SH7606 Datasheet, PDF (83/532 Pages) Renesas Technology Corp – 32-Bit RISC Microcomputer SuperH™ RISC engine Family / SH7606 Series

Section 3 Cache
Read Miss: An external bus cycle starts and the entry is updated. The way to be replaced is shown
in table 3.1. Data is updated in units of 16 bytes by updating the entry. When the desired
instruction or data is loaded from external memory to the cache, the instruction or data is
transferred to the CPU in parallel. When it is loaded to the cache, the U bit is cleared to 0, the V
bit is set to 1, the LRU bits are updated so that they point to the most recently hit way. When the U
bit of 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.
3.3.3 Write Access
Write Hit: In a write access in write-back mode, the data is written to the cache and no external
memory write cycle is generated. The U bit of the entry that has been written to is set to 1, and the
LRU bits are 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 generated. The U bit
of the entry that has been written to is not updated, and the LRU bits are updated to indicate that
the hit way is the most recently hit way.
Write Miss: In write-back mode, an external write cycle starts when a write miss occurs, and the
entry is updated. The way to be replaced is shown in table 3.1. When the U bit of the entry which
is to be replaced by entry updating is 1, the cache-update cycle starts after the entry has been
transferred to the write-back buffer. Data is written to the cache and the U bit and the V bit are set
to 1. The LRU bits are updated to indicate that the replaced way is the most recently updated way.
After the cache has completed its update cycle, the write-back buffer writes the entry back to the
memory. Transfer is in 16-byte units. In write-through mode, no write to cache occurs in a write
miss; the write is only to the external memory.
3.3.4 Write-Back Buffer
When the U bit of the entry to be replaced in write-back mode is 1, the entry must be written back
to the external memory. To increase performance, the entry to be replaced is first transferred to the
write-back buffer and fetching of new entries to the cache takes priority over writing back to the
external memory. After the fetching of new entries to the cache completes, the write-back buffer
writes the entry back to the external memory. During the write-back cycles, the cache can be
accessed. The write-back buffer can hold one line of cache data (16 bytes) and its physical
address. Figure 3.3 shows the configuration of the write-back buffer.
Rev. 4.00 Sep. 13, 2007 Page 57 of 502
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