82801BA Datasheet, PDF (107/498 Pages) Intel Corporation – Intel 82801BA I/O Controller Hub 2 (ICH2) and Intel 82801BAM I/O Controller Hub 2 Mobile

Functional Description
5.7.4
Modes of Operation
Fully Nested Mode
In this mode, interrupt requests are ordered in priority from 0 through 7, with 0 being the highest.
When an interrupt is acknowledged, the highest priority request is determined and its vector placed
on the bus. Additionally, the ISR for the interrupt is set. This ISR bit remains set until: the
processor issues an EOI command immediately before returning from the service routine; or if in
AEOI mode, on the trailing edge of the second INTA#. While the ISR bit is set, all further
interrupts of the same or lower priority are inhibited; higher levels will generate another interrupt.
Interrupt priorities can be changed in the rotating priority mode.
Special Fully Nested Mode
This mode will be 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 will be 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 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.
• 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 will have 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, IRQ6 will be 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. Polled 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
82801BA ICH2 and 82801BAM ICH2-M Datasheet
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