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Raven PCI Bridge ASIC
EOI Register
2
Interrupt Acknowledge Register
8259 Mode
Current Task Priority Level
2-90
Each processor has a private EOI register which is used to signal the end
of processing for a particular interrupt event. If multiple nested interrupts
are in service, the EOI command terminates the interrupt service of the
highest priority source. Once an interrupt is acknowledged, only sources
of higher priority will be allowed to interrupt the processor until the EOI
command is received. This register should always be written with a value
of zero which is the nonspecific EOI command.
Upon receipt of an interrupt signal, the processor may read this register to
retrieve the vector of the interrupt source which caused the interrupt.
The 8259 mode bits control the use of an external 8259 pair for PC-AT
compatibility. Following a reset, this mode is set for pass-through, which
essentially disables the advanced controller and passes an 8259 input on
external interrupt source 0 directly through to processor 0. During interrupt
controller initialization this channel should be programmed for mixed
mode in order to take advantage of the interrupt delivery modes.
Each processor has a separate Current Task Priority Level register. The
system software uses this register to indicate the relative priority of the task
running on the corresponding processor. The interrupt controller will not
deliver an interrupt to a processor unless it has a priority level which is
greater than the current task priority level of that processor. This value is
also used in determining the destination for interrupts which are delivered
using the distributed deliver mode.
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