8259A Datasheet, PDF (3/24 Pages) Intel Corporation – PROGRAMMABLE INTERRUPT CONTROLLER

FUNCTIONAL DESCRIPTION
Interrupts in Microcomputer Systems
Microcomputer system design requires that I O de-
vices such as keyboards displays sensors and oth-
er components receive servicing in a an efficient
manner so that large amounts of the total system
tasks can be assumed by the microcomputer with
little or no effect on throughput
The most common method of servicing such devic-
es is the Polled approach This is where the proces-
sor must test each device in sequence and in effect
‘‘ask’’ each one if it needs servicing It is easy to see
that a large portion of the main program is looping
through this continuous polling cycle and that such a
method would have a serious detrimental effect on
system throughput thus limiting the tasks that could
be assumed by the microcomputer and reducing the
cost effectiveness of using such devices
A more desirable method would be one that would
allow the microprocessor to be executing its main
program and only stop to service peripheral devices
when it is told to do so by the device itself In effect
the method would provide an external asynchronous
input that would inform the processor that it should
complete whatever instruction that is currently being
executed and fetch a new routine that will service
the requesting device Once this servicing is com-
plete however the processor would resume exactly
where it left off
This method is called Interrupt It is easy to see that
system throughput would drastically increase and
thus more tasks could be assumed by the micro-
computer to further enhance its cost effectiveness
The Programmable Interrupt Controller (PIC) func-
tions as an overall manager in an Interrupt-Driven
system environment It accepts requests from the
peripheral equipment determines which of the in-
coming requests is of the highest importance (priori-
ty) ascertains whether the incoming request has a
higher priority value than the level currently being
serviced and issues an interrupt to the CPU based
on this determination
Each peripheral device or structure usually has a
special program or ‘‘routine’’ that is associated with
its specific functional or operational requirements
this is referred to as a ‘‘service routine’’ The PIC
after issuing an Interrupt to the CPU must somehow
input information into the CPU that can ‘‘point’’ the
Program Counter to the service routine associated
with the requesting device This ‘‘pointer’’ is an ad-
dress in a vectoring table and will often be referred
to in this document as vectoring data
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Figure 3a Polled Method
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Figure 3b Interrupt Method
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