Motorola MPC8260 User Manual
MOTOROLA
Chapter 8. The 60x Bus
8-31
Part III. The Hardware Interface
Figure 8-11. Data Tenure Terminated by Assertion of TEA
MPC8260 interprets the following bus transactions as bus errors:
¥
Direct-store transactions, as indicated by the assertion of XATS.
¥
Bus errors asserted by slaves (internal or external).
8.6 Memory CoherencyÑMEI Protocol
The MPC8260 provides dedicated hardware to ensure memory coherency by snooping bus
transactions, by maintaining information about the status of data in a cache block, and by
the address retry capability. Each data cache block includes status bits that support the
modiÞed/exclusive/invalid, or MEI, cache-coherency protocol.
transactions, by maintaining information about the status of data in a cache block, and by
the address retry capability. Each data cache block includes status bits that support the
modiÞed/exclusive/invalid, or MEI, cache-coherency protocol.
Asserting the global (GBL) output signal indicates whether the current transaction must be
snooped by other snooping devices on the bus. Address bus masters assert GBL to indicate
that the current transaction is a global access (that is, an access to memory shared by more
than one device). If GBL is not asserted for the transaction, that transaction is not snooped.
When other devices detect the GBL input asserted, they must respond by snooping any
addresses broadcast. Normally, GBL reßects the M bit value speciÞed for the memory
reference in the corresponding translation descriptor. Care must be taken to minimize the
number of pages marked as global, because the retry protocol discussed in the previous
section used to enforce coherency can require signiÞcant bus bandwidth.
snooped by other snooping devices on the bus. Address bus masters assert GBL to indicate
that the current transaction is a global access (that is, an access to memory shared by more
than one device). If GBL is not asserted for the transaction, that transaction is not snooped.
When other devices detect the GBL input asserted, they must respond by snooping any
addresses broadcast. Normally, GBL reßects the M bit value speciÞed for the memory
reference in the corresponding translation descriptor. Care must be taken to minimize the
number of pages marked as global, because the retry protocol discussed in the previous
section used to enforce coherency can require signiÞcant bus bandwidth.
CLKOUT
ADDR + ATTR
TS
AACK
DBG
TA
Data
TEA
For Single
For Burst