82374EB Datasheet, PDF (160/208 Pages) Intel Corporation – SYSTEM COMPONENT (ESC)

82374EB 82374SB
9 6 2 THE SPECIAL FULLY NESTED MODE
This mode will be used in the case of a big system where cascading is used and the priority has to be
conserved within each slave In this case the special fully nested mode will be programmed to the master
(using ICW4) This mode is similar to the normal nested mode with the following exceptions
1 When an interrupt request from a certain slave is in service this slave is not locked out from the master’s
priority logic and further interrupt requests from higher priority IRQ’s within the slave will be recognized by
the master and will initiate interrupts to the processor (In the normal nested mode a slave is masked out
when its request is in service and no higher requests from the same slave can be serviced )
2 When exiting the Interrupt Service routine the software has to check whether the interrupt serviced was the
only one from that slave This is done by sending a non-specific End of Interrupt (EOI) command to the
slave and then reading its In-Service register and checking for zero If it is empty a non-specific EOI can be
sent to the master too If not no EOI should be sent
9 6 3 AUTOMATIC ROTATION (EQUAL PRIORITY DEVICES)
In some applications there are a number of interrupting devices of equal priority Automatic rotation mode
provides for a sequential 8-way rotation In this mode a device receives the lowest priority after being serviced
In the worst case a device requesting an interrupt will have to wait until each of seven other devices are
serviced at most once Figure 21 shows an example of automatic rotation
Figure 21 Automatic Rotation Mode Example
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