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r - - - - - - - - . . . . . . - - - - - - - - - - - - - - - - - - - - - - -
---,
- - - PORT C BIT SELECT
BIT 03 02 01
o
0
0
0
1 0 0
1
2
0
1
0
3 0 1
1
4
1
0
0
5
1
0
1
6
1
1
0
7
1
1
1
- - - - - - 0 - BIT SET/RESET CONTROL WORD
' - - - - - -
~
--J
Figure 3-8.
Bit Set/Reset Control Word Format
INTERRUPT CONTROL PROGRAMMING
An 8259A Programmable Interrupt Controller device provides priority
reso 1 ut i on for eight interrupt 1 eve 1 s.
An interrupt mat ri x on the
Arn96/4116 provides for connecting the eight interrupt inputs to 24
possible interrupt sources.
The 8259A has the capability to determine
which unmasked input has the highest priority.
A higher priority input
Hill
interrupt any lower pri ori ty input be i ng scrv iced.
A lo\'/cr
priority interrupt will be held for later
servic~.
NOTE
When using Multibus interrupts, the 8259A must be
used in the edge trigger mode.
INITIALIZATION
The 8259A accepts two types of command words, Initial ization Command
Words
(ICWs)
and Operation Command Words
(OCWs).
Before normal
operation can occur, the 8259A must be initialized.
Initialization
consists of three ICWs, ICWl, ICW2, and ICW4.
The ICWs are shown in
figure 3-9.
The ICWI format for use \'Jith the Am96/4116 should be 13 hex for edge
triggering or IB hex for level triggering, written to address FFC8.
Bit 0 should always be a 1, because ICW4 is required for a 16-bit
microprocessor.
Since only one 8259A is used, bit 1 should always be
set.
Bit 2 is a don1t care and bit 3 determines the type of triggering
at the interrupt inputs.
The Initialization Control Word is defined as
ICWI when address bit 1 is zero and ICW bit 4 is set.
3-12
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