Intel 80C186XL, 80C188XL user manual INT0

Models: 80C186XL 80C188XL

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INTERRUPT CONTROL UNIT

INT

 

INT0

8259A

V

CC

or

 

 

 

82C59A

 

 

INTA

 

INTA0

 

 

Interrupt

 

 

Control

 

 

Unit

INT

 

INT1

8259A

VCC

or

 

 

82C59A

 

 

INTA

 

INTA1

 

 

A1211-A0

Figure 8-2. Using External 8259A Modules in Cascade Mode

8.3.3.1Special 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

Page 203
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Intel 80C186XL, 80C188XL user manual INT0