Cascade Mode - Intel 460GX Software Developer’s Manual

15.2.3.4 Specific Rotation (Specific Priority)
The programmer can change priorities by programming the bottom priority and thus fixing all
other priorities. For example, if IRQ5 is programmed as the bottom priority device, then IRQ6 will
be the highest priority device.
The Set Priority Command is issued in OCW2 where: R=1, SL=1; LO-L2 is the binary priority
level code of the bottom priority device. See the register description for the bit definitions.
Note that, in this mode, internal status is updated by software control during OCW2. However, it is
independent of the End of Interrupt (EOI) command (also executed by OCW2). 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).
15.2.3.5 Poll Command
The Polled 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.
The 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 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 Interrupt Controller treats the next I/O
read pulse to the Interrupt Controller as an interrupt acknowledge, sets the appropriate IS bit if
there is a request, and reads the priority level. Interrupts are frozen from the I/O write to the I/O
read.
This mode is useful if there is a routine command common to several levels so that the INTA#
sequence is not needed (saves ROM space).
15.2.4

Cascade Mode

The Interrupt Controllers in the IFB are interconnected in a cascade configuration with one master
and one slave. This configuration can handle up to 15 separate priority levels.
The master controls the slaves through a three line internal cascade bus. When the master drives
010b on the cascade bus, this bus acts like a chip select to the slave controller.
In a cascade configuration, the slave interrupt outputs are connected to the master interrupt request
inputs. When a slave request line is activated and afterwards acknowledged, the master will enable
the corresponding slave to release the interrupt vector address during the second INTA# cycle of
the interrupt acknowledge sequence.
Each Interrupt Controller in the cascaded system must follow a separate initialization sequence and
can be programmed to work in a different mode. An EOI Command must be issued twice: once for
the master and once for the slave.
Intel® 460GX Chipset Software Developer's Manual
PCI/LPC Bridge Description
15-5