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4.3.4
Write-Back Buffer
In order to give priority to data reads to the cache and improve performance, the SH7750 has a
write-back buffer which holds the relevant cache entry when it becomes necessary to purge a dirty
cache entry into external memory as the result of a cache miss. The write-back buffer contains one
cache line of data and the physical address of the purge destination.
Physical address bits [28:5]
4.3.5
Write-Through Buffer
The SH7750 has a 64-bit buffer for holding write data when writing data in write-through mode or
writing to a non-cacheable area. This allows the CPU to proceed to the next operation as soon as
the write to the write-through buffer is completed, without waiting for completion of the write to
external memory.
4.3.6
RAM Mode
Setting CCR.ORA to 1 enables 8 kbytes of the operand cache to be used as RAM. The operand
cache entries used as RAM are entries 128 to 255 and 384 to 511 . Other entries can still be used
as cache. RAM can be accessed using addresses H'7C00 0000 to H'7FFF FFFF. Byte-, word-,
longword-, and quadword-size data reads and writes can be performed in the operand cache RAM
area. Instruction fetches cannot be performed in this area.
An example of RAM use is shown below. Here, the 4 kbytes comprising OC entries 128 to 256
are designated as RAM area 1, and the 4 kbytes comprising OC entries 384 to 511 as RAM area 2.
LW0
Figure 4.3 Configuration of Write-Back Buffer
Physical address bits [28:0]
Figure 4.4 Configuration of Write-Through Buffer
LW1
LW2
LW3
LW0
LW4
LW5
LW6
LW1
Rev. 2.0, 03/99, page 69 of 396
LW7
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