SH7708 Datasheet, PDF (116/633 Pages) Renesas Technology Corp – SuperH™ RISC engine

Address array (ways 0–3)
Data array (ways 0–3)
Entry 0 V U Address
Entry 1
0 LW0 LW1 LW2 LW3
1
LRU section
0
1
•
•
•
•
•
•
•
•
•
•
•
•
Entry 127
24 (1 + 1 + 22) bits
127
128 (32 × 4) bits
LW0–LW3: Longword data 0–3
127
6 bits
Figure 5.1 Cache Structure
Address Array: The V bit indicates whether the entry data is valid. When the V bit is 1, data is
valid; when 0, data is not valid. The U bit indicates whether the entry has been written to in write-
back mode. When the U bit is 1, the entry has been written to; when 0, it has not. The address tag
holds the physical address used in the external memory access. It is composed of 22 bits (address
bits 31–10) used for comparison during cache searches.
In the SH7708 Series, the top three of the 32 physical address bits are used as shadow bits (see
section 10), and therefore in a normal replace operation the top three bits of the vector address are
cleared to 0.
The V and U bits are initialized to 0 by a power-on reset, but are not initialized by a manual reset.
The tag address is not initialized by either a power-on or manual reset.
Data Array: Holds a 16-byte instruction or data. Entries are registered in the cache in line units
(16 bytes). The data array is not initialized by a power-on or manual reset.
LRU: With the 4-way set associative system, up to four instructions or data with the same entry
address (address bits 10–4) can be registered in the cache. When an entry is registered, the LRU
shows which of the four ways it is recorded in. There are six LRU bits, controlled by hardware. A
least-recently-used (LRU) algorithm is used to select the way.
In normal mode, four ways are used as cache and six LRU bits indicate the way to be replaced
(table 5.2). If a bit pattern other than those listed in table 5.2 is set in the LRU bits by software, the
cache will not function correctly. When modifying the LRU bits by software, set one of the
patterns listed in table 5.2.
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