Tandy 1000
inter
8259AJ8259A-2/8259A-8
Technical Reference Manual
FUNCTIONAL DESCRIPTION
Interrupts
in
Microcomputer
Systems
Microcomputer
system
design
requires that 1/0 devices
such
as keyboards,
displays,
sensors and
other
com·
ponents
receive
servicing
in an
efficient
manner
so that
large
amounts
of
the
total
system
tasks
can
be
assumed
by
the
microcomputer
with
little
or
no
effect
on
through·
put.
The
most
common
method
of
servicing
such
devices is
the
Polled
approach. This is where
the
processor
must
test
each device in sequence and in
effect
"ask"
each
one
if it
needs servicing.
It
is easy to see that a large par·
tion
of
the
main program is
looping
through
this
con·
tinuous
polling
cycle
and that
such
a
method
would
have a serious,
detrimental
effect
on
system
through·
put,
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
ifs
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
fefch
a
new
routine
that
will
service
the
requesfing
device. Once
this
servicing
is complete, however, the
processor
would
resume
exactly
where
it
lelf
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
microcompufer
to
further
enhance
its
cost
effectiveness.
The Programmable
Interrupt
Controller
(PIC)
functions
as an overall manager in an Interrupt-Driven
system
environment.
It
accepts
requests
from
the
peripheral
equipment,
determines
which
of
the
incoming
requests
is
of
the
highest
importance
(priority),
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
strucfure
usually
has a special
program or
"roufine"
thaf
is
associated
with
its
specific
functional
or
operational
requirements;
this
is referred
to
as a
"service
roufine".
The PIC,
after
issuing
an Infer-
rupt
fo
the
CPU,
must
somehow
inpuf
information
into
the CPU
fhat
can
"poinf"
the
Program
Counter
to
fhe
service roUfine
associated
with
the
requesting
device.
This
"poinfer"
is an
address
in a
vectoring
table
and
will
often
be referred to, in
this
document,
as
vectoring
data.
The
8259A
The 8259A Is a
device
specifically
designed
for
use
in
real
time,
inferrupt
driven
microcomputer
systems.
It
manages
eight
levels
or
requests and has
built-in
fea·
tures
for
expandability
to
other
8259A's (up
to
64 levels).
It
is
programmed
by
the
sysfem's
software
as an I/O
peripheral.
A
selection
of
priority
modes
is
available
to
the
programmer
so
that
the
manner In
which
the
re-
quests
are
procellsed
by
the
8259A can be
configured
to
match
his
system
requirements.
The
priority
modes can
be changed
or
reconfigured
dynamically
at any
time
dur-
ing
the
main
program. This
means
that
the
complete
interrupt
structure
can
be
defined
as required, based
on
the
total
system
environment.
Figure 38. Polled
Method
Figure 3b.
Interrupt
Method
2-122
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