Digital Systems - Chapter12.pdf - po ang - Januszek66

CHAPTER 12

MEMORY DEVICES

■

OUTLINE

12-1

Memory Terminology

12-13

Dynamic RAM (DRAM)

12-2

General Memory Operation

12-14

Dynamic RAM Structure

and Operation

12-3

CPU–Memory Connections

12-15

DRAM Read/Write Cycles

12-4

Read-Only Memories

12-16

DRAM Refreshing

12-5

ROM Architecture

12-17

DRAM Technology

12-6

ROM Timing

12-18

Expanding Word Size and

Capacity

12-7

Types of ROMs

12-8

Flash Memory

12-19

Special Memory Functions

12-9

ROM Applications

12-20

Troubleshooting RAM

Systems

12-10

Semiconductor RAM

12-11

RAM Architecture

12-21

Testing ROM

12-12

Static RAM (SRAM)

■ OBJECTIVES

Upon completion of this chapter, you will be able to:

■

Understand and correctly use the terminology associated with memory

systems.

Describe the difference between read/write memory and read-only

memory.

■

Discuss the difference between volatile and nonvolatile memory.

■

Determine the capacity of a memory device from its inputs and

outputs.

■

Outline the steps that occur when the CPU reads from or writes to

memory.

■

Distinguish among the various types of ROMs and cite some common

applications.

■

Understand and describe the organization and operation of static and

dynamic RAMs.

■

Compare the relative advantages and disadvantages of EPROM,

EEPROM, and flash memory.

■

Combine memory ICs to form memory modules with larger word size

and/or capacity.

■

Use the test results on a RAM or ROM system to determine possible

faults in the memory system.

■

■ INTRODUCTION

A major advantage of digital over analog systems is the ability to store eas-

ily large quantities of digital information and data for short or long peri-

ods. This memory capability is what makes digital systems so versatile and

adaptable to many situations. For example, in a digital computer, the inter-

nal main memory stores instructions that tell the computer what to do un-

der all possible circumstances so that the computer will do its job with a

minimum amount of human intervention.

This chapter is devoted to a study of the most commonly used types

of memory devices and systems. We have already become very familiar

with the flip-flop, which is an electronic memory device. We have also

seen how groups of FFs called registers can be used to store information

and how this information can be transferred to other locations. FF regis-

ters are high-speed memory elements that are used extensively in the

internal operations of a digital computer, where digital information is

continually being moved from one location to another. Advances in LSI

and VLSI technology have made it possible to obtain large numbers of

785

786

C HAPTER 12/ M EMORY D EVICES

FFs on a single chip arranged in various memory-array formats. These

bipolar and MOS semiconductor memories are the fastest memory

devices available, and their cost has been continuously decreasing as LSI

technology improves.

Digital data can also be stored as charges on capacitors, and a very im-

portant type of semiconductor memory uses this principle to obtain high-

density storage at low power-requirement levels.

Semiconductor memories are used as the main memory of a computer

(Figure 12-1), where fast operation is important. A computer’s main memory—

also called its working memory —is in constant communication with the

central processing unit (CPU) as a program of instructions is being

executed. A program and any data used by the program reside in the main

memory while the computer is working on that program. RAM and ROM

(to be defined shortly) make up main memory.

Another form of storage in a computer is performed by auxiliary

memory (Figure 12-1), which is separate from the main working memory.

Auxiliary memory—also called mass storage —has the capacity to store

massive amounts of data without the need for electrical power. Auxiliary

memory operates at a much slower speed than main memory, and it

stores programs and data that are not currently being used by the CPU.

This information is transferred to the main memory when the computer

needs it. Common auxiliary memory devices are magnetic disk and com-

pact disk (CD).

We will take a detailed look at the characteristics of the most common

memory devices used as the internal memory of a computer. First, we de-

fine some of the common terms used in memory systems.

FIGURE 12-1 A computer

system normally uses high-

speed main memory and

slower external auxiliary

memory.

Computer

Main

memory

(semiconductor)

Arithmetic

unit

Control

unit

Central processor (CPU)

Auxiliary mass

storage

(magnetic, optical)

12-1

MEMORY TERMINOLOGY

The study of memory devices and systems is filled with terminology that can

sometimes be overwhelming to a student. Before we get into any compre-

hensive discussion of memories, it would be helpful if you had the meaning

787

S ECTION 12-1/ M EMORY T ERMINOLOGY

of some of the more basic terms under your belt. Other new terms will be de-

fined as they appear in the chapter.

Memory Cell. A device or an electrical circuit used to store a single bit

(0 or 1). Examples of memory cells include a flip-flop, a charged capaci-

tor, and a single spot on magnetic tape or disk.

■

Memory Word. A group of bits (cells) in a memory that represents in-

structions or data of some type. For example, a register consisting of

eight FFs can be considered to be a memory that is storing an eight-bit

word. Word sizes in modern computers typically range from 8 to 64 bits,

depending on the size of the computer.

■

Byte. A special term used for a group of eight bits. A byte always consists

of eight bits. Word sizes can be expressed in bytes as well as in bits. For

example, a word size of eight bits is also a word size of one byte, a word

size of 16 bits is two bytes, and so on.

■

Capacity. A way of specifying how many bits can be stored in a particu-

lar memory device or complete memory system. To illustrate, suppose

that we have a memory that can store 4096 20-bit words. This represents

a total capacity of 81,920 bits. We could also express this memory’s ca-

pacity as When expressed this way, the first number (4096) is

the number of words, and the second number (20) is the number of bits

per word (word size). The number of words in a memory is often a multi-

ple of 1024. It is common to use the designation “1K” to represent 1024

■

4096

20.

2 10 when referring to memory capacity. Thus, a memory that has a stor-

age capacity of is actually a memory. The develop-

ment of larger memories has brought about the designation “1M” or “1

meg” to represent 2 20

4096

1,048,576. Thus, a memory that has a capacity of

is actually one with a capacity of

2,097,152

The designation

“giga” refers to 2 30

1,073,741,824.

EXAMPLE 12-1A

A certain semiconductor memory chip is specified as How many

words can be stored on this chip? What is the word size? How many total bits

can this chip store?

2 K

Solution

1024

2048 words

Each word is eight bits (one byte). The total number of bits is therefore

2048

16,384 bits

EXAMPLE 12-1B

Which memory stores the most bits: a

memory or a memory that

stores 1M words at a word size of 16 bits?

Solution

1,048,576

41,943,040 bits

1,048,576

16,777,216 bits

The

memory stores more bits.

788

C HAPTER 12/ M EMORY D EVICES

Density. Another term for capacity. When we say that one memory device

has a greater density than another, we mean that it can store more bits in

the same amount of space. It is more dense.

■

Address. A number that identifies the location of a word in memory.

Each word stored in a memory device or system has a unique address.

Addresses always exist in a digital system as a binary number, although

octal, hexadecimal, and decimal numbers are often used to represent the

address for convenience. Figure 12-2 illustrates a small memory consist-

ing of eight words. Each of these eight words has a specific address rep-

resented as a three-bit number ranging from 000 to 111. Whenever we re-

fer to a specific word location in memory, we use its address code to

identify it.

■

Read Operation. The operation whereby the binary word stored in a spe-

cific memory location (address) is sensed and then transferred to an-

other device. For example, if we want to use word 4 of the memory of

Figure 12-2 for some purpose, we must perform a read operation on

address 100. The read operation is often called a fetch operation because

a word is being fetched from memory. We will use both terms inter-

changeably.

■

Write Operation. The operation whereby a new word is placed into a par-

ticular memory location. It is also referred to as a store operation.

Whenever a new word is written into a memory location, it replaces the

word that was previously stored there.

■

Access Time. A measure of a memory device’s operating speed. It is the

amount of time required to perform a read operation. More specifically,

it is the time between the memory receiving a new address input and the

data becoming available at the memory output. The symbol t ACC is used

for access time.

■

Volatile Memory. Any type of memory that requires the application of

electrical power in order to store information. If the electrical power is

removed, all information stored in the memory will be lost. Many semi-

conductor memories are volatile, while all magnetic memories are non-

volatile, which means that they can store information without electrical

power.

■

Random-Access Memory (RAM). Memory in which the actual physical

location of a memory word has no effect on how long it takes to read

■

FIGURE 12-2 Each word

location has a specific

binary address.

Addresses

000

Word 0

001

010

011

100

Word 1

Word 2

Word 3

Word 4

101

110

111

Word 5

Word 6

Word 7

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