RadiSys EPC-8A, Hardware Reference Manual

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EPC-8A Hardware Reference
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masked after doing the EOI to the interrupt controller. Remember to re-enable them
prior to leaving the interrupt handler.
• 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.
Start of Loop
• Determine the source of the interrupt or event. This can be done by reading the VME
Interrupt State register which should be ANDed with the VME Interrupt Enable
register. As described above, the VME Event State register and VME Event enable
register may also be potential sources for the generation of IRQ10. Keep in mind that
all pending interrupts must be handled.
• If the interrupt is a VMEbus interrupt 1–7;
• Acknowledge the interrupt to the VMEbus device generating the interrupt as follows:
1. Set the IACK bit in the VME Modifier register.
2. Establish a byte-ordering for the status/ID to be read. Whether this is an 8-bit or
16-bit read is dependent on the card issuing the interrupt.
3. The address modifiers and transfer length are dependent on the hardware
generating the interrupt.
4. Perform a read of the VMEbus where the address being read reflects the interrupt
level being responded to. Address lines A3–A1 must reflect the interrupt level in
binary form. Multiply the interrupt level by 2 and use that as the address of the
read operation.
5. After the read operation, clear the IACK bit in the VME Modifier register.
• If the interrupt is a VXIbus message interrupt, the interrupt is acknowledged and
cleared by reading the appropriate register(s), followed by setting the WRDY bit in
the VME Response register.
• Call your interrupt handling routine.
• Upon returning from the interrupt handling routine, go back to the beginning of the
loop until no more interrupts are active. In other words, you must handle all other
active interrupts. This includes all other interrupts and errors which come in prior to
calling the interrupt handling routine as well as any new interrupts and errors which
may occur during this process. Only when all interrupts and error conditions are
handled may you return from the overall interrupt handler. Again, if you miss any
interrupts or errors, no other interrupts or errors are recognized.
Protected-Mode Direct VMEbus Accesses
Addresses above 256 MB map directly onto the VMEbus. When direct “protected-mode”
addressing of A24 or A16 space, the high-order nibble is used to define the access mode
and byte ordering. For A32 space, the high-order 2 bits define the access mode leaving 30
bits available for addressing. Thus, only the first 1 Gigabyte of VMEbus A32 space is