MOTOROLA
Chapter 6. MPC860 Instruction Set
6-19
Part II. PowerPC Microprocessor Module
The
lwarx and stwcx. instructions require the EA to be aligned. Exception handling
software should not attempt to emulate a misaligned lwarx or stwcx. instruction, because
there is no correct way to deÞne the address associated with the reservation.
In general, the lwarx and stwcx. instructions should be used only in system programs,
which can be invoked by application programs as needed.
At most, one reservation exists simultaneously on any processor. The address associated
with the reservation can be changed by a subsequent lwarx instruction. The conditional
store is performed based upon the existence of a reservation established by the preceding
lwarx, regardless of whether the address generated by the lwarx matches that generated by
the stwcx. instruction. A reservation held by the processor is cleared by one of the
following:
¥
Executing an stwcx. instruction to any address
¥
Attempt by another device to modify a location in the reservation granularity
(16 bytes)
In write-through mode, lwarx and stwcx. do not cause a DSI exception.
The sync instruction guarantees that previously fetched instructions Þnish before any
subsequent instructions are dispatched to the execution units. It does not affect fetching;
instructions continue to be fetched up to the instruction queue limit, but dispatch stalls until
the sync Þnishes.
The original purpose of the sync instruction was to synchronize coherent memory with
other processors in a multiprocessor system; it makes sure that memory as seen by one
processor is the same as memory seen by the other processors, and broadcasts a special
signal to signal that the action is taking place. However, the MPC860 does not support this
enforcement of coherency in a multiprocessor system, and it broadcasts no special
synchronization signal. The MPC860 simply expects other processors not to rely on
coherency of memory that it has cached in copy-back mode.
The only case where a sync instruction would be useful in an MPC8xx system is if software
modiÞed the page table structure associated with the SMMU only and needed to guarantee
that data accesses after that instruction would be executed in the new data context.
However, this is an unexpected special case; isync would work here, but the pipeline need
not be ßushed in this case, so sync is sufÞcient.
6.2.5 PowerPC VEA Instructions
The PowerPC VEA describes the semantics of the memory model that can be assumed by
software processes, and includes descriptions of the cache model, cache control
instructions, address aliasing, and other related issues.
Содержание MPC860 PowerQUICC
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