Renesas SH7781 - Manual (Page 253)

8. Caches
Rev.1.00 Jan. 10, 2008 Page 223 of 1658
REJ09B0261-0100
write-back buffer is then written back to external memory.
8.3.2 Prefetch Operation
When the Operand Cache (OC) is enabled (OCE = 1 in CCR) and data is prefetched from a
cacheable area, the cache operates as follows:
1. The tag, V bit, U bit, and LRU bits on each way are read from the cache line indexed by virtual
address bits [12:5].
2. The tag, read from each way, is compared with bits [28:10] of the physical address resulting
from virtual address translation by the MMU:
⎯ If there is a way whose tag matches and its V bit is 1, see No. 3.
⎯ If there is no way whose tag matches and the V bit is 1, and the U bit of the way which is
selected to replace using the LRU bits is 0, see No. 4.
⎯ If there is no way whose tag matches and the V bit is 1, and the U bit of the way which is
selected to replace using the LRU bits is 1, see No. 5.
3. Cache hit
Then the LRU bits are updated to indicate the hitted way is the latest one.
4. Cache miss (no write-back)
Data is read into the cache line on the way, which is selected to replace, from the physical
address space corresponding to the virtual address. Data reading is performed, using the
wraparound method, in order from the quad-word data (8 bytes) including the cache-missed
data. In the prefetch operation the CPU doesn't wait the data arrives. While the one cache line
of data is being read, the CPU can execute the next processing. When reading of one line of
data is completed, the tag corresponding to the physical address is recorded in the cache, 1 is
written to the V bit and 0 is written to the U bit on the way. And the LRU bit is updated to
indicate the way is latest one.
5. Cache miss (with write-back)
The tag and data field of the cache line on the way which is selected to replace are saved in the
write-back buffer. Then data is read into the cache line on the way which is selected to replace
from the physical address space corresponding to the virtual address. Data reading is
performed, using the wraparound method, in order from the quad-word data (8 bytes)
including the cache-missed data. In the prefetch operation the CPU doesn't wait the data
arrives. While the one cache line of data is being read, the CPU can execute the next
processing. And the LRU bits are updated to indicate the way is latest one. The data in the
write-back buffer is then written back to external memory.