Core Complex Bus (CCB)
PowerPC e500 Core Family Reference Manual, Rev. 1
Freescale Semiconductor
13-7
The system ensures that the ordering barrier established by the ORDER command between any
pre- and post-mbar bus transactions (excluding instruction fetches) is honored in any system
queues and out to the transactions’ destinations. If transaction ordering does not occur naturally or
is not easily controlled in the system, a simple method could be to not complete the ORDER
command on the bus (similar to the SYNC command) until all prior bus transactions have
completed or to withhold bus grant for any further transactions until such completion.
13.4 Address Streaming Mode
Address streaming mode (selected by setting HID1[ASTE]) provides a way to increase address
bus throughput on the CCB. Address streaming is useful for systems that must normally extend
the address tenure by delaying address acknowledge after transfer start, thereby reducing bus
transactions during a given period, as in the following examples:
•
A system where addresses cannot be decoded or accepted immediately after transfer start
by the system
•
A snooping system where address acknowledge must be delayed to allow snooping caches
(including the L1 caches of the core complex in certain clock modes) to process a snoop
transaction
Note that address streaming, as defined here, differs from address pipelining, which is the issue of
multiple address tenures independent of whether associated data tenures were started or
completed.
Address streaming allows one additional bus transaction from the same bus master to start on the
address bus during a current address tenure. This mode effectively overlaps and staggers two
address tenures from the same bus master at any given time. It also effectively pipelines address
tenures with respect to the address acknowledge/retry window.
13.5 L2 Cache Support
The e500 implements specific instructions to selectively lock and unlock lines in its L1 caches or
in an L2 cache. To facilitate locking and unlocking of a front-side L2 cache (usually located
directly on the CCB), the core complex provides an address lock attribute (CL) on the bus, which
can be used in conjunction with the internal transfer type, tt[0:4], encodings to identify which
addresses to lock or unlock.
13.5.1 L2 Locking
When the core complex executes an instruction to lock a line in an L2 cache (dcbtls, dcbtstls, or
icbtls, with CT = 1), it performs the associated bus operation as a burst read transaction with the
lock attribute asserted. A front-side L2 cache may recognize this transaction as a direction to
establish the cache line (if not already valid) and to mark it as locked. Note that this is a complete
Содержание PowerPC e500 Core
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