Instruction and Data Caches
MPC885 PowerQUICC Family Reference Manual, Rev. 2
7-26
Freescale Semiconductor
7.6.5
Data Accesses to Caching-Inhibited Memory Regions
For load misses to caching-inhibited memory regions, the data is read from memory but not placed in the
cache and the cache status is not affected.
For store misses to caching-inhibited memory regions, the data is written to memory but not placed in the
cache and the cache status is not affected.
It is considered a programming error if a load, store, or dcbz targeting a caching-inhibited memory region
results in a cache hit. The PowerPC architecture allows the result of such programming errors to be
boundedly undefined. Software must ensure that data from a caching-inhibited regions have not been
previously loaded into the data cache, or, if they have, that those blocks have been flushed from the cache.
Whenever the memory/cache attributes of any memory region are changed (for example, from
caching-allowed to caching-inhibited), it is critical that the cache contents reflect the new attributes.
Therefore, when changing memory region attributes (in the MMU) the user must perform the procedures
described in
Section 7.5.5, “Updating Code and Memory Region Attributes.”
7.6.6
Atomic Memory References
The PowerPC architecture defines the Load Word and Reserve Indexed (lwarx) and the Store Word
Conditional Indexed (stwcx.) instructions to provide an atomic update function for a single, aligned word
of memory. These instructions can be used to develop a rich set of multiprocessor synchronization
primitives. For detailed information about these instructions, refer to
Section 5.2.4.6, “Memory
Synchronization Instructions—UISA,”
in this book and Chapter 8, “Instruction Set,” in the Programming
Environments Manual.
The lwarx instruction performs a load word from memory operation and creates a reservation for the
16-byte section of memory that contains the accessed word. The reservation granularity is 16 bytes. The
lwarx instruction makes a nonspecific reservation with respect to the executing processor and a specific
reservation with respect to other masters. This means that any subsequent stwcx. executed by the same
processor, regardless of address, will cancel the reservation. Also, any bus write operation from another
processor to an address that matches the reservation address will cancel the reservation.
The stwcx. instruction does not check the reservation for a matching address. The stwcx. instruction is
only required to determine whether a reservation exists. The stwcx. instruction performs a store word
operation only if the reservation exists. If the reservation has been cancelled for any reason, the stwcx.
instruction fails and clears the CR0[EQ] bit in the condition register. The architectural intent is to follow
the lwarx/stwcx. instruction pair with a conditional branch which checks to see whether the stwcx.
instruction failed.
Note that atomic memory references constructed using lwarx/stwcx. instructions depend on the presence
of a coherent memory system for correct operation. These instructions should not be expected to provide
atomic access to noncoherent memory. Since the MPC885 does not snoop external bus activity, provision
is made to cancel a reservation inside the MPC885 by using the CR and KR input signals. The state of the
reservation is always presented onto the RSV output signal. This can be used by external agents to
determine when an internal condition has caused a change in the reservation state. See
Section 13.4.9,
“Memory Reservation,”
for more information. Internal to the MPC885, the data cache has snoop logic to
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