
VXIbus Interface
The state registers can also be read by non-interrupt-handler software to poll for the
state of these signals.
The enable registers allow one to mask selectively these 22 status signals. A 0 status
bit and a corresponding 1 enable bit causes the PC architecture IRQ10 interrupt to be
asserted.
Unlike the 22 input conditions, which are level sensitive inputs, the PC architecture
defines the PC interrupts, such as IRQ10, as edge sensitive. This requires special
attention if you are writing your own interrupt handlers. Because IRQ10 is edge trig-
gered, you could miss an incoming interrupt/event that occurs when IRQ10 is dis-
abled, meaning that your software needs to test for and handle all pending inter-
rupts/events before you leave from the IRQ10 interrupt handler. To do this correctly,
follow the following steps. These steps assume the reader is familiar with the pro-
gramming of the 8259 interrupt controller in the PC architecture.
1. Depending on your environment, you may wish to switch to another
stack (a must under DOS), and may wish to save the state of the VME
modifier and address registers if you will be using them.
2. To prevent reentry to the interrupt handler, mask off all the inter-
rupts/events or mask off the IRQ10 interrupt. (Reenable what you have
masked off at the end of the interrupt handler.)
3. Acknowledge the interrupt by sending end-of-interrupt to both 8259
interrupt controllers.
4. Find an enabled pending interrupt/event.
7
7
5. If an enabled pending VXIbus interrupt is found, do an interrupt-
acknowledge cycle by setting the IACK bit in the VME modifier register
and performing a VMEbus read, setting address bits A3-A1 to denote the
interrupt number. This returns the status/ID value from the interrupter.
For the other controllable conditions (message, sticky BERR, watchdog
timer), you may follow the instructions earlier in this chapter to remove
these interrupting conditions.
6. Perform
application-dependent
handling of the interrupt/event.
7. If there are still enabled pending interrupts/events, go to step 4. If not,
return from the IRQ10 interrupt handler.
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