Raven Interrupt Controller Implementation
http://www.motorola.com/computer/literature 2-77
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Dynamically Changing I/O Interrupt Configuration
The interrupt controller provides a mechanism for safely changing the
vector, priority, or destination of I/O interrupt sources. This is provided to
support systems which allow dynamic configuration of I/O devices. In
order to change the vector, priority, or destination of an active interrupt
source, the following sequence should be performed:
1. Mask the source using the MASK bit in the vector/priority register.
2. Wait for the activity bit (ACT) for that source to be cleared.
3. Make the desired changes.
4. Unmask the source.
This sequence ensures that the vector, priority, destination, and mask
information remain valid until all processing of pending interrupts is
complete.
EOI Register
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.
Interrupt Acknowledge Register
Upon receipt of an interrupt signal, the processor may read this register to
retrieve the vector of the interrupt source which caused the interrupt.
8259 Mode
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
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