Special Fully Nested Mode - Intel 80C186EA User Manual

INTERRUPT CONTROL UNIT
8259A
82C59A
8259A
82C59A
Figure 8-2. Using External 8259A Modules in Cascade Mode
8.3.3.1

Special Fully Nested Mode

Special fully nested mode is an optional feature normally used with cascade mode. It is applicable
only to INT0 and INT1. In special fully nested mode, an interrupt request is serviced even if its
In-Service bit is set.
In cascade mode, an 8259A controls up to eight external interrupts that share a single interrupt
input pin. Special fully nested mode allows the 8259A's priority structure to be maintained. For
example, assume that the CPU is servicing a low-priority interrupt from the 8259A. While the
interrupt handler is executing, the 8259A receives a higher priority interrupt from one of its sourc-
es. The 8259A applies its own priority criteria to that interrupt and asserts its interrupt to the In-
terrupt Control Unit. Special fully nested mode allows the higher priority interrupt to be serviced
even though the In-Service bit for that source is already set. A higher priority interrupt has pre-
empted a lower priority interrupt, and interrupt nesting is fully maintained.
Special fully nested mode can also be used without cascade mode. In this case, it allows a single
external interrupt pin (either INT0 or INT1) to preempt itself.
8-8
INT
or
INTA
INT
or
INTA
INT0
V CC
INTA0
INT1
V CC
INTA1
Interrupt
Control
Unit
A1211-A0