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APPENDIX 4A 151

APPENDIX 4A PERFORMANCE CHARACTERISTICS
OF TWO-LEVEL MEMORIES

In this chapter, reference is made to a cache that acts as a buffer between main
memory and processor, creating a two-level internal memory. This two-level archi-
tecture exploits a property known as locality to provide improved performance over
a comparable one-level memory.
The main memory cache mechanism is part of the computer architecture, im-
plemented in hardware and typically invisible to the operating system.There are two
other instances of a two-level memory approach that also exploit locality and that
are, at least partially, implemented in the operating system: virtual memory and the
disk cache (Table 4.7). Virtual memory is explored in Chapter 8; disk cache is be-
yond the scope of this book but is examined in [STAL09]. In this appendix, we look
at some of the performance characteristics of two-level memories that are common
to all three approaches.
Locality

The basis for the performance advantage of a two-level memory is a principle
known as locality of reference [DENN68]. This principle states that memory refer-
ences tend to cluster. Over a long period of time, the clusters in use change, but over
a short period of time, the processor is primarily working with fixed clusters of
memory references.
Intuitively, the principle of locality makes sense. Consider the following line of
reasoning:
1. Except for branch and call instructions, which constitute only a small fraction
of all program instructions, program execution is sequential. Hence, in most
cases, the next instruction to be fetched immediately follows the last instruc-
tion fetched.
2. It is rare to have a long uninterrupted sequence of procedure calls followed by
the corresponding sequence of returns. Rather, a program remains confined to a

Table 4.7 Characteristics of Two-Level Memories
Main Memory Virtual Memory
Cache (paging) Disk Cache

6
6
Typical access time 5 : 1 (main memory 10 : 1 (main memory 10 : 1 (main memory
ratios vs. cache) vs. disk) vs. disk)
Memory management Implemented by Combination of hardware System software
system special hardware and system software
Typical block or page 4 to 128 bytes 64 to 4096 bytes (virtual 64 to 4096 bytes
size (cache block) memory page) (disk block or pages)
Access of processor Direct access Indirect access Indirect access
to second level