MC68030 Datasheet, PDF (136/602 Pages) Motorola, Inc – ENHANCED 32-BIT MICROPROCESSOR

On-Chip Cache Memories
FFF A
CCC 3
2 10 1
LONG-WORD
SELECT
TAG
INDEX
AA AAAAAAAAAAAAAAAAAAAAAA
2 2 2 2 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 0 0 0 0 ACCESS ADDRESS
3 2 10 9 87 6 5 4 3 2 1 0 9 8 7 6 5 4 3 2 1 0
TAG V V V V
1 OF 16
SELECT
TAG REPLACE
VALID
COMPARATOR
LINE HIT
ENTRY HIT
DATA FROM DATA
CACHE DATA BUS
DATA TO
EXECUTION UNIT
CACHE CONTROL LOGIC
CACHE SIZE = 64 (LONG WORDS)
LINE SIZE = 4 (LONG WORDS)
SET SIZE = 1
Figure 6-3. On-Chip Data Cache Organization
6.1.2.1 WRITE ALLOCATION. The supervisor program can configure the data cache for
either of two types of allocation for data cache entries that miss on write cycles. The state
of the write allocation (WA) bit in the cache control register specifies either no write
allocation or write allocation with partial validation of the data entries in the cache on writes.
When no write allocation is selected (WA=0), write cycles that miss do not alter the data
cache contents. In this mode, the processor does not replace entries in the cache during
write operations. The cache is updated only during a write hit.
When write allocation is selected (WA=1), the processor always updates the data cache on
cachable write cycles, but only validates an updated entry that hits or an entry that is
updated with long-word data that is long-word aligned. When a tag miss occurs on a write
of long-word data that is long-word aligned, the corresponding tag is replaced, and only the
long word being written is marked as valid. The other three entries in the cache line are
invalidated when a tag miss occurs on a misaligned long-word write or on a byte or word
write, the data is not written in the cache, the tag is unaltered, and the valid bit(s) are cleared.
Thus, an aligned long-word data write may replace a previously valid entry; whereas, a
misaligned data write or a write of data that is not long word may invalidate a previously valid
entry or entries.
MOTOROLA
MC68030 USER’S MANUAL
6-7