Tandy 1000 HX Technical Reference Manual page 125

8259A
FUNCTIONAL DESCRIPTION
Interrupts in Microcomputer Systems
Microcomputer system design requires that 1.0 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.
Figure 3a. Polled Method
Figure 3b. Interrupt Method
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