Pci To Local Memory Read Buffering - Motorola MPC8240 User Manual

Table 12-1 summarizes the snooping behavior of PCI accesses to local memory that hit in
one of the internal buffers.
Table 12-1. Snooping Behavior Caused by a Hit in an Internal Buffer
Read
Read (not locked)
Read (first access of
a locked transfer)
Read (not locked)
Read (first access of
a locked transfer)
Write
Write
Write
1
Only reads can start an exclusive access (locked transfer). The
first locked transfer must be snooped so that the cache line in the
L1 is invalidated.

12.1.3.1 PCI to Local Memory Read Buffering

The following subsections describe the PCMRB buffer and capability of the MPC8240 to
perform speculative PCI reads from local memory.
12.1.3.1.1 PCI-to-Local-Memory-Read Buffers (PCMRBs)
When a PCI device initiates a read from local memory, the address is snooped on the
peripheral logic bus (provided snooping is enabled). If the memory interface is available,
the memory access is started simultaneously with the snoop. If the snoop results in a hit in
the L1 cache, the MPC8240 cancels the local memory access.
Depending on the outcome of the snoop, the requested data is latched into either one of the
32-byte PCI-to-local-memory-read buffers (PCMRBs), or into both the copyback buffer
and a PCMRB (as described in Section 12.1.1, "Processor Core/Local Memory Buffers")
as follows:
• If the snoop hits in the L1, the copyback data is written to the copyback buffer and
copied to the PCMRB when the copyback buffer is flushed to memory. The data is
forwarded to the PCI bus from the PCMRB, and to local memory from the copyback
buffer.
• If the snoop does not hit in the L1, a PCMRB is filled from local memory with the
entire corresponding cache line, regardless of whether the starting address of the
PCI-initiated transaction was at a cache-line boundary. The data is forwarded to the
PCI bus from the PCMRB as the PCMRB is loaded (the CCU doesn't wait for the
PCMRB to be full).
PCI Transaction Hit in Internal Buffer
1
1
Chapter 12. Central Control Unit
Snoop Required
Copyback
PCMRB
PCMRB
PCMWB
PCMWB
Copyback
PCMRB
PCMWB
Internal Buffers
Yes
No
Yes
No
Yes
Yes
Yes
No
12-7