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CACHE SUBSYSTEMS
cache or
by
copying all cache writes both to the main memory and to
all
other caches
that share the same memory (a technique known
as
broadcasting). Hardware trans-
parent systems are illustrated in Figure
7-7.
• Non-cacheable memory - Cache coherency
is
maintained
by
designating shared
memory
as
non-cacheable. In such a system, all accesses to shared memory are cache
misses, because the shared memory
is
never copied into the cache. The non-cacheable
memory can be identified using chip-select logic or high-address bits. Figure
7-8
illus-
trates non-cacheable memory.
Software can offset the reduction in the hit rate caused
by
non-cacheable memory
by
using the string move instruction (REP MOVS) to copy data between non-cacheable
memory and cacheable memory and
by
mapping shared memory accesses to the
cacheable
locatio~s.
This technique
is
especially appropriate for systems in which
copying
is
necessary for other reasons (as in some implementations of UNlX for
example).
• Cache flushing - A cache flush writes
any
altered data to the main memory (if this
has not been done with write-through) and clears the contents of the cache.
If
all the
caches in the system are flushed before a device writes to shared memory, the poten-
tial
for
stale data in any cache
is
eliminated.
Combinations of various cache coherency techniques may offer the optimal solution for a
particular system. For example, a system might use hardware transparency for time-
critical
110
operations such
as
paging and non-cache able memory for slower I/O such as
printing.
7.4 EFFICIENCY AND PERFORMANCE
The measurement of cache effectiveness
is
divided into two topics: efficiency and per-
formance. Cache efficiency
is
its ability to maintain the most used code and data
requested
by
the microprocessor. Efficiency
is
measured in terms of hit rate. Perfor-
mance
is
a measurement of the speed in which a microprocessor can perform a given
OTHER
BUS
CACHE
MASTER
MAIN
J
MEMORY
,
i3B6~
OX
CPU
CACHE
-
231732i7-7
Figure
7-7.
Hardware
Transparency
7-11