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FLUSH transaction:
When the operand cache is enabled, the FLUSH transaction checks the
operand cache and if the hit line is dirty, then the data is written back to the external memory. If
the transaction is not hit to the cache or the hit entry is not dirty, it is no-operation.
8.5.2 Prefetch
Operation
The SH-4A supports a prefetch instruction to reduce the cache fill penalty incurred as the result of
a cache miss. If it is known that a cache miss will result from a read or write operation, it is
possible to fill the cache with data beforehand by means of the prefetch instruction to prevent a
cache miss due to the read or write operation, and so improve software performance. If a prefetch
instruction is executed for data already held in the cache, or if the prefetch address results in a
UTLB miss or a protection violation, the result is no operation, and an exception is not generated.
Details of the prefetch instruction are given in section 10, Instruction Descriptions.
•
Prefetch instruction (OC)
: PREF @Rn
•
Prefetch instruction (IC)
: PREFI @Rn
8.6
Memory-Mapped Cache Configuration
To enable the IC and OC to be managed by software, the IC contents can be read from or written
to by a program in the P2 area by means of a MOV instruction in privileged mode. Operation is
not guaranteed if access is made from a program in another area. In this case, execute one of the
following three methods for executing a branch to the P0, U0, P1, or P3 area.
1. Execute a branch using the RTE instruction.
2. Execute a branch to the P0, U0, P1, or P3 area after executing the ICBI instruction for any
address (including non-cacheable area).
3. If the MC bit in IRMCR is 0 (initial value) before making an access to the memory-mapped
IC, the specific instruction does not need to be executed. However, note that the CPU
processing performance will be lowered because the instruction fetch is performed again for
the next instruction after making an access to the memory-mapped IC.
Note that the method 3 may not be guaranteed in the future SuperH Series. Therefore, it is
recommended that the method 1 or 2 should be used for being compatible with the future SuperH
Series.
In privileged mode, the OC contents can be read from or written to by a program in the P1 or P2
area by means of a MOV instruction. Operation is not guaranteed if access is made from a
program in another area. The IC and OC are allocated to the P4 area in the virtual address space.
Only data accesses can be used on both the IC address array and data array and the OC address
array and data array, and accesses are always longword-size. Instruction fetches cannot be
performed in these areas. For reserved bits, a write value of 0 should be specified and the read
value is undefined.
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