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8-29
INTERRUPT CONTROL UNIT
8.5.2 Interrupt Vectoring in Slave Mode
In Slave mode, the external 8259A module acts as the master interrupt controller. Therefore, in-
terrupt acknowledge cycles are required for every interrupt, including those from integrated pe-
ripherals. During the first interrupt acknowledge cycle, the external 8259A determines which
slave interrupt controller has the highest priority interrupt request. It then drives that slave’s ad-
dress onto its CAS2:0
pins (Figure 8-20). External logic must decode the correct slave address
from the CAS2:0
pins to drive the SELECT pin.
Figure 8-20. Interrupt Vectoring in Slave Mode
The SELECT pin is the slave-select input to the Interrupt Control Unit. During the second inter-
rupt acknowledge cycle, the highest-priority slave interrupt controller transfers the interrupt type
of its highest priority interrupt to the CPU. If the Interrupt Control Unit is the highest-priority
slave, it passes the interrupt type to the CPU internally; however, the interrupt acknowledge cycle
still must occur for the benefit of the external 8259A module.
T1 T2 T3 T4
CLKOUT
TI TI T1 T2 T3
LOCK
CAS2:0
S2:0
INTA0
SELECT
NOTES:
1. INT1/SELECT has the SELECT function in slave mode.
2. INT2/INTA0 has the INTA0 function in slave mode.
3. Cascade address is driven by the external 8259A.
4. SELECT must be driven before phase 2 of T2 of the second INTA.
5. SELECT read by processor.
6. ALE is generated for each INTA.
7. RD is inactive.
Slave Cascade Address From 8259A
T4
INTA INTA
A1199-A0