RadiSys EPC-5A, Hardware & Software Reference Manual

Page: 89 / 156

Chapter 6: The VMEbus Interface
Page 83
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Keep in mind that while PC/AT interrupts are edge sensitive, VMEbus interrupts
are level sensitive. As such, you must ensure that
1) The 8259 interrupt controller is enabled to capture interrupts before a
VMEbus interrupt occurs (otherwise VMEbus interrupts will be totally
missed) and
2) You must handle all pending VMEbus interrupts before returning from the
interrupt handler.
•
When an interrupt occurs, first acknowledge the interrupt to the PC/AT 8259
interrupt controllers by sending both interrupt controllers an End-of-Interrupt
(EOI).
•
You must make sure that your interrupt handler code is not re-entered while
dispatching interrupts. Either all interrupts should be disabled or IRQ10 should
be masked after doing the EOI to the interrupt controller. Remember to
re-enable them prior to leaving the interrupt handler.
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If you are using DOS, you may need to switch to an internal stack. This may or
may not be necessary in other environments and applications. You should also
store the state of the VMEbus (i.e., current byte ordering, bus mappings and
address modifiers) if you expect the state to change. Be sure to restore the state
before leaving the interrupt handler.