SH7708 Datasheet, PDF (120/633 Pages) Renesas Technology Corp – SuperH™ RISC engine

5.3.2 Read Access
Read Hit: In a read access, instructions and data are transferred from the cache to the CPU. The
transfer unit is 32 bits. The LRU is updated.
Read Miss: An external bus cycle starts and the entry is updated. The way replaced is the one
least recently used. 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 in the cache, the U bit is
cleared to 0 and the V bit is set to 1. When the U bit of a replaced entry in write-back mode is 1,
the cache fill cycle starts after the entry is transferred to the write-back buffer. After the cache
completes its fill cycle, the write-back buffer writes back the entry to memory. The write-back unit
is 16 bytes.
5.3.3 Write Access
Write Hit: In a write access in write-back mode, data is written to the cache and the U bit of the
entry written is set to 1. Writing occurs only to the cache; no external memory write cycle is
issued. In write-through mode, data is written to the cache and an external memory write cycle is
issued.
Write Miss: In write-back mode, an external bus cycle starts when a write miss occurs and an
entry with its U bit set to 1 is replaced. The way to be replaced is the one least recently used.
When the U bit of the entry to be replaced is 1, the cache fill cycle starts after the entry is
transferred to the write-back buffer. After the cache completes its fill cycle, the write-back buffer
writes back the entry to memory. The write-back unit is 16 bytes. Data is written to the cache and
the U bit is set to 1. In write-through mode, no write to cache occurs in a write miss; the write is
only to external memory.
5.3.4 Write-Back Buffer
When the U bit of the entry to be replaced in write-back mode is 1, it must be written back to
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
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 5.4 shows the
configuration of the write-back buffer.
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