Sun Microelectronics
18
UltraSPARC User’s Manual
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When the I-MMU maps the fetch as noncacheable.
The instruction cache snoops stores from other processors or DMA transfers, but
it is not updated by stores in the same processor, except for block commit stores
(see Section 13.6.4, “Block Load and Store Instructions,” on page 230). The
FLUSH instruction can be used to maintain coherency. Block commit stores up-
date the I-Cache but do not flush instructions that have already been prefetched
into the pipeline. A FLUSH, DONE, or RETRY instruction can be used to flush
the pipeline. For block copies that must maintain I-Cache coherency, it is more ef-
ficient to use block commit stores in the loop, followed by a single FLUSH in-
struction to flush the pipeline.
Note:
The size of each I-Cache set is the same as the page size in UltraSPARC-I
and UltraSPARC-II; thus, the virtual index bits equal the physical index bits.
3.1.1.2 Data Cache (D-Cache)
The D-Cache is a write-through, nonallocating-on-write-miss 16-Kb direct
mapped cache with two 16-byte sub-blocks per line. Data accesses bypass the
data cache when the D-Cache enable bit in the LSU_Control_Register is clear (see
Section A.6, “LSU_Control_Register,” on page 306). Load misses will not allocate
in the D-Cache if the D-MMU enable bit in the LSU_Control_Register is clear or
the access is mapped by the D-MMU as virtual noncacheable.
Note:
A noncacheable access may access data in the D-Cache from an earlier
cacheable access to the same physical block, unless the D-Cache is disabled.
Software must flush the D-Cache when changing a physical page from cacheable
to noncacheable (see Section 5.2, “Cache Flushing”).
3.1.2 Level-2 PIPT External Cache (E-Cache)
UltraSPARC’s level-2 (external) cache (the E-Cache) is physically indexed, physi-
cally tagged (PIPT). This cache has no references to virtual address and context
information. The operating system needs no knowledge of such caches after ini-
tialization, except for stable storage management and error handling.
Memory accesses must be cacheable in the E-Cache to allow use of UltraSPARC’s
ECC checking. As a result, there is no E-Cache enable bit in the
LSU_Control_Register.
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