
Sun Microelectronics
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7. UltraSPARC External Interfaces
2.
After software clears BUSY in the Interrupt Vector Receive register,
UltraSPARC sends a P_IAK reply. UltraSPARC supports only one
outstanding P_INT_REQ transaction; SC can send the next P_INT_REQ
request on the cycle after the P_IAK reply.
When UltraSPARC sends an interrupt:
1.
If SC can deliver the interrupt transaction to the target (that is, if the target
UltraSPARC does not have another outstanding interrupt), SC issues an
S_WAB reply to the sending UltraSPARC, commanding it to drive the
interrupt data on SYSDATA. UltraSPARC clears the BUSY and NACK bits
in the Interrupt Vector Dispatch Register.
2.
If SC cannot deliver the interrupt (because the target has an outstanding
interrupt), SC should issue an S_INAK to the sending UltraSPARC.
UltraSPARC clears the BUSY bit and sets the NACK bit in its Interrupt
Vector Dispatch Register. In this case, software can retry later after some
backoff period.
7.12.1 Extended Interrupt Target ID
During an interrupt send, UltraSPARC also passes PA<20:19> to create an extend-
ed MID<6:5> field. (See Chapter 9, “Interrupt Handling.”) This may be useful for
extending the interrupt send domain. This extended MID is not present any-
where else, however; for example, in the P_REPLYs or other address packets.
7.12.2 P_IAK Assertion
After UltraSPARC receives an interrupt (P_INT_REQ), it waits until software
clears the BUSY bit in the Interrupt Vector Receive Register and then asserts
P_IAK. This informs SC that UltraSPARC is ready to receive another interrupt.
Software can clear the BUSY bit in the Interrupt Vector Receive Register at any
time. UltraSPARC issues P_IAK only when the BUSY bit is cleared following a
P_INT_REQ that has not been P_IAKed.
7.13 P_REPLY and S_REPLY
7.13.1 P_REPLY
P_REPLY is a 5-bit physical interface between each UltraSPARC and the SC. Each
UltraSPARC drives the P_REPLY pins radially to SC. Figure 7-22 shows the
P_REPLY packet format.
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