6-9
CACHE SUBSYSTEM
Given an equal amount of cache memory as in the direct mapped example, the set associative
cache has half as many locations, and the extra address bit becomes part of the tag field. Because
the set-associative cache has several places for a block with the same cache index, the hit rate is
increased. The set associative cache performs more efficiently than a direct mapped cache, but it
needs a wider tag field and additional logic to determine which set should receive the data. This
function is determined by the replacement policy, which is covered later in this section.
Sector Buffering: Another cache configuration uses a sector buffer and is sometimes called a sub-
block cache. The cache is a number of sectors, and the sectors in turn are a number of blocks.
Each block can have its own valid bit, but only one tag address exists per sector. When a word is
accessed whose sector is in the cache but the block is not, then the block is fetched from the main
memory. Sector buffering has its own trade-offs associated with miss ratios and bus utilization.
Having smaller blocks increases the miss ratio, but reduces the number of external bus accesses.
Conversely, having a large number of blocks increases the hit ratio but also increases the external
bus utilization.
Figure 6-4
shows the cache organization in sector buffering.
Figure 6-4. Sector Buffer Cache Organization
The Intel486 processor’s on-board cache is organized 4-way set associative with a line size of 16
bytes. The 8-Kbyte cache is organized as four 2-Kbyte sets. Each 2-Kbyte set is comprised of 128
16-byte lines.
Figure 6-5
shows the cache organization. An application can achieve an extremely
high hit rate with the 4-way associativity. The cache is transparent so that the Intel486 processor
remains software-compatible with its non-cache predecessors.
TAG 1
BLOCK 1.1
BLOCK 1.2
BLOCK 1.3
...
BLOCK 1.N
TAG 2
BLOCK 2.1
BLOCK 2.2
BLOCK 2.3
...
BLOCK 2.N
...
...
...
...
...
...
TAG M
BLOCK M.1
BLOCK M.2
BLOCK M.3
...
...
TAG per sector
Blocks per sector
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