307013-003 Datasheet, PDF (137/848 Pages) Intel Corporation – Intel I/O Controller Hub 7

Functional Description
5.9.4.2
5.9.4.3
5.9.4.4
5.9.4.5
Special Fully-Nested Mode
This mode is used in the case of a system where cascading is used, and the priority has
to be conserved within each slave. In this case, the special fully-nested mode is
programmed to the master controller. This mode is similar to the fully-nested mode
with the following exceptions:
• 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 interrupts within the slave are recognized by the master and initiate
interrupts to the processor. In the normal-nested mode, a slave is masked out
when its request is in service.
• When exiting the Interrupt Service routine, software has to check whether the
interrupt serviced was the only one from that slave. This is done by sending a Non-
Specific EOI command to the slave and then reading its ISR. If it is 0, a non-
specific EOI can also be sent to the master.
Automatic Rotation Mode (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 has to wait until each of seven other devices are serviced at
most once.
There are two ways to accomplish automatic rotation using OCW2; the Rotation on
Non-Specific EOI Command (R=1, SL=0, EOI=1) and the rotate in automatic EOI mode
which is set by (R=1, SL=0, EOI=0).
Specific Rotation Mode (Specific Priority)
Software can change interrupt priorities by programming the bottom priority. For
example, if IRQ5 is programmed as the bottom priority device, then IRQ6 is the highest
priority device. The Set Priority Command is issued in OCW2 to accomplish this, where:
R=1, SL=1, and LO–L2 is the binary priority level code of the bottom priority device.
In this mode, internal status is updated by software control during OCW2. However, it
is independent of the EOI command. Priority changes can be executed during an EOI
command by using the Rotate on Specific EOI Command in OCW2 (R=1, SL=1, EOI=1
and LO–L2=IRQ level to receive bottom priority.
Poll Mode
Poll mode can be used to conserve space in the interrupt vector table. Multiple
interrupts that can be serviced by one interrupt service routine do not need separate
vectors if the service routine uses the poll command. Poll mode can also be used to
expand the number of interrupts. The polling interrupt service routine can call the
appropriate service routine, instead of providing the interrupt vectors in the vector
table. In this mode, the INTR output is not used and the microprocessor internal
Interrupt Enable flip-flop is reset, disabling its interrupt input. Service to devices is
achieved by software using a Poll command.
The Poll command is issued by setting P=1 in OCW3. The PIC treats its next I/O read as
an interrupt acknowledge, sets the appropriate ISR bit if there is a request, and reads
the priority level. Interrupts are frozen from the OCW3 write to the I/O read. The byte
returned during the I/O read contains a 1 in bit 7 if there is an interrupt, and the binary
code of the highest priority level in bits 2:0.
Intel ^® ICH7 Family Datasheet
137