The 502 is able to run a number of programmes at the same time. It is essential that it should be able to do this, for it is inevitable that, when processing is required to proceed in coincidence with the occurence of a number of independent external events, that is to say when the time allowed for computing is the real time duration of the event, the instant at which each programme may be required cannot be determined in advance.
The programmes are normally interrelated, in that they correspond to the component parts of the system under surveillance, and they must therefore be brought into use as the various parts of the system come into play. This implies that control of the 502 must be automatically transferred at any time, from one programme to another, in response to changing external circumstances.
This automatic transfer is executed in the 502 by means of a facility known as Programme Break.
In any complex system, there will be further factors to be considered when control is transferred from one programme to another. The programmes will have greater of lesser degrees of time dependence, according to their urgency, or immediacy, with which the corresponding parts of the system are required to operate. This time dependence may be extremely variable. Furthermore, there will be wide variations in the level of activity of each programme. The mean level of activity of any one programme will generally be low, but there can be periods, which may be cyclic or random in recurrence, when the level of activity is high.
These unpredictable factors demand that the control of the computer must be exercised by the programme which is most deserving of attention at any given time. In the 502 on-line data-processer, each of the programmes dealing with different system components is allocated a Priority Rating on the basis of its time dependence and activity level. A change in external circumstances which requires the action of a programme having a higher priority rating than the current one generates a Break Stimulus. This initiates the programme interruption.
When a suitable break stimulus occurs, control is transferred to the programme of highest priority by means of the Programme Control Unit. The process is entirely automatic and takes under 10 microseconds. If a programme prescribes the use of a peripheral device which is unavailable, provision is made for Automatic Programme Suppression. The unit will arrange for the programme of next highest priority to run until the device becomes ready.
With this type of organisation the individual programmes may be written as independent entities and without reference to the speeds of the peripheral devices.
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