ACT apricot - Initialization Command Words

INTERRUPT CONTROLLER
(c) Enable the priority of the interrupt request lines to
be changed.
(d) Allow the status of the bits of the three internal
registers within the PIC (IRR, ISR and IMR) to be
analyzed.
The first series of command words are called
Initialization Command Words and the second series,
Operational Command Words.
Initialization Command Words
Three Initialization Command Words (ICWs) are required
to set the PIC into an initial operating condition. The three
words have to be issued in a fixed sequence, and if any
changes to the initial operating condition are required, the
whole sequence must be reprogrammed. Once initialized;
the priority of the interrupt requests are automatically
assigned from IRO (highest priority) through to IR7 (lowest
priority);
the PIC is able to accept interrupt requests.
The initialization programming sequence is ICWl first,
ICW2 next,
and finally ICW 4. ICW3 is not required since
the system operates using a single PIC.
ICWl defines two parameters when the PIC is used with
an 8086 CPU. These are as follows:
(a) The way the PIC senses an active interrupt request
(either edge or level sensitive interrupt request).
(b) Whether there is more than one PIC operating
within the processing environment.
Due to the fact that the two interrupt request lines from
the Expansion Slots are connected to both Expansion Slots
(in effect in wire-ORed fashion), the level sensitive interrupt
mode must be adopted. (In the edge sensitive mode, a
transition on an interrupt request line from a device
connected to one of the Expansion Slots would be
undetected by the PIC, if the interrupt request line is
already raised
to logic high by a device connected to the
second Expansion Slot).