Designed for the job

502 is basically a very fast general purpose computing machine having storage, arithmetic, input, output and control facilities, but is specifically designed for real-time, on-line data-processing appliations.

Priority processing

Features essential to on-line operations of varying time dependence permit the simultaneous running of a number of programmes on a time-division basis. Programmes dealing with different components of the system under surveillance are allocated[,] priority ratings according to the urgency with which they must be obeyed.

Programme Interruption

The 502 incorporates a facility known as Programme Break which enables a high priority programme to take precedence over one of lower priority in response to an external stimulus.

Compound immediate access store

The internal storage system of the 502 contains a fast section with a very short cycle time, but of limited capacity, and a larger section of slightly longer cycle time. The capacity of the main section may be extended to any practical limit.

Instructions may be taken for either store, and may operate on data from either store.


Elliott ultra-high-speed logic elements, assembled in a fully parallel pure binary system of an intrinsically elegant design, impart to the 502 the maximum attainable operating speed.

Operation time for simple arithmetical functions on operands in the fast store is only 2.0 microseconds, including access time of instructions. Long functions such as multiplication and division take about 10 microseconds only. When both instructions and operands are obtained from the main store, the corresponding times are of the order of 7 and 14 microseconds.

Short words

The short word length of 20 bits, used throughout the 502, is entirely adequate for the majority of on-line control applications. The instuction code includes facilities for double-length multiplication, division and shifting functions.

Automatic data transfers

Data transfers between the 502 and its peripheral equipment are aided by the provision of a semi-automatic transfer facility, which allows transfers of data to take place on demand of the peripheral equipment.

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Page created by Bill Purvis, last update: 20th November, 2003