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80C186EC/80C188EC Microprocessor User's Manual

80C186EC/80C188EC Microprocessor User's Manual

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INTERRUPT CONTROL UNIT<br />

Initialization begins with the writing of ICW1. ICW1 is accessed whenever a write to the 8259A<br />

module occurs with A0=0 (MPICP0 or SPICP0) and data bit D4=1. The following actions occur<br />

within the 8259A module when ICW1 is written:<br />

• the edge detection circuit is reset<br />

• the Interrupt Mask Register is cleared<br />

• the IR7 line is assigned lowest priority (default)<br />

• the slave mode address is set to 7<br />

• Special Mask Mode is cleared<br />

• the Status Read bits are set to select the Interrupt Request Register<br />

Initialization continues with the successive writing of ICW2, ICW3 and ICW4. The remainder of<br />

this section describes the Initialization Command Words in detail.<br />

8-22

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