UNIT 3 MEMORY SYSTEM Memory System - IGNOU

[Pages:32]UNIT 3 MEMORY SYSTEM

Structure

3.0 Introduction 3.1 Objectives 3.2 Memory Types and Storage Devices

3.2.1 Semiconductor (Main) Memory 3.2.2 Magnetic Memory 3.2.3 Optical Memory 3.3 Memory Hierarchy & their Needs 3.4 Summary 3.5 Answers to Check Your Progress 3.6 Further Readings

3.0 INTRODUCTION

Page No.

43 46 46 52 56 63 67 69 70 74

To understand how data is processed, by a computer, we can draw a simple analogy between computers and humans.

Suppose a student asks a teacher "what happens when 15 is multiplied by 8". S/He receives the answer 120 from teacher. In the case of a computer, this process can be described as follows:

The teachers brain receives the question through his/her ears (analogous to a computers input device), processes the question with the help of his brains information processing and analytical ability (analogous to computers CPU) and gives the answer through the mouth (analogous to a computers output device). Further, just as the teacher can write down the answer on a sheet of paper or blackboard, the computer can also print the answer on a paper through a printer attached to it.

Every modern computer system consists of three basic sections: 1. Input device (i.e. Keyboard, mouse or scanner etc.) 2. Processor (or CPU):

Control unit (CU) Arithmetic and Logic Unit (ALU) Memory unit 3. Output device (Visual Display Unit (Monitor/screen) or printer etc.)

The basic parts of a computers are shown in Figure 3.1

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Basics of Computer Hardware

Central Processing Unit Memory (main)

Input Devices

Control unit

Output-devices

The Memory unit is an important component of a computer where all the data and information are stored in the form of binary digits

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Arithmetic-Logic unit

Figure 3.1: Basic Parts of a Computer

Input devices such as a keyboard, mouse or scanner are used to enter input (data and/or instructions), directly into the computer.

The CPU is like the human brain; it has a memory and just like there is a faculty in the brain that regulates the functioning of all parts of the body, a computer has a control-unit (CU), which controls its entire operation including its input and output devices.

Processing of data is done in the arithmetic and logic unit (ALU). It performs Arithmetic and logic operations such as addition, subtraction, multiplication and division. Here the word "Logic" is used because a computer (unlike a calculator) has the capacity to do logical operations also, such as compare two numbers, and find out which of the two numbers is greater.

The Memory unit is an important component of a computer where all the data and information are stored in the form of binary digits (combination of 0s and 1s) and retrieved whenever necessary. Computer systems use a variety of devices for storing instructions and data. The computer memory is the place where the computer holds data and programs that are in use. Computer memory refers to the physical devices in a computer. If our computers CPU had to constantly access the hard drive to retrieve every piece of data it requires, the operation will be very slow. On the other hand, when the data or information is kept in memory the CPU can access it much more quickly. From the time the computer is turned on until the time it is shut down, the CPU is constantly using the memory system. The act of entering data into a storage location is called a memory write operation, and the act of retrieving data from a storage location is called a memory read operation. Data and instructions are moved, to and from memory, in bunches of word length. These memory devices are categorised according to access time, storage capacity and cost-per-bit of storage.

Based on these criteria memory is broadly categorised into two types: Primary or main memory (also called semiconductor memory). Secondary or auxiliary memory (magnetic memory/Optical memory).

The Table 3.1 summarizes the difference between Primary (or main) memory and secondary (or auxiliary) memory.

Table: 3.1 : Difference between Memories

Memory System

Access Time Storage capacity Cost/bit of storage

Primary memory Secondary memory

Faster Slower

Smaller higher

High Low

That is, Primary memory (i.e. RAM, ROM etc.) have faster access time, smaller storage capacity, and higher cost per bit of storage, as compared to secondary memory.

Based on access time, storage capacity and cost/bit storage, the memory devices (such as RAM, ROM, Hard-disk, Floppy disk, Magnetic disk, Magnetic Tape, CD-ROM, and DVD etc.) can be categorized into three kinds of memory systems: Semiconductor memory such as RAM, ROM etc Magnetic memory such as Hard-disk, Floppy disk, and Magnetic tapes Optical memory such as CD-ROM, DVD etc

A Central Processing Unit (CPU) in a computer system is an extremely fast device as compared to the main memory, but it can not work on its own. It depends on the main memory that sends data and instruction when required for processing.

In other words, we can say that even memories with smaller size (i.e. primary memory) have very high access time (time taken by CPU to access a location in memory), which reduces the overall speed of a computer.

To reduce the cost of a large sized memory, a special type of high speed memory, known as cache memory can be used in between the CPU and the main memory

Computer memory can also be categorized on the basis of Volatile and Non-Volatile characteristics. Non-volatile Memory is a type of computer memory which can retain the stored information even if the power is shut down. Examples of Non-Volatile Memory are Read-only memory (ROM), flash memory, optical discs etc. On the other hand, a Volatile memory loses its content when the power goes off. Random Access Memory (RAM), which is the most common form of Primary Storage, is a type of volatile memory. When the computer is shut down, everything contained in RAM is lost.

To reduce the idle time of the CPU and increase the speed of processing, a fast memory can be used. This can be done by using a large size of main memory. But the cost of main memories of relatively large size is still very high. To reduce the cost of a large sized memory, a special type of high speed memory, known as cache memory can be used in between the CPU and the main memory (as shown in Figure 3.2). Thus the cost versus access time leads to a hierarchy of computer memory, where the memory is organized into a hierarchy, known as the memory hierarchy. This includes CPU registers, Cache memory, main memory and various secondary storage devices. Although the memory hierarchy is organized in such a way to minimize the cost, without compromising the overall speed of access.

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Basics of Computer Hardware

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Main Memory

Cache Memory

CPU

Figure 3.2: Memory Organization

In this unit we will mainly focus on storage organization and storage devices such as disk/tape, CD/DVD and ROM/PROM etc. We also discuss the memory hierarchy which includes main memory and high speed memory such as cache memory.

3.1 OBJECTIVES

After going through this unit, you should be able to: differentiate between types of memory and needs of the memory system; differentiate between various types of memories such as semiconductor, magnetic

and Optical memory; describe the various secondary storage devices such as Hard-disk, floppy disk,

CD-ROM, DVD-ROM etc; and describe the importance of the memory hierarchy.

3.2 MEMORY AND STORAGE DEVICES

Memory is an important component of a computer where all the data and information are stored in the form of binary digits (combination of 0s and 1s) and retrieved whenever necessary.

There are two main functions of the memory: To store programs, data and information into the computer. To store the results of computation.

A computer system uses a variety of devices for storing the instructions and data. When you want to execute a computer program, the program has to be in memory. Any input data needed for processing by that program should also be in memory. All the intermediate results and outputs from the program are stored in the memory until the machine is turned off.

The storage devices of a computer system are ranked according to the following criteria:

1. Access time: This is the time required to locate and retrieve stored data from the storage unit in response to a program instruction. That is the time interval between the read/write request and the availability of the data. A fast access time is always preferred.

2. Storage capacity: It is the amount of data that can be stored in the storage unit. A large capacity is preferred.

3. Cost per bit of storage: It is the cost of a storage unit for a given storage capacity. Low cost per bit of storage is always preferred. The final goal is to minimize this cost.

Based on above mentioned criteria, at present the following three kinds of memory system are commonly used in modern computers:

Table 3.2 : Memory and its Purposes

Sl.No. Types of Memory Purpose

1

Processor's

These are the small set of high speed registers which

internal (CPU)

are internal to a processor and are used as temporary

memories

locations where actual processing is done.

2

Primary (main) It is a fast and large memory but is slower than

memory

processor memory. Primary memory has faster

access time, smaller storage capacity and higher cost

per bit storage.

This memory is accessed directly by the processor. It

stores programs and data which are currently needed

by the CPU. The CPU communicates directly with

the main memory. The size of the main memory is

kept small because of its high cost. It is a volatile

type of memory.

3

Secondary (or Secondary memory is mainly used for bulk storage

auxiliary)

(mass storage) of programs, data and other

memory

information. It has much larger capacity than main

memory but is slower.

It is non-volatile type of memory. It stores system

software, compiler, assembler and useful packages,,

large data files etc.

Thus from above discussions, we can summarize the following points:

Secondary memory cannot be accessed directly by the CPU. First the information of these memories (which is needed by the CPU for current processing) is transferred to the main memory and then the information can be accessed as the information of main memory. Hard-disk and floppy disks are the most common secondary memories used in computers.

Secondary storage systems must offer large storage capacities, low cost per bit and medium access times. Magnetic media (such as floppy disks and hard disks) have been used for such purposes for a long time. But audio and video media, either in compressed form or uncompressed form, require higher storage capacity than the other media forms and the storage cost for such media is significantly higher.

Optical storage devices offer a higher storage density at a lower cost. A CD-ROM can be used as an optical storage device. Many software companies offer both operating system and application software on CD-ROMs today. This technology has

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been the main catalyst for the development of multimedia in computing because it is used in multimedia external devices such as video recorders and digital recorders (Digital Audio Tape) which can be used for multimedia systems.

Removable disk, tape cartridges are other forms of secondary storage devices used for back-up purposes having higher storage density and higher transfer rate.

There is another type of high speed memory, known as Cache memory, which is used to increase the speed of processing by making current programs and data available to the CPU at a rapid rate. Cache memory is a relatively small, high speed memory that stores the most recent used instructions or data. It acts as a high-speed buffer between main memory and the CPU.The cache memory is placed in between CPU and main memory. Access time is the time it takes a device or program to locate information and make it available to the computer for further processing. Cache memory access time is about 0.5 to 2.5 ns which is much less than that of the main memory. The access time of main memory is about 50-70 ns. Because of its very high cost, the capacity of the cache memory deployed is 2 to 3 percent of that of the main memory. The access time of mass storage devices such as hard disks are measured in milliseconds (ms).

The most common memory hierarchy is shown in Figure 3.3 :

CPU

Cache Memory

Primary Memory

Secondary Memory

Figure 3.3 : Memory Hierarchies

Now let us start with the memory organization of primary storage. A primary or internal storage section is basic to all computers. Figure 3.4 compares the different types of memory in terms of capacity, access speed, cost per bit of storage as follows:

Small, fast, expensive

Registers Cache Main Memory

Magnetic Disks/Virtual memory Magnetic Tapes, Remote Access etc.

Large, slow, cheap

Figure 3.4 : Storage media in terms of cost, Speed and capacity trade-offs

All the memory devices can be categorized into three main categories: Semiconductor (or Main) memory Magnetic memory Optical memory

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The Figure 3.5 illustrates the storage cost, speed and capacity of these memories. Note that cost increases with faster access speeds but decreases with access capacity.

Faster access time Primary (Semiconductor) Storage Capacity decreases

Memory

Primary storage

Cost per bit increases

Magnetic Disks Magnetic Tapes

Secondary storage

Optical Disks

Figure 3.5: Storage media cost, Speed and capacity trade-offs

You can note down the following points from the Figure 3.5 Semiconductor memories are used mainly for primary storage. It stores programs and

data which are currently needed by the CPU. The semiconductor memory is an electronic, static device. There are no moving parts

in it. Some examples of semiconductor memory are RAM, ROM etc. The semiconductor memory is faster, compact and lighter. It consumes less power. The magnetic and optical memories are slow compared to semiconductor memory.

But they are cheaper than semiconductor memory. They are not static devices. They are either in the form of a rotating disk or tape. All computers contain both semiconductor as well as magnetic memory. The examples of magnetic memory are Hard-disk, floppy disk, magnetic disk and tapes.

The Figure 3.6 shows a relationship between the access-time and capacity of various

types of memory.

Capacity

Register

0.5-2 ns (nanosecond)

Cache memory

0.5-2.5 ns

RAM

50-70 ns

Mass storage

5-20 ms (millisecond)

Access time Figure 3.6 : Capacity vs. access-time

Optical recording techniques have been recently used to store data on the surface of a coated disk. Information is written to or read from an optical disk using a laser beam. An example of this kind of serial access memory is a CDROM (Compact Disk Read-Only Memory). Only one surface of an optical disk is used to stored data. An optical disk has very high storage capacity, up to 20 GB. It is relatively inexpensive and has a long life of

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Basics of Computer Hardware

at least 15-20 years. Better optical recording methods which records data on multiple layers on a disk surface have been recently introduced. This storage device is known as DVD-ROM (Digital Versatile Disk Read-Only Memory). The main drawback of the optical disk system is its slow average access time. Table 3.3 shows the some characteristics of the discussed various memory technologies.

Table 3.3: Characteristics of Memory Technologies

Technology

Semiconductor Memories Magnetic Memories Optical Memories

Nature of Access Mode Volatile/

storage

Nonvolatile

medium

Electronic

Magnetic

Optical (laser beam)

Random (or Direct) Sequential/Rand om Random

Volatile Non-volatile Non-volatile

Access Average cost

Time (Rs/bit)

(in sec)

10-8

10-2

10-1

10-6

1

10-7

Note that there are two basic methods of accessing information from various memory devices : Sequential or serial access, or Direct or Random access

A Sequential-access memory device reads data in sequence. In other words, information on a serial device can only be retrieved in the same sequence in which it is stored. Data is recorded one after another in a predetermined sequence (such as in numeric order) on a storage medium. Sequential processing is quite suitable for such applications like preparation of monthly pay slips, or monthly electronic bills etc., where each address needs to be accessed in turn. If you are working with a sequential access device and information is stored at the last address, then data stored at the last address cannot be accessed until all preceding locations in the sequence have been traversed. That is locating an individual item of data requires searching the recorded data on the tape until the desired item is located.

A sequential-access memory such as magnetic tape is organized by arranging memory cells in a linear sequence. These do not have unique storage address that can be directly addressed. Instead, data is presented serially for writing and is retrieved serially during a read.

Read/write head

Figure 3.7: Sequential Access Memory 50

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