IBM 704

computer 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.

[Or did the 704 actually come *before* the 650?]

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.

[Was the 704 scientific, business or general purpose?

Difference between 704 and 709?]

(1996-01-24)