Write Operation - Hitachi SH7750 Hardware Manual

4.3.3

Write Operation

When the OC is enabled (CCR.OCE = 1) and data is written by means of an effective address to 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 → (3c)
• If the tag does not match, the V bit is 1, and the U bit is 1 → (3e)
3a. Cache hit (copy-back)
A data write in accordance with the access size (quadword/longword/word/byte) is performed
for the data indexed by bits [4:0] of the effective address of the data field of the cache line
indexed by effective address bits [13:5]. Then 1 is set in the U bit.
3b. Cache hit (write-through)
A data write in accordance with the access size (quadword/longword/word/byte) is performed
for the data indexed by bits [4:0] of the effective address of the data field of the cache line
indexed by effective address bits [13:5]. A write is also performed to the corresponding
external memory using the specified access size.
3c. Cache miss (no copy-back/write-back)
A data write in accordance with the access size (quadword/longword/word/byte) is performed
for the data indexed by bits [4:0] of the effective address of the data field of the cache line
indexed by effective address bits [13:5]. Then, data is read into the cache line from the external
memory space corresponding to the effective address. Data reading is performed, using the
wraparound method, in order from the longword data corresponding to the effective address,
and one cache line of data is read excluding the written data. During this time, the CPU can
execute the next processing. When reading of one line of data is completed, the tag
corresponding to the effective address is recorded in the cache, and 1 is written to the V bit and
U bit.
3d. Cache miss (write-through)
A write of the specified access size is performed to the external memory corresponding to the
effective address. In this case, a write to cache is not performed.
Rev. 6.0, 07/02, page 104 of 986
Copy-back Write-through
→ (3a) → (3b)
→ (3c) → (3d)
→ (3c) → (3d)
→ (3d)
→ (3d)