80C186EC/80C188EC Microprocessor User's Manual

INTERRUPT CONTROL UNIT
Table 8-2. OCW2 Instruction Field Decoding (Continued)
R SL EOI Command
1 0 1 Rotate on Non-Specific EOI Command
1 1 0 Set Priority (Specific Rotation) *
1 1 1 Rotate on Specific EOI Command *
* These commands use the L2:0 field
The Rotate in Automatic EOI Mode commands control priority rotation when the 8259A module
is programmed (in ICW4) for Automatic EOI Mode. When Rotate in Automatic EOI Mode is set,
priority rotates automatically at the end of the interrupt acknowledge cycle. Automatic priority
rotation in Automatic EOI Mode is canceled by issuing the clear command (R=0, SL=0, EOI=0).
The Non-Specific EOI Command resets the highest-priority In-Service bit. The Rotate on Non-
Specific EOI Command resets the highest-priority In-Service bit and assigns the corresponding
IR line the lowest priority.
The Specific EOI Command resets the In-Service bit for the IR line specified in the L2:0 field of
OCW2. The Rotate on Specific EOI Command resets the In-Service bit for the IR line specified
in the L2:0 field of OCW2 and assigns that line the lowest priority.
The Set Priority Command (Specific Rotation) assigns the lowest priority to the IR line specified
in L2:0 of OCW2.
Bits D4 and D3 are part of the address for the OCW2 register. D4 and D3 must always be programmed
to zero. The L2:0 bits are “don’t care” when they are not used by an OCW2 instruction.
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