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118 CHAPTER 4 / CACHE MEMORY

used as a buffer to hold data temporarily that is to be read out to disk. Such a tech-
2
nique, sometimes referred to as a disk cache, improves performance in two ways:
• Disk writes are clustered. Instead of many small transfers of data, we have a
few large transfers of data. This improves disk performance and minimizes
processor involvement.
• Some data destined for write-out may be referenced by a program before the
next dump to disk. In that case, the data are retrieved rapidly from the soft-
ware cache rather than slowly from the disk.
Appendix 4A examines the performance implications of multilevel memory
structures.

4.2 CACHE MEMORY PRINCIPLES

Cache memory is intended to give memory speed approaching that of the fastest
memories available, and at the same time provide a large memory size at the price
of less expensive types of semiconductor memories. The concept is illustrated in
Figure 4.3a. There is a relatively large and slow main memory together with a
smaller, faster cache memory. The cache contains a copy of portions of main mem-
ory. When the processor attempts to read a word of memory, a check is made to

Block Transfer
Word Transfer

CPU Cache Main memory
Fast Slow

(a) Single cache

Level 1 Level 2 Level 3 Main
CPU
(L1) cache (L2) cache (L3) cache memory
Fastest Fast
Less Slow
fast

(b) Three-level cache organization
Figure 4.3 Cache and Main Memory

2 Disk cache is generally a purely software technique and is not examined in this book. See [STAL09] for
a discussion.