Instruction and Data Caches
MPC885 PowerQUICC Family Reference Manual, Rev. 2
Freescale Semiconductor
7-19
7.4.5
Data Cache Block Flush (dcbf)
The effective address is computed, translated, and checked for protection violations as defined 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 block is in the modified-valid state, the modified block is written
back to memory and the cache block is placed in the invalid state. If the address hits in the cache, and the
cache block is in the unmodified-valid state, the cache block is placed in the invalid state. If the address
misses in the cache, no action is taken.
If a bus error occurs while executing the dcbf instruction, DC_CST[CCER1] is set and a machine check
exception is generated. The data of the cache block flagged by the bus error is retrieved from the copyback
buffer, not from the data cache. See
Section 7.3.2.1, “Reading Data Cache Tags and Copyback Buffer,”
for
more information.
The function of this instruction is independent of the memory/cache access attributes. The dcbf instruction
executes regardless of whether the cache is disabled or the cache block is locked.
7.4.6
Data Cache Block Invalidate (dcbi)
The effective address is computed, translated, and checked for protection violations as defined in the
PowerPC architecture. This instruction is treated as a store with respect to address translation and memory
protection.
If the address hits in the cache, the cache block is placed in the invalid state, regardless of whether the data
is modified. If the address misses in the cache, no action is taken. Because this instruction may effectively
destroy modified data, it is privileged (that is, dcbi is available only to programs at the supervisor privilege
level, MSR[PR] = 0).
The function of this instruction is independent of the memory/cache access attributes. The dcbi instruction
executes regardless of whether the cache is disabled or the cache block is locked.
7.5
Instruction Cache Operations
When the instruction MMU is enabled (MSR[IR] = 1), the instruction cache operates as defined by the
memory/cache access attributes. When the instruction MMU is disabled (MSR[IR] = 0), the instruction
cache operates as defined by the default instruction memory access attributes. The default state of the
caching-inhibited/caching-allowed attribute is determined by MI_CTR[CIDEF], and the entire memory
space defaults to the guarded attribute. See
Chapter 8, “Memory Management Unit,”
for more information.
An instruction cache access begins with an instruction fetch request from the instruction sequencer in the
MPC8xx core. As shown in
Figure 7-1
, bits 20–27 of the instruction address provide the index to select a
set (0–255) within the instruction cache array. The tags from each way of the set are compared against
bits 0–19 of the instruction address. If a match is found and the matched entry is valid, it is a cache hit. If
no tag matches or the matched tag is not valid, it is a cache miss.
Содержание PowerQUICC MPC870
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