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7.4 Cache Refill
A physical cache line in the R3900 Processor Core comprises 4 words for the instruction cache and 1 word for
the data cache. The refill size can be designated independently of the line size. The refill size can be
4/8/16/32 words for the instruction cache, and 1/4/8/16/32 words for the data cache. In a burst read
operation, data or instructions of the designated refill size are read. However, when the data cache refill size is
set to one word (Config register DCBR = 0), a single read operation is performed.
Both caches are refilled from the head of the refill boundary.
Regardless of the refill size, tags are updated one physical line at a time.
Additional explanation : If an instruction changing the cache configuration (MTC0 to modify the Config
register, or any CACHE instruction) is executed during a refill cycle, the new configuration takes
effect after the refill cycle in progress is completed. Note that instruction cache invalidation is
possible only while the instruction cache is disabled.
Missed word
4 words
(a) Instruction cache
Refill size boundary
Refill start word
Refill size
Figure 7-6. Cache refill
Missed word
1 word
(b) Data cache
Refill size boundary
Refill start word