MOTOROLA
Chapter 8. Instruction and Data Caches
8-19
Part II. PowerPC Microprocessor Module
The MPC860 loads the data into the cache when the effective address hits in the TLB, is
permitted load access from the addressed page, and is directed at a caching-allowed page.
Otherwise, the MPC860 treats these instructions as no-ops. The data brought into the cache
as a result of this instruction is validated in the same manner that a load instruction would
be (that is, it is marked as unmodiÞed-valid). Note that the successful execution of the dcbt
(or dcbtst) instruction affects the state of the TLB and cache LRU bits.
8.4.3 Data Cache Block Zero (dcbz)
The effective address is computed, translated, and checked for protection violations as
deÞned in the PowerPC architecture. The dcbz instruction is treated as a store to the
addressed byte with respect to address translation and protection.
If the block containing the byte addressed by the EA is in the data cache, all bytes are
cleared, and the tag is marked as modiÞed-valid. If the block containing the byte addressed
by the EA is not in the data cache and the corresponding page is caching-allowed, the block
is established in the data cache without fetching the block from main memory, and all bytes
of the block are cleared, and the tag is marked as modiÞed-valid.
The dcbz instruction executes regardless of whether the cache block is locked, but if the
cache is disabled, an alignment exception is generated. If the page containing the byte
addressed by the EA is caching-inhibited or write-through, then the system alignment
exception handler is invoked.
8.4.4 Data Cache Block Store (dcbst)
The effective address is computed, translated, and checked for protection violations as
deÞned in the PowerPC architecture. This instruction is treated as a load with respect to
address translation and memory protection.
If the address hits in the cache and the cache block is in the unmodiÞed-valid state, no
action is taken. If the address hits in the cache and the cache block is in the modiÞed-valid
state, the modiÞed block is written back to memory and the cache block is placed in the
unmodiÞed-valid state.
If a bus error occurs while executing the dcbst instruction, DC_CST[CCER1] is set and a
machine check exception is generated. The data of the cache block ßagged by the bus error
is retrieved from the copyback buffer, not from the data cache. See Section 8.3.2.1,
ÒReading Data Cache Tags and Copyback Buffer,Ó for more information.
The function of this instruction is independent of the memory/cache access attributes. The
dcbst instruction executes regardless of whether the cache is disabled or the cache block is
locked.
Содержание MPC860 PowerQUICC
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