IBM 704

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IBM 704

A large, scientific computer made by IBM and used by the largest commercial, government and educational institutions.

The IBM 704 had 36-bit memory words, 15-bit addresses and instructions with one address. A few index register instructions had the infamous 15-bit decrement field in addition to the 15-bit address.

The 704, and IBM 709 which had the same basic architecture, represented a substantial step forward from the IBM 650's magnetic drum storage as they provided random access at electronic speed to core storage, typically 32k words of 36 bits each.

A typical 700 series installation would be in a specially built room of perhaps 1000 to 2000 square feet, with cables running under a raised floor and substantial air conditioning. There might be up to eight magnetic tape transports, each about 3 x 3 x 6 feet, on one or two "channels." The 1/2 inch tape had seven tracks and moved at 150 inches per second, giving a read/write speed of 15,000 six bit characters (plus parity) per second.

In the centre would be the operator's console consisting of cabinets and tables for storage of tapes and boxes of cards; and a card reader, a card punch, and a line printer, each perhaps 4 x 4 x 5 feet in dimension. Small jobs could be entered via punched cards at the console, but as a rule the user jobs were transferred from cards to magnetic tape by off-line equipment and only control information was entered at the console (see SPOOL). Before each job, the operating system was loaded from a read-only system tape (because the system in core could have been corrupted by the previous user), and then the user's program, in the form of card images on the input tape, would be run. Program output would be written to another tape (typically on another channel) for printing off-line.

Well run installations would transfer the user's cards to tape, run the job, and print the output tape with a turnaround time of one to four hours.

The processing unit typically occupied a position symmetric but opposite the operator's console. Physically the largest of the units, it included a glass enclosure a few feet in dimension in which could be seen the "core" about one foot on each side. The 36-bit word could hold two 18-bit addresses called the "Contents of the Address Register" (CAR) and the "Contents of the Decrement Register" (CDR).

On the opposite side of the floor from the tape drives and operator's console would be a desk and bookshelves for the ever-present (24 hours a day) "field engineer" dressed in, you guessed it, a grey flannel suit and tie. The maintenance of the many thousands of vacuum tubes, each with limited lifetime, and the cleaning, lubrication, and adjustment of mechanical equipment, was augmented by a constant flow of bug reports, change orders to both hardware and software, and hand-holding for worried users.

The 704 was oriented toward scientific work and included floating point hardware and the first Fortran implementation. Its hardware was the basis for the requirement in some programming languages that loops must be executed at least once.

The IBM 705 was the business counterpart of the 704. The 705 was a decimal machine with a circular register which could hold several variables (numbers, values) at the same time.

Very few 700 series computers remained in service by 1965, but the IBM 7090, using transistors but similar in logical structure, remained an important machine until the production of the earliest integrated circuits.

This article is provided by FOLDOC - Free Online Dictionary of Computing (
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