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LRU (SH7751R only)
In a 2-way set-associative system, up to two entry addresses (among addresses 12 to 15) can
register the same data in cache. The LRU bit indicates to which way the entry is to be
registered among the two ways. There is one LRU bit in each entry, and it is controlled by
hardware. The LRU (Last Recently Used) algorithm that selects the most recently accessed
way is used for way selection. The LRU bit is initialized to 0 by a power-on reset, but is not
initialized by a manual reset. The LRU bit cannot be read from or written to by software.
4.4.2
Read Operation
When the IC is enabled (CCR.ICE = 1) and instruction fetches are performed by means of an
effective address from a cacheable area, the instruction cache operates as follows:
1. The tag and V bit are read from the cache line indexed by effective address bits [12:5].
2. The tag is compared with bits [28:10] of the address resulting from effective address
translation by the MMU:
•
If the tag matches and the V bit is 1
•
If the tag matches and the V bit is 0
•
If the tag does not match and the V bit is 0
•
If the tag does not match and the V bit is 1
3a. Cache hit
The data indexed by effective address bits [4:2] is read as an instruction from the data field of
the cache line indexed by effective address bits [12:5].
3b. Cache miss
Data is read into the cache line from the external memory space corresponding to the effective
address. Data reading is performed, using the wraparound method, in order from the longword
data corresponding to the effective address, and when the corresponding data arrives in the
cache, the read data is returned to the CPU as an instruction. When reading of one line of data
is completed, the tag corresponding to the effective address is recorded in the cache, and 1 is
written to the V bit.
4.4.3
IC Index Mode
Setting CCR.IIX to 1 enables IC indexing to be performed using bit [25] of the effective address.
This is called IC index mode. In normal mode, with CCR.IIX cleared to 0, IC indexing is
performed using bits [12:5] of the effective address. Using index mode allows the IC to be handled
as two areas by means of effective address bit [25], providing efficient use of the cache.
Rev. 3.0, 04/02, page 102 of 1064
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