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UNIT-2 : MEMORY

UNIT STRUCTURE

2.1 Learning Objectives 2.2 Introduction 2.3 Introduction to Computer Memories 2.4 Primary memory 2.4.1 Random Access Memory(RAM) 2.4.2 Read Only Memory(ROM) 2.5 Secondary Memory 2.6 Let Us Sum Up 2.7 Further Readings 2.8 Answers to Check Your Progress 2.9 Possible Questions

2.1 LEARNING OBJECTIVES

After going through this unit, you will be able to- • Describe about computer memories • Identify the types of memory • Narrate about the difference in type of memories • Identify commercial memories

2.2 INTRODUCTION

In the previous unit you have come across the fundamental concepts of a computer organisation, and hardware, etc. In this unit you are going to be introduced with the computer memories. This unit mainly deals with the different types of memories used in a computer and their functions. This unit also highlights on some commercial memories. So let us now first start with the section introduction to'.

26 Fundamentals of Computer and Management Computer Memory Unit-2 2.3 INTRODUCTION TO COMPUTER MEMORIES

As mentioned in the previous unit a complete computer system consists of both software and hardware. Now software is the set of instructions according to which the computer system works. These instructions are required to be stored in a device from where we can recall later whenever we need to run any software. Also it is important to store the outcome of any task performed on a computer system or the result of any execution. Also it becomes sometimes most important to create a base and store it properly for any further use. All these data are stored in a particular device of a computer system which is known as Memory. Allihe data are stored in digital format in the storing device or element of the computer system i.e. in the memory. These memory devices may be either meant for temporary storage or permanent storage. Both these type are important to run the computer system smoothly and efficiently. The temporary memories are also known as the , as it is capable of storing till when the power is ON, after that it loses all its data. Permanent memories are called the Non volatile memory as it can retain data even after the power is switched OFF.

The term memory usually refers to a form of storage and sometimes other forms of fast but temporary storage. Similarly, storage today more commonly refers to mass storage such as optical discs, forms of like hard disks and other types. All these are slower than memory, but of a more permanent nature. Generally, the term Primary Memory is used to represent the memory and the term Secondary Memory is used to represent the storage. *

CHECK YOUR PROGRESS-1

Fill in the blanks: (i) Software is a set of , according to which a computer works. (ii) The storing device of a computer system is called the

ill) Computer memory can be classified as &

Fundamentals of Computer and Management 27 Unit-2 Computer Memory 2.4 PRIMARY MEMORY

Primary memory also known as memory,is the only one directly accessible to the CPU. The CPU continuously reads instructions stored there and executes them.Any data actively operated on is also stored there in uniform manner. It is a form of semiconductor storage known as random access memory(RAM) and sometimes other forms offaM but temporary storage. It is small-sized, light, but quite expensive. This type of memory is divided into the following two types:

• RAM (Random Access Memory) • ROM (Read Only Memory) ^

2.4.1 Random Access Memory(RAM)

RAM stands for Random Access Memory. This means Information can be retrieve and store by the computer at any order. RAM gives your computer a temporary place to process electronic data. This means that, RAM chips continue to store information only as long as computer has electrical power.

In other words, when you shut off your computer, all the data stored in RAM are lost. All actual starts with the CPU ().

There are generally two broad categories of random access memory: • DRAM memories {Dynamic Random Access Memory) • SRAM memories {Static Random Access Memory)

/; It is a miniaturized Similar to a , a memory chip is an electronic circuit that (/C) made of millions of transistors and . In the most has been manufactured in the common form of computer memory,dynamic random access memory surface of a substrate (DRAM), a transistor and a are paired to create a memory of a semiconductor cell, which represents a single of data. The capacitor holds the bit material. of information — a 0 or a 1. The transistor acts as a that lets i8 Fundamentals of Computer and Management Computer Memory Unit-2 the control circuitry on the memory chip read the capacitor or change

its .

Given that the capacitors become discharged they must be constantly recharged (the exact term is refresh) at regular intervals, known as the refresh cycle. DRAM memories for example require refresh cycles of around 15 nanoseconds (ns).

DRAM--Dynamic RAM

Dynamic random access memory(DRAM) is the most common kind of random access memory (RAM) for personal and workstations. Memory is the network of electrically-charged points in which a computer stores quickly accessible data in the form of Os and '1's. Random access means that the PC can access any part of the memory or space directly rather than having to proceed sequentially from some starting place. DRAM is dynamic in that, unlike static RAM (SRAM), it needs to have its storage cells refreshed or given a new electronic charge every few milliseconds. Static RAM does not need refreshing because it operates on the principle, of moving current that is switched in one of two directions rather than a storage cell that holds a charge in place. Static RAM is generally used for memory, which can be accessed more quickly than DRAM. DRAM stores each bit in a storage cell consisting of a capacitor and a transistor. Capacitors tend to lose their charge rather quickly; thds, the need for recharging. A variety of other RAM interfaces to the computer exist, such as EDO RAM and SDRAM.

SRAM - Static RAM

» . ■ Static RAM is a type of RAM that holds its data without external refresh, for as long as power is supplied to the circuit. This is contrasted to dynamic RAM(DRAM), which rpust berrefreshed many times per second in order to hold its data contents. SRAMs are used for specific applications within the PC,where their strengths outweigh • their weaknesses compared to DRAM: . Fundamentals of Computer and Management 29 Unit-2 Computer Memory • Simplicity: SRAMs don't require external refresh circuitry or other work in order for them to keep their data intact. • Speed: SRAM is faster than DRAM.

In contrast, SRAMs have the following weaknesses, compared to

DRAMs:

• Cost: SRAM is, for byte, several times more expensive than

DRAM. • Size: SRAMs take up much more space than DRAMs (which is part of why the cost is higher).

These advantages and disadvantages taken together obviously show that performance-wise, SRAM is superior to DRAM, and we would use it exclusively if only we could do so economically. Unfortunately, 32 MB of SRAM would be prohibitively large and costly, which is why DRAM is used for system memory. SRAMs are used instead for level 1 cache and level 2 cache memory, for which it is perfectly suited; cache memory needs to be very fast, and not very large.

SRAM is manufactured in a way rather similar to how processors are: highly-integrated transistor patterns photo-etched into silicon. Each SRAM bit is comprised of between four and six transistors, which is why SRAM takes up much more space compared to DRAM, which uses only one (plus a capacitor). Because an SRAM chip is comprised of thousand^ or millions of identical cells, it is much easier to make than a CPU, which is a large die with a non-repetitive structure. This is one reason why RAM chips cost much less than processors do.

2.4.2 Read Only Memory(ROM)

One major type of memory that is used in PCs Is called read - only memory or ROM for short. ROM is a type of memory that normally can only be read, as opposed to RAM which can be both read and :0 Fundamentals of Computer and Management Computer Memory Unit-2 written. There are two main reasons that read-only memory is used

for certain functions within the PC:

• Permanence: The values stored in ROM are always there, whether the power is on or not. A ROM can be removed from the PC, stored for an indefinite period of time, and then replaced, and the data it contains will still be there. For this reason, it is called non-volatile storage. A hard disk is also non-volatile, for the same reason, but regular RAM is not.

• Security: The fact that ROM cannot easily be modified provides a measure of security against accidental (or malicious) changes to its contents. You are not going to find viruses infecting true ROMs,for example; it's just not possible. (It's technically possible with erasable , though in practice never seen)

Read-only memory is most commonly used to store system-level programs that we want to have available to the PC at all times. The most common example is the system BIOS program, which is stored in a ROM called (amazingly enough)the system BIOS ROM. Having this in a permanent ROM means it is available when the power is turned on so that the PC can use it to boot up the system. Remember that when you first turn on the PC the system memory is empty, so there has to be something for the PC to use when it starts up. See this section for a description of the system BIOS ROM; see here for a description of the system boot sequence.

While the whole point of a ROM is supposed to be that the contents cannot be changed, there are times when being able to change the contents of a ROM can be very useful. There are several ROM variants that can be changed under certain circumstances; these can be thought of as "mostly read-only memory". The following are the different types of ROMs with a description of their relative modifiability:

• ROM: A regular ROM is constructed from hard-wired logic, encoded in the silicon itself, much the way that a processor is. It is designed to perform a specific function and cannot be changed. Fundamentals of Cqmputer and Management . -^1 Unii-2 Computer Memory This is inflexible and so regular ROMs are only used generally for programs that are static(not changing often) and mass-produced. This product is analogous to a commercial software CD-ROM that you purchase in a store.

• Programmable ROM (PROM): This is a type of ROM that can be programmed using special equipment; it can be written to, but only once. This is useful for companies that make their own ROMs from software they write, because when they change their code they can create new PROMs without requiring expensive equipment. This is similar to the way a CD-ROM recorder works by letting you "burn" programs onto blanks once and then letting you read from them many times. In fact, programming a PROM is also called burning,just like buming a CD-R,and it is comparable in terms of its flexibility.

• Erasable Programmable ROM(EPROM): An EPROM is a ROM that can be erased and reprogrammed. A little glass window is installed in the top of the ROM package, through which you can actually see the chip that holds the memory. Ultraviolet light of a specific frequency can be shined through this window for a specified period of time, which will erase the EPROM and allow it to be reprogrammed again. Obviously this is much more useful than a regular PROM, but it does require the erasing light. Continuing the "CD" analogy, this technology is analogous to a reusable CD-RW.

• Electrically Erasable Programmable ROM (EEPROM): The ' next level of eras-ability is the EEPROM, which can be erased under software control. This is the most flexible type of ROM, and is now commonly used for holding BIOS programs. When you hear reference to a "flash BIOS" or doing a BIOS upgrade by "flashing", this refers to reprogramming the BIOS EEPROM with a special software program. Here we are blurring the line a bit between what "read-only" really means, but remember that this rewriting is done maybe once a year or so, compared to real read-write memory (RAM) where rewriting is done often many times per second. ;2 Fundamentals of Computer and Management Computer Memory Unit-2

CHECK YOUR PROGRESS-2

1. Which memory is the only one directly accessible to CPU?

2. What are the two basic type of RAM?

3. Write down the name of different types of ROM.

2.5 SECONDARY MEMORY

Secondary memory or storage provides the facility of storing information and programs permanently. It differs from primary storage in that it is not directly accessible by the CPU.The computer usually uses its input/output channels to access secondary storage and transfers desired data using intermediate area in primary storage. Secondary storage does not lose the data when the device is powered down; it is non-volatile. It is typically also.an order of magnitude fess expensive than primary storage. Consequently, modern computer systems typically have an order of * . ■ ' . 1 s magnitude more secondary storage than primary storage and the data is kept for a longer time there. The following figure shows the commonly used secondary storage devices and their classification.

Fundamentals of Computer and Management 33 Unit-2 Computer Memory Secondary Storage Devices

Sequential Direct Access Devices Access Devices

Magnetic Disks Optica Disks Mamory Storage Devices _i_

Floppy Hard D- Worm CD-RW DVD Disks Disks ROM (CD-R) Drive Card

Zip Disk Disk Winchester Pack Disk Fig. 2.1 :Secondary storage devices and their classification

In modem computers, hard disks are usually used as secondary storage. The time taken to access a given byte of information stored on a hard disk Is typically a few thousandths of a second, or milliseconds. By contrast, the time taken to access a given byte of information stored in random access memory is measured in thousand-millionths of a second, or nanoseconds. Some other examples of secondary storage technologies are: Flash Memory (e.g. USB sticks or keys). Floppy Disks, . tape. Punch Cards, standalone RAM disks, and Zip drives.

The characteristics that are used for the classification of various types of storage are:

Storage Capacity: It is a representative of the size of the memory. The capacity of internal memory and main memory can be expressed in terms of number of words or . The storage capacity of external memory is normally measured in terms of bytes.

Access Modes: A memory is considered to consist of various memory locations. The information from these memory locations can be accessed in the following ways:

Fundamentals of Computer and Management Computer Memory Unit-2 • Random Access: It is the mode in which memory locations can be accessed in any order in the same amount of time. Ferrite and Semi- - conductor memories, which generaliy constitute main memory, are of this nature. The storage location can be accessed independently and there exist separate access mechanism for each iocation. • Sequential Access: Sequential access also known as serial access is a type of method of retrieving data from a storage device. With sequential access, the device must read or move through all information up to the point it is attempting to read or write. A is an example of a sequential access drive, where the drive must move the tape forward or backwards until it reaches its

destination. • Direct Access: A direct access storage device is one in which any location in the device may be selected at random, access to the information stored is direct i.e. no need of scanning a series of records and approximately equal access time is required for each location. Hard disk, optical disk are the examples of direct access storage device.

Access Time: The access time is the time required between the requests made for a read or write operation till the time the data is made available or written at the requested location. Normally it is measured for read operation. The access time depends on the physical characteristics and access mode used for that device.

Validity of information: Some memories lose information over a period of time. For example, there can be some memories where the stored data bit value 1 looses its strength to become 0 over a period of time. These kinds of memories require refreshing. The memories, which require refreshing, are termed as dynamic memories. In contrast, the memories, which do not require refreshing, are called static memories. Another factor, which can destroy the contents, is the presence and absence of electrrcity. The memories, which lose their content on failure of power are termed as volatile memories; those, which do not, are called non-volatile. Magnetic memories are non-volatile and semi-conductor main memories are volatile in nature. Fundamentals of Computer and Management 35 Unit-2 Computer Memory

Sequential Access

Random Access

Fig. 2.2: Shows methods of accessing from Sequential and Direct storage Device

Physical Characteristics: In this respect the memory devices can be

V categorized into four main categories viz. electronic, magnetic, mechanical and optical. One of the requirements for a storage device is that it should exhibit two well-defined physical states, such as 0 and 1 that can be represented in those two states. The data transfer rate of the memory depends on how quickly the state can be recognized and altered. The following table lists some of the memory technologies along with their physical and other important characteristics.

Access Access Performance Physical nature of Technology . time(in sec) Mode of Storage storage medium

Semiconductor Random Volatile memories, 10-8 Electronic

Magnetic disk 10-2 Direct Norvvolatile Magnetic

Magnetic tape 10-^ Sequential Non-volatile Magnetic

Compact disk approx 10-' Direct Non-volatile Optical ROM

Fig. 2.3: Characteristics of some memory technologies

From the point of view of the technology used, the storage can be classified as;

16 Fundamentals of Computer and Management Computer Memory ^ Unit-2 Semiconductor storage : The uses semi conductor based integrated circuits to store information. A semiconductor memory chip may contain millions of tiny transistors or capacitors. Both volatile and non-volatile forms of semiconductor memory exist. In modern computers, the primary storage almost exclusively consists of dynamic volatile semiconductor memory or dynamic random access memory. d/lagnetlc storage: Magnetic storage uses different patterns of magneti zation on a magnetically coated surface to store information. Magnetic storage is non-volatile. The information is accessed using one or more read/write heads which may contain one or more recording transducers. In modern computers, magnetic storage used are Magnetic disk, . Hard disk. Magnetic tape etc: .

Optical storage : , in the case ©f typical optical discs, uses tiny pits etched on the surface of a circular disc to store information, and reads this information by illuminating the surface with a and observing the reflection. storage is non-volatile and sequential access. The following forms are currently in common use: • CD, CD-ROM, DVD, Blue-ray Disc, HD DVD( Read only storage, used for mass distribution of digital information such as music, video, computer programs) • CD-R, DVD-R, DVD+R (Write once storage, used for tertiary and off-line storage)

• CD-RW, DVD-RW, DVD+RW, DVD-RAM.(Slow write, fast read storage, used for tertiary and off-line storage}

Paper tape and punch cards have been used to store information for automatic processing since the 1890s, long before general-purpose computefs existed. lnformation#was recprded by punching holes into the paper or cardboard medium, and was read by mechanically (or, later, optically) sensing whether a particular location on the medium was solid or contained'a hole." , ,

Fundamentals of Computer and Management 37 Unit-2 Computer Memory

CHECK YOUR PROGRESS-3

1. Give some examples of secondary storage device technology.

2. What is access time?

3. What do you mean by semiconductor storage?

4. CD, CD-ROM, DVD, Blue-ray Disc, HD DVD comes in which category of storage?

2.6 LET US SUM UP

• The main data storage devices of a computer system are known as Memory. • All data are stored in digital format in a memory of a computer system. • Computer Memory can be categorized in two basic categories, viz- Primary memory & Secondary memory. • Primary memory also known as memory is the only one directly accessible to the CPU. • Primary memory is of two basic types- RAM and ROM.

38 Fundamentals of Computer and Management Computer Memory • . Unit-2 • Secondary memory or storage provides the facility of storing inforrhation and programs permanently. It differs from primary storage in that it is not directly accessible by the CPU. • Some examples of secondary storage technologies are: Hard disks, Flash Memory (e.g. USB sticks or keys), Floppy Disks, Magnetic tape. Paper tape, Punch Cards, standalone RAM disks, and Zip

drives. • The Semiconductor memory uses semiconductor base^d integrated circuits to store information. • Magnetic storage uses different patterns of magnetization on a magnetically coated surface to store information. • Optical storage, in the case of typical optical discs, uses tiny pits etched on the surface of a circular disc to store information, and reads this information by illumii^ating the surface with a laser diode and observing the reflection.

2.7 FURTHER READINGS

• Computer Fundamentals Architecture and drganizatlon- B. Ram (New Age International(P) Ltd.). • Introduction to Computer Science- ITL Education Solution Limited (Pearson Education). • Computer Fundamentals- P K SInha (BPS Publication).

• DOEACC O & A Level Books.

2.8 ANSWERS TO CHECK YOUR PROGRESS

CHECK YOUR PROGRESS-1 (i) instruction . (ii) memory (iii) primary, secondary

CHECK YOUR PROGReSS-2 1. Primary Memory 2. DRAM and SRAM 3. ROM, PROM, EPROM, EEPROM > Fundamentals of Computer ai]d Management 39 Unit-2 Computer Memory

CHECK YOUR PROGRESS-3 1. Hard disk, Flash Memory, Floppy Disks, Magnetic tape. Paper tape, Punch Cards, standalone RAM disks, arid Zip drives. 2. The access time is the time required between the requests made for a read or write operation till the time the data Is made available or written at the requested location. 3. Semiconductor storage means that storage technology which uses semiconductor based 10 for storing. 4. Optical storage

2.9 POSSIBE QUESTIONS

1. Write down about the basic difference between primary memory and secondary memory.

2. Write about the DRAM.

3. Write a short note on SRAM. 4. What are the different types of ROM? Write a few lines about them. 5. Write a short note on secondary memory.

/0 Fundamentals of Computer and Management