Sun Microelectronics
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7. UltraSPARC External Interfaces
• P_SNACK if the block is not present in the E-Cache or the writeback buffer.
The P_SACK or P_SACKD reply indicates that UltraSPARC is ready to transfer
the requested data. SC initiates the data transfer by sending S_CRAB. If NDP=0
and the block was not present in the cache, UltraSPARC drives undefined data in
response to the S_CRAB.
UltraSPARC responds more quickly if NDP=0; SC should assert NDP only in sys-
tems that do not support Dtags. Section 7.10, “S_REQ,” on page 111 for more tim-
ing information.
UltraSPARC supports one outstanding coherent system request. SC can send its
next coherent request on the cycle after the S_CRAB reply.
7.8 Non-Cached Data Transactions
This section specifies the non-cached data transactions; that is, transactions is-
sued while the MMU is disabled or to non-physical cacheable pages. UltraSPARC
does not cache data associated with these transactions.
7.8.1 NonCachedRead (P_NCRD_REQ)
Noncached Read. Generated by an UltraSPARC by a load or instruction fetch
from a noncached address space, or by SC to read an UltraSPARC’s port_ID reg-
ister on behalf of another processor.
This transaction reads either 1, 2, 4, 8, or 16 bytes; the byte location is specified
with a bytemask in the request packet. The address is aligned on a 16-byte
boundary. The bytemask is aligned on a natural boundary.
SC sends an S_RAS (Read ACK Single) reply, which directs the requesting
UltraSPARC to receive the data from SYSDATA.
SC can send P_NCRD_REQ to UltraSPARC in order to service an interprocessor
read request. The transaction sequence is as follows:
1. UltraSPARC
1
sends P_NCRD_REQ to SC in order to read the port_ID of
UltraSPARC
2
2. SC forwards the P_NCRD_REQ to UltraSPARC
2
3. UltraSPARC
2
responds to SC with P_RAS, indicating that it is ready to
drive the requested data
4. SC responds to UltraSPARC
2
by sending S_SRS
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