Motorola MPC750 User Manual page 345

In general, an address tenure on the bus is followed strictly in order by its associated data
tenure. Transactions pipelined by the MPC750 complete strictly in order. However, the
MPC750 can run bus transactions out of order only when the external system allows the
MPC750 to perform a cache-line-snoop-push-out operation (or other write transaction, if
pending in the MPC750 write queues) between the address and data tenures of a read
operation through the use of DBWO. This effectively envelopes the write operation within
the read operation. Figure 8-22 shows how the DBWO signal is used to perform an
enveloped write transaction.
Write Address
(1) (2).-_ _
--"LJ
LJ
~
LJ
Enveloped Write
Transaction
(2)
r - - - - - - - - - . . ,
(1)..--_ _ _ _ _ _
--"LJ LJ
OBB
~~
____________
~!I~
____________
~r-
OBWO
L S
Figure 8-22. Data Bus Write Only Transaction
Note that although the MPC750 can pipeline any write transaction behind the read
transaction, special care should be used when using the enveloped write feature. It is
envisioned that most system implementations will not need this capability; for these
applications, DBWO should remain negated. In systems where this capability is needed,
DBWO should be asserted under the following scenario:
1. The MPC750 initiates a read transaction (either single-beat or burst) by completing
the read address tenure with no address retry.
2. Then, the MPC750 initiates a write transaction by completing the write address
tenure, with no address retry.
3. At this point, if DBWO is asserted with a qualified data bus grant to the MPC750,
the MPC750 asserts DBB and drives the write data onto the data bus, out of order
with respect to the address pipeline. The write transaction concludes with the
MPC750 negating DBB.
4. The next qualified data bus grant signals the MPC750 to complete the outstanding
read transaction by latching the data on the bus. This assertion of DBG should not
be accompanied by an asserted DBWO.
8-38
MPC750 RISC Microprocessor User's Manual