Chapter 12. Central Control Unit
12-7
Internal Buffers
Table 12-1 summarizes the snooping behavior of PCI accesses to local memory that hit in
one of the internal buffers.
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).
Table 12-1. Snooping Behavior Caused by a Hit in an Internal Buffer
PCI Transaction
Hit in Internal Buffer
Snoop Required
Read
Copyback
Yes
Read (not locked)
PCMRB
No
Read (first access of
a locked transfer)
1
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.
PCMRB
Yes
Read (not locked)
PCMWB
No
Read (first access of
a locked transfer)
PCMWB
Yes
Write
Copyback
Yes
Write
PCMRB
Yes
Write
PCMWB
No
Summary of Contents for MPC8240
Page 1: ...MPC8240UM D Rev 1 1 2001 MPC8240 Integrated Processor User s Manual ...
Page 38: ...xviii MPC8240 Integrated Processor User s Manual TABLES Table Number Title Page Number ...
Page 48: ...xlviii MPC8240 Integrated Processor User s Manual Acronyms and Abbreviations ...
Page 312: ...6 94 MPC8240 Integrated Processor User s Manual ROM Flash Interface Operation ...
Page 348: ...7 36 MPC8240 Integrated Processor User s Manual PCI Host and Agent Modes ...
Page 372: ...8 24 MPC8240 Integrated Processor User s Manual DMA Register Descriptions ...
Page 394: ...9 22 MPC8240 Integrated Processor User s Manual I2O Interface ...
Page 412: ...10 18 MPC8240 Integrated Processor User s Manual Programming Guidelines ...
Page 454: ...12 14 MPC8240 Integrated Processor User s Manual Internal Arbitration ...
Page 466: ...13 12 MPC8240 Integrated Processor User s Manual Exception Latencies ...
Page 516: ...16 14 Watchpoint Trigger Applications ...
Page 538: ...B 16 MPC8240 Integrated Processor User s Manual Setting the Endian Mode of Operation ...
Page 546: ...C 8 MPC8240 Integrated Processor User s Manual ...
Page 640: ...INDEX Index 16 MPC8240 Integrated Processor User s Manual ...