. Mauchly.

All During 1946 the ENIAC remained at the Moore School, working out numerical solutions to problems in such fields as atomic energy and ballistic trajectories. Dismantling at the Moore School began in the winter, and the first units arrived at Aberdeen Proving Ground in January 1947. The ENIAC became operational again in August 1947.

The ENIAC's first few years at the Aberdeen Proving Ground were difficult ones for the operating and maintenance crews. The computer represented the largest collection of interconnected electronic circuitry then in existence, and its thousands of components had to remain operational simultaneously. The result was a huge preventive-maintenance and testing program, which, in the end, led to some major modifications of the system.

Tubes were life-tested, and statistical data on the failures were compiled. This information led to many improvements in vacuum tubes themselves. Procurement of large quantities of improved, reliable tubes, however, became a difficult problem. Power-line fluctuations and power failures made continuous operation directly off transformer mains an impossibility. The substantial quantity of heat which had to be dissipated into the warm, humid Aberdeen atmosphere created a heat-removal problem of major proportions. Down times were long; error-free running periods were short.

Programming new problems meant weeks of checking and set-up time, for the ENIAC was designed as a general-purpose computer with logical changes provided by plug-and-socket connections between accumulators, function tables, and input-output units. However, the ENIAC's primary area of application was ballistics--mainly the differential equations of motion.

In view of this, the ENIAC was converted into an internally stored fixed-program computer when the late Dr. John von Neumann of the Institute for Advanced Study at Princeton suggested that code selection be made by means of switches so that cable connections could remain fixed for most standard trajectory problems. After that, considerable time was saved when problems were changed.

The ENIAC performed arithmetic and transfer operations simultaneously. Concurrent operation caused programming difficulties. A converter code was devised to enable serial operation. Each function table, as a result of these changes, became available for the storage of 600 two-decimal digit instructions.

Those revolutionary modifications, installed early in 1948, converted ENIAC into a serial instruction execution machine with internal parallel transfer of decimal information. The original pluggable connections came to be regarded as permanent wiring by most BRL personnel.

By February 1949, when the ENIAC completed the computation for Project Chore, an Ordnance Corps contract with the University of Chicago, operating difficulties had been reduced to a minimum. Running times were longer, down times shorter and reduced in number. The Chore contract and others completed during this period proved the ENIAC's worth. Other machines, among them the Bush differential analyzer and the Bell relay calculator, would have required a prohibitive length of time to complete the problems that were assigned to the ENIAC, and the latter was much faster than any digital system then in existence.

For example, a skilled person with a desk calculator could compute a 60- second trajectory in about 20 hours. The analog differential analyzer produced the same result in 15 minutes. ENIAC required 30 seconds--just half the time of the projectile's flight.

The ENIAC led the computer field during the period 1949 through 1952 when it served as the main computation workhorse for the solution of the scientific problems of the Nation. It surpassed all other existing computers put together whenever it came to problems involving a large number of arithmetic operations. It was the major instrument for the computation of all ballistic tables for the U.S. Army and Air Force.

In addition to ballistics, the ENIAC's field of application included weather prediction, atomic-energy calculations, cosmic-ray studies, thermal ignition, random-number studies, wind-tunnel design, and other scientific uses. It is recalled that no electronic computers were being applied to commercial problems until about 1951.

EDVAC and ORDVAC, both faster than ENIAC, began to share the Computing Laboratory's work load with the ENIAC in 1953. It became apparent almost immediately that the ENIAC would have to be modified if it were to remain competitive, economical, and efficient. Modifications, based on new developments in the computer art, were again made on the ENIAC.

In addition to an independent motor-generator set, which eliminated the power troubles, a high-speed electronic shifter, which reduced by 80 percent the time required for numerical shifting and eliminated numerous tubes and program units, was installed early in 1952. Later, in July 1953, a 100-word static magnetic-core memory was added to the system.

The core storage unit, the first operational unit of its kind, was built by the Burroughs Corporation. The Binary coded decimal, excess three, system of number representation was used. It was operated successfully three days after its arrival at BRL and continued in service until the ENIAC was retired.

To provide for the additional memory capacity, the ENIAC was equipped with a new function-table selector, a special memory-address selector, and special pulse-shaping circuits. Three new orders were added to the converter code for use with the new memory.

Despite these modernizations and the fact that trouble-free operating time remained at about 100 hours a week during the last 6 years of the ENIAC's use, its operating costs were far above those of the EDVAC and ORDVAC. The ENIAC was no longer competitive from an economic point of view. The work load gradually was shifted to the other machines, and at 11:45 p.m. on October 2, 1955, the power to ENIAC was removed.

The late Dr. von Neumann suggested that attempts be made to preserve at least some of the ENIAC at the Smithsonian Institution at Washington, DC. So far, efforts at preservation have had several concrete results. An operational ENIAC accumulator unit has been shipped to the United States Military Academy at West Point, NY, for display in the Academy museum. The Smithsonian will display portions of the ENIAC as soon as space becomes available.