SH7721 datasheet(269/1524 Pages) RENESAS

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Section 5 Cache

Rev. 3.00 Jan. 18, 2008 Page 207 of 1458

REJ09B0033-0300

5.3.2

Read Access

(1)

Read Hit

In a read access, instructions and data are transferred from the cache to the CPU. The LRU i

updated to indicate that the hit way is the most recently hit way.

(2)

Read Miss

An external bus cycle starts and the entry is updated. The way to be replaced is shown in table 5.4.

Entries are updated in 16-byte units. When the desired instruction or data that caused the miss is

loaded from external memory to the cache, the instruction or data is transferred to the CPU in

parallel with being loaded to the cache. When it is loaded to the cache, the U bit is cleared to 0 and

the V bit is set to 1 to indicate that the hit way is the most recently hit way. When the U bit for the

entry which is to be replaced by entry updating in write-back mode is 1, the cache-update cycle

starts after the entry is transferred to the write-back buffer. After the cache completes its update

cycle, the write-back buffer writes the entry back to the memory. Transfer is in 16-byte units.

5.3.3

Prefetch Operation

(1)

Prefetch Hit

The LRU is updated to indicate that the hit way is the most recently hit way. The other contents of

the cache are not changed. Instructions and data are not transferred from the cache to the CPU.

(2)

Prefetch Miss

Instructions and data are not transferred from the cache to the CPU. The way that is to be replaced

is shown in table 5.3. The other operations are the same as those for a read miss.

5.3.4

Write Access

(1)

Write Hit

In a write access in write-back mode, the data is written to the cache and no external memory

write cycle is issued. The U bit of the entry that has been written to is set to 1, and the LRU is

updated to indicate that the hit way is the most recently hit way. In write-through mode, the data is

written to the cache and an external memory write cycle is issued. The U bit of the entry that has

been written to is not updated, and the LRU is updated to indicate that the hit way is the most

recently hit way.

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