Chapter 9. System Interface Operation
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MPX Bus Protocol
source data to processor B, DBG to processor B may be asserted, and the data source can
continue to stream another data tenure.
The negation of ARTRY for that address tenure indicates that the address tenure associated
with the data tenure about to be granted was not retried by a snooping device. Because of
address pipelining, an assertion of ARTRY may not be for the data tenure about to be
granted; therefore, it may not affect data bus grant qualiTcation.
One bus clock cycle after accepting a qualiTed data bus grant, the MPC7400 begins driving
or sampling the data bus and sampling the transfer acknowledge signals.
9.6.2.1.2 Data Streaming Constraints for Data Bus Arbitration in MPX Bus
Mode
Data streaming from one data tenure to the next in MPX mode is supported only for data
tenures in which the source of the data is the same. A dead cycle must be placed in between
two adjacent data tenures when the data is driven by two different devices. For example, if
the Trst data transfer is a processor read from memory and the second data transfer is a
processor write to memory, data streaming is not supported; a minimum of one dead cycle
must be inserted between the Tnal TA of the read and the Trst TA of the write. In this
example, the earliest cycle that the processor accepts DBG as a qualiTed DBG is the cycle
after the Tnal TA of the read.
In addition, data streaming from one data transfer to a second is only allowed if the Trst
transfer is a multiple-beat transfer (requiring at least two TAs). If the Trst data transfer is a
single-beat transfer (only one TA), then the earliest that the processor will accept a DBG as
a qualiTed DBG is the cycle after the single TA assertion.
Note that with these two constraints, it is possible to stream from a multiple-beat
transaction into a single-beat transaction. While this may not affect overall system
performance, it may simplify the design of the data bus arbiter and memory controller.
9.6.2.2 Data Bus Transfers
The MPC7400 in MPX bus mode expects data transactions to be carried out in 64-bit beats
just like in the 60x bus interface. Four-beat burst transfers are used by the MPC7400 to
transfer cache lines into or out of its internal cache. Two-beat burst transfers are used for
caching-inhibited or write-through 128-bit AltiVec loads or stores. Non-burst transfers are
performed for non-AltiVec caching-inhibited or write-through operations. The MPC7400
does not directly support dynamic interfacing to subsystems with less than a 64-bit wide
data path, and it does not support a static 32-bit data bus mode. (See Section 9.6.1.3,
òTransfer Attributes in MPX Bus Modeó).
Note that the operation of the data and data parity signals is the same as that in the 60x bus
mode (see Section 9.4.2, òData Transfer Signals and Protocol,ó) except as noted in the
following subsections.