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INTERRUPT CONTROL UNIT
Initialization begins with the writing of ICW1. ICW1 is accessed whenever a write to the 8259A
module occurs with A0=0 (MPICP0 or SPICP0) and data bit D4=1. The following actions occur
within the 8259A module when ICW1 is written:
•
the edge detection circuit is reset
•
the Interrupt Mask Register is cleared
•
the IR7 line is assigned lowest priority (default)
•
the slave mode address is set to 7
•
Special Mask Mode is cleared
•
the Status Read bits are set to select the Interrupt Request Register
Initialization continues with the successive writing of ICW2, ICW3 and ICW4. The remainder of
this section describes the Initialization Command Words in detail.
8-22
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