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The operand cache consists of 512 cache lines, each composed of a 19-bit tag, V bit, U bit, and 32-
byte data.
• Tag
Stores the upper 19 bits of the 29-bit external memory address of the data line to be cached.
The tag is not initialized by a power-on or manual reset.
• V bit (validity bit)
Indicates that valid data is stored in the cache line. When this bit is 1, the cache line data is
valid. The V bit is initialized to 0 by a power-on reset, but retains its value in a manual reset.
• U bit (dirty bit)
The U bit is set to 1 if data is written to the cache line while the cache is being used in copy-
back mode. That is, the U bit indicates a mismatch between the data in the cache line and the
data in external memory. The U bit is never set to 1 while the cache is being used in write-
through mode, unless it is modified by accessing the memory-mapped cache (see section 4.5,
Memory-Mapped Cache Configuration). The U bit is initialized to 0 by a power-on reset, but
retains its value in a manual reset.
• Data field
The data field holds 32 bytes (256 bits) of data per cache line. The data array is not initialized
by a power-on or manual reset.
4.3.2
Read Operation
When the OC is enabled (CCR.OCE = 1) and data is read by means of an effective address from a
cacheable area, the cache operates as follows:
1. The tag, V bit, and U bit are read from the cache line indexed by effective address bits [13:5].
2. The tag is compared with bits [28:10] of the address resulting from effective address
translation by the MMU:
• If the tag matches and the V bit is 1
• If the tag matches and the V bit is 0
• If the tag does not match and the V bit is 0
• If the tag does not match, the V bit is 1, and the U bit is 0 → (3b)
• If the tag does not match, the V bit is 1, and the U bit is 1 → (3c)
Rev. 4.0, 04/00, page 66 of 850
→ (3a)
→ (3b)
→ (3b)
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