
MOTOROLA
Chapter 4. Instruction and Data Cache Operation
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4-19
Cache Coherency—MEI Protocol
4.7.4
Coherency Precautions
The G2 core supports a three-state coherency protocol that supports the modified,
exclusive, and invalid (MEI) cache states. This protocol is a compatible subset of the MESI
four-state protocol and operates coherently in systems that contain four-state caches. In
addition, the G2 core does not broadcast cache operations caused by cache instructions.
They are intended for the management of the local cache but not for other caches in the
system.
4.7.4.1
Coherency in Single-Processor Systems
The following situations concerning coherency can be encountered within a
single-processor system:
Load or store to a caching-inhibited page (WIM = 0bx1x) and a cache hit occurs.
Caching is inhibited for this page (I = 1)—Load or store operations to a
caching-inhibited page that hit in the cache cause boundedly undefined results.
Store to a page marked write-through (WIM = 0b10x) and a cache read hit to a
modified cache block.
This page is marked as write-through (W = 1)—The G2 core pushes the modified
cache block to memory and the block remains marked modified (M).
Note that when WIM bits are changed, it is critical that the cache contents reflect the new
WIM bit settings. For example, if a block or page that had allowed caching becomes
caching-inhibited, software should ensure that the appropriate cache blocks are flushed to
memory and invalidated.
4.7.5
Load and Store Coherency Summary
Table 4-4 provides a summary of memory coherency actions performed by the G2 core on
load operations. Noncacheable cases are not part of this table.
Table 4-5 provides an overview of memory coherency actions on store operations. This
table does not include noncacheable or write-through cases. The read-with-intent-to-
modify (RWITM) examples involve selecting a replacement class and casting-out modified
data that may have resided in that replacement class.
Table 4-4. Memory Coherency Actions on Load Operations
Cache State
Bus Operation
core_artry
Action
M
None
Don’t care
Read from cache
E
None
Don’t care
Read from cache
I
Read
Negated
Load data and mark E
I
Read
Asserted
Retry read operation
F
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