CHAPTER 6
COMPUTER MEMORIES
INTRODUCTION
The memory of a computer holds (stores) program instructions (what to do),
data (information), operands (affected, manipulated, or operated upon data), and
calculations (ALU results). The CPU controls the information stored in memory.
Information is fetched, manipulated (under program control) and/or written (or
written back) into memory for immediate or later use. The internal memory of a
computer is also referred to as main memory, global memory, main storage, or
primary storage. Do not confuse it with secondary or auxiliary memory (also called
mass storage) provided by various peripheral devices. In newer computers you also
will encounter a number of small and independent local memories that are used for
a variety of purposes by embedded microprocessors. You have already learned
about cache memory that lies between the CPU and main memory.
After completing this chapter, you should be able to:
l Describe the organization of memory
. Describe the operation of main memory
l Recognize the types of memory and describe how they function
TOPIC 1—MEMORY ORGANIZATION
AND OPERATION
The main memory of a computer is used for storing
programs,
data, calculations, and operands.
Memory is used in all types of computer systems includ-
ing mainframes, minicomputers, and microcomputers.
The amount of main memory each type of computer has
varies according to the configuration. A wide variety
of memory types is being used. To simplify our
discussion, we have divided memory into two general
categories: read/write (random access) memory and
read-only memory. Within the read/write group, we
discuss magnetic (core and film) memories and semi-
conductor (static and dynamic) memories. Read-only
memory can be subdivided into factory programmed
parts called read-only memory (ROM) and user pro-
grammable devices called programmable read-only
memory (PROM). This classification system is illus-
trated in figure 6-1. Let’s take a look at some of the termi-
nology used with regard to the computer’s memory.
TERMINOLOGY
The following terms need to be explained at this
point:
l Memory —Memory generally refers to the actual
hardware where the programs, data, calculations, or
operands are stored.
l Memory address —A memory address is a
particular location of a larger memory array. Usually
one memory address contains one word of data. A
word is one packet of information for the computer and
is usually composed of many bits. Computers exist that
use 1-bit words, 8-bit words, 16-bit words, 32-bit
words, and 64-bit words. Handling computer data in
8-bit words is so common that the 8-bit word has its own
name, the byte. Half of a byte is called a nibble (4 bits).
l Capacity (memory size ) —Capacity is an
important aspect of system performance; it is a useful
and convenient way to describe the size of memory. At
the individual part level, a computer’s memory may be
6-1
