CAlllNTEfWAL
4.
PROGRAMMING THE 8259A
Programming the 8259A is accomplished by using two
types of command words: Initialization Command
Words (lCWs) and Operational Command Words
(OCWs). All the modes and commands explained in the
previous section, "Operation
of
the 8259A", are pro·
grammable using the ICWs and OCWs (see Appendix A
mana vvores).
I
ne
IvVVS
are
usee
10
sen·up me
ne\.;tll>·
sary conditions and modes for proper 8259A operation.
Figure
20
shows the initialization
flOW
of the 8259A.
Both
ICW1
and ICW2 must be issued for any form of
8259A operation. However, ICW3 and ICW4 are used
only
if
designated so in ICW1. Determining the neces·
sity
and use of each ICW is covered shortly in individual
groupings. Note that, once intialized, if any program·
ming changes
within
the ICWs are
to
be made, the entire
ICW sequence must
be
reprogrammed, not just an
indio
viduallCW.
Certain internal set-up conditions occur automatically
within the 8259A after the first ICW has been issued.
These are:
A.
Sequencer logic is set
to
accept the remain
ng
ICWs
as designated in ICW1.
B.
The
ISR
(In·Servlce Register) and IMR (Interrupt Mask
Register) are both cleared.
The ICW programming format. Figure
21,
shows bit
designation and a short definition
of
each
ICW.
With the
ICW format as reference, the functions of each ICW will
now be explained individually.
Figur.
20. Initialization
FI_
Initialization Command Word Format
,""',
1
'C,,"«NEEDlO
o
1\10
Ie".
"'£(0(0
I
'
St"'CLt
'----.
__
O_"_CA_SC_AD_'
_¥O_D'
__
_I
To Next Page
Loading...
