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7. UltraSPARC External Interfaces
perform any tag match on its Etag for S_CPD_REQ, in order to accelerate its
P_REPLY. In this case, the SC’s copyback request is itself an error, indicating that
the Dtags do not accurately reflect the state of the processor’s E-Cache.
7.17.2.9 Target ID<4:0>
This field is only used in the interrupt request packet. It contains the Port ID of
the destination UltraSPARC to which the interrupt packet is to be delivered.
7.17.2.10 Parity
The parity bit is bit 35 of SYSADDR; it protects SYSADDR<34:0} with odd parity.
That is, if the sum of the ‘1’ bits on bits 34:0 is even, Parity is set to 1; otherwise,
Parity is set to 0.
7.18 WriteInvalidate
If UltraSPARC sets the IVA bit in a P_WRI_REQ transaction, the it expects SC to
send an S_INV_REQ for the associated line. In systems with Dtags, the Dtags will
correctly indicate to SC whether or not to send S_INV_REQ to the requestor; in
this case, SC can ignore the IVA bit. In system without Dtags, however, SC must
send the requesting UltraSPARC an S_INV_REQ if IVA=1 in a P_WRI_REQ.
7.18.1 Using the IVA bit in a P_WRI_REQ
UltraSPARC can issue a cache-coherent block store that will guarantee all caches
are invalid when it completes. In this case, SC must issue S_INV_REQ to all ap-
propriate caches, including the master that issued the P_WRI_REQ. This is be-
cause the issuer cannot invalidate the line until the P_WRI_REQ has entered the
memory order, in case there are pending S_REQs coming to that line.
In systems that do not support Dtags, UltraSPARC sets the IVA (Invalidate Advi-
sory) bit to indicate that it needs an S_INV_REQ in order for its P_WRI_REQ to
complete. UltraSPARC can set IVA when it is not needed, but IVA should never
be clear when it should be set.
Since P_WRI_REQs can be outstanding with coherent read misses, there is a pos-
sible race condition if they are to the same address. (The P_WRI_REQs and coher-
ent read misses can complete out of order.) UltraSPARC resolves this by:
•
Restricting the issue of some transactions during pending P_WRI_REQs, and
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