Chapter 9. System Interface Operation
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MPX Bus Protocol
9.6.2.2.8 Data Tenure Reordering in MPX Bus Only
The MPC7400 allows data tenures to be executed out of order with respect to their
corresponding address tenures in MPX bus mode. The system must supply an index with
each data bus grant to indicate which of the masters outstanding data tenures is being
serviced. The data transfer index inputs, DTI[0:2], provide this function.
The DTI[0:2] signals act as a pointer into the queue of outstanding transactions within the
MPC7400. The MPC7400 supports up to six outstanding transactions. DTI[0:2] for a given
data bus tenure is driven by the system on the cycle prior to the associated DBG. The
MPC7400 continually samples DTI[0:2] and qualiTes the pointer on the subsequent cycle
if the data bus grant is qualiTed.
A DTI value of 0b000 indicates that the data tenure for the oldest remaining transaction is
to be serviced; a value of 0b001 indicates that the second oldest remaining transaction is to
be serviced, etc. A value of 0b101 selects the sixth and oldest MPC7400 transaction. The
system tracks the status of the MPC7400 queues by monitoring the TS and DBG. The
MPC7400 adds a new transaction to the tail of its queue with each assertion of TS for an
address and data transaction and each assertion of HIT for data-only transactions. It
removes an entry if the transaction is retried with ARTRY, or when it receives a qualiTed
DBG. When a transaction is removed from the queue, all transactions newer than the
transaction removed shift forward.
The system must not provide an illegal DTI value the cycle before any qualiTed data bus
grant when the processor has one or more valid data transactions queued. An illegal DTI
value is deTned as one of the following:
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A DTI value greater than 0b101
A DTI value that does not have any valid corresponding data transaction unless the
DTI value is 0b000. For instance, if only two data transactions are queued, a DTI
value of 0b010 or greater is illegal.
The processors behavior is boundedly undeTned if an illegal DTI value is detected at this
time.
9.6.2.3 Data Termination Phase in MPX Bus Mode
Three signals are used to terminate the individual data beats of the data tenure and the data
tenure itselfTA, TEA and ARTRY. In MPX bus mode, they function as in the 60x bus
mode.