Specifications

service routine is begun. Note that those operations all occur auto-
matically and that no device-polling is required to determine which
service routine to execute.
5. 7.2 microseconds is the time interval between the central Processor’s
receiving the interrupt command and the fetching of the first inStruC-
tion. This assumes there were no NPR transfers during this time.
6. The device service routine can resume the interrupted process by
executing the RTI (Return from interrupt) instrudion which ‘ipops”
the processor stack back into the PC and PS. This requires 4.5
microseconds if there are no intervening NPR’s.
7, A device service routine can be interrupted in turn by a sufficiently
high priority bus request any time after completion of its first in-
struction.
8. lf such an interrupt occurs, the PC and PS of the device service .
routine are aUtOITX3tiCally pushed into the stack and the new device
routine initiated as above. This “nesting” of priority interrupts can
go on to any level, limited only by the core available for the stack.
More commonly, this process will nest only four levels deep since
there are four levels of BR signals. An example of nested device
servicing is shown in Figure 2-2. A rough core map is given for each
step of the process. The SP points to the top word of the stack as
shown.
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