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SILICON GATE MOS 8259
8259 PRELIMINARY
FUNCTIONAL DESCRIPTION
I
INTERRUPT
REQUESTS
8259
IRQ IRQ IRQ
IRQ
IRQ IRQ IRQ IRQ
765
4
~
2 1 0
SLAVE
PROG.
ADDRESS BUS (16)
8259
System Interface.
In
microcomputer systems,
the
rate
at
which a peripheral
device
or
devices can be serviced determines
the
total
amount
of system tasks
that
can be assigned
to
the
control of
the
microprocessor.
The
higher
the
throughput
the
more jobs
the
microcomputer can
do
and
the
more cost effective it be-
comes. Interrupts have long been accepted as a key
to
im-
proving system
throughput
by servicing a peripheral device
only when
the
device has requested it
to
do
so. Efficient
managing of
the
interrupt requests
to
the
CPU
will have a
significant effect
on
the
overall cost effectiveness
of
the
microcomputer system.
The
8259
Programmable Interrupt Controller
is
a single-chip
device
that
can manage eight levels
of
requests and has built-
in
featuresfor expandability
to
other
8259s (up
to
64
levels).
It
is programmed
by
the
systems software
as
an I/O periphe-
ral. A selection
of
priority algorithms
is
available
to
the
pro-
grammer so
that
the
manner in which
the
requests are pro-
cessed by
the
8259
can be configured
to
match his system
requirements. The priority assignments and algorithms can
be changed
or
reconfigured dynamically
at
any
time during
the
main program. This means
that
the
oomplete interrupt
structure can
be
defined as required, based
on
the
total
sys-
tem environment.
The system interface
is
the
same as
other
peripheral devices
in
the
MC5-80. A special
input
is
provided (SP)
to
program
the 8259 as a slave or master device when expanding
to
more
than
eight levels. Basically
the
master accepts INT
in-
puts from
the
slaves and issues a composite request
to
the
8080A; when it receives
the
INTA from
the
8228
it puts
the first
byte
on
the
CALL
on
the
bus.
On
subsequent
I
NT
As
the
interrupting slave puts
out
the
address
of
the
vector.
~
ADDR
ESS
BUS
(16)
~
\
CONTROL
BUS
\
INT
REQ
\
DATA
BUS
(8)
\
.",-
to/'..
,t./'..
,t.
:lo
---
---
---
---
I----
--
~
-
1--
---
~
-
--I-
~
--
I--
-
I-
-
--
---I--
I----
(
~v'7
~V7
r--
~,7
"''...1''1
~v'7
"'v'7
""'
'"
8259
8259
8259
SLAVE 2
p.-
I--
SLAVE 1
'"
MASTER
p...t-
""'
'"
Sf
SP
SP
G!O
1
fIll
f
11
G!O
f f f
fIlII
lJJ
J
1111
,
21
20
19 18 17 16 15 14
13 12
11
10 9 8 7 6
5 4 3
2 1
0
I
INTERRUPT
REQUESTS
Cascading
the
8259
22
Level Controller (Expandable
to
64
levels).
5-174