What is semiconductor (Main) Memory, What is Optical Memories & WHAT IS MEMORY HIERARCHY AND THEIR NEEDS

What is semiconductor (Main) Memory

·         All computers except very small computers contain both semiconductor as well as magnetic memory.

·        All modern computers use semiconductor memory as its main memory (or primary memory). Semiconductor memory is known as Random access memory (RAM) because          any part of the memory can be accessed for reading and writing.

·         It stores programs and data which are currently needed by the CPU.

·         Another part of main memory is Read Only Memory (ROM). ROMs are those memories on which it is not possible to write the data. They can only be read.

·         Thus RAM and ROM memories are used as the main memory of the computer.

·         The Main memory holds the programs and data required by the CPU for carrying out its operations.

·         The primary (main) storage is a semiconductor device that is built using integrated circuits. The data is stored in binary form in main memory. Numeric as well as non-               numeric data can be represented in binary form. With two binary digits, we can represent 4 different characters. With three binary digits, we can represent 8                              different characters. Computers internally use eight binary digits to represent characters and digits (A binary digit is referred to as a bit and 8 bits are called a byte). 256            characters can be represented by a byte.

    The capacity of a computer‟s memory is usually expressed in terms of bytes. Computer codes such as ASCII (American Standard Code for Information Interchange) use           various arrangements of bits to form bytes that represent the numbers 0 to 9, the letters of the alphabet, and many other characters.

Storage capacities are frequently measured in Kilobytes (KB), Megabytes (MB), Gigabytes (GB), or Terabytes (TB). Table summarizes the commonly used names with abbreviations and number of bytes for these storage capacities.

                                      Table: Commonly used names, Abbreviations and storage capacity in bytes.

                                                

Types of Main Memory

Memory can be of various types like Random Access Memory (RAM) and Read-Only Memory (ROM). Figure summarizes the different types of main memory.

   

RAM (Random Access Memory)

•   The Read and write memory (R/W memory) of a computer is called a RAM. The user can write information into RAM and read information from it. It is called random access   since any memory location can be accessed in a random manner for reading and writing. The access time is the same for each memory location. It usually refers to  "temporary" memory, which means that when the system is shut down, the memory is lost.

Both static and dynamic RAMs use CMOS technology. CMOS devices consume less power. Static RAMs hold information in a flip-flop circuit consisting of two cross coupled inverters. In a RAM the memory cell must be associated with a read and write facility. Six (6) transistors are needed per memory cell in a static RAM. Dynamic RAMs required fewer transistors per memory cell. The following are commonly used RAM chips:

EDO (Extended Data Output RAM): In an EDO RAM any memory access stores 256 bytes of data into latches. The latches hold next 256 bytes of information, so that in most programs which are sequentially executed, the data are available without wait states.

·     

     SDRAM (Synchronous DRAM) and SGRAM (Synchronous Graphics RAM):

These RAM chips use the same clock rate as the CPU uses. As a result the memory chips remain ready to transfer data when the CPU expects them to be busy. SDRAM is often used as mass storage whereas SGRAM is used as a high end graphics memory.

Dual-Ported DRAM: These types of RAM allow one to access two memory locations simultaneously. Sometimes it is also called video RAM (or VRAM). WRAM (Window RAM) is a special version of VRAM, which is commonly used in

PCs running WINDOWS and WINDOWS applications.

   SIMM and DIMM: These stand for single-Inline and Double Inline Memory Modules. These are small printed circuit cards, on which several DRAM memory chips are placed. Such cards are plugged into the system board of the computer.

ROM (Read Only Memory)

        A Read-Only memory (ROM) is a non-volatile memory, i.e., the information stored in it is not lost even if the power supply goes off. Thus a Read Only Memory (ROM) is         one in which information is stored permanently.

 Unlike RAM, the information from ROM can only be READ and it is not possible to WRITE fresh information to it. That is, the CPU can only fetch or READ instructions  from ROM. This is the reason why it is called ROM. Computers almost always contain a small amount of Read-Only memory (ROM). It is much cheaper compared to  RAMs when produced in large volumes.

  ROM is used for storing a special set of instruction, which the computer needs when it starts up (boots up).

 The contents of ROMs are decided by the manufacturers. The contents are permanently stored in a ROM at the time of manufacture.

         From the programming mode point of view, we have

          Masked-programmed

          User-programmed

 ROMs in which contents are written at the time of IC manufacture are called mask-programmed ROMs. PROM, EPROM and EEPROM or any other kind of PROM are user  programmable ROMs. If we simply write (or say) ROM it means masked programmed.

An example of a ROM is the Toshiba mask ROM, TCS 534000.

PROM (Programmable ROM)

·         A variation of ROM chip is programmable read only memory (PROM). A PROM is a memory chip on which data can be written only once.

·         ROM chips are supplied by computer manufacturer and it is not possible for a user to modify the programs stored inside the ROM chip. However, in case of PROM, it is       possible for a user to customize a system by storing own program in a PROM chip.

·        Once a program has been written on to a PROM chip, the recorded information cannot be changed i.e., the PROM becomes a ROM and it is only possible to read the           stored information.

·         PROM is also a non-volatile memory i.e. the stored information remains even if power is switched off.

·         The basic difference between PROM and a ROM is that a PROM is manufactured as blank memory, whereas a ROM is programmed during the manufacturing process.

To write data on a PROM chip, you need a special device called a PROM programmer or a PROM burner. The process of programming a PROM is sometimes called burning the PROM.

Magnetic Memory

In the above section we have seen various types of semiconductor RAMs. These high speed semiconductor storage devices (i.e. RAMs) are expensive. So we need some inexpensive media for storage. Also semiconductor memory has the following limitations:

1)      Limited Capacity: Semiconductor (primary) memory of today‟s computers is not sufficient, since most of the data processing organizations deal with a large volume of data.

2)      Volatile Memory: Semiconductor memory is volatile in nature. But there is always a need to store data on a permanent basis.

Thus there is a need of additional memory, that is inexpensive, non-volatile in nature and has large capacity. Magnetic material is inexpensive and long lasting, so it is an ideal choice for us. Magnetic memory is a permanent non-volatile, type of memory. Now-a-days, we are not using floppy disk.

A modern computer uses the following two types of magnetic memory:

(i)     Magnetic Disks: Hard disks and Floppy disks.

     (ii)   Magnetic Tapes : Magnetic disks are the most common form of secondary storage because they provide fast access and high storage capacities at a reasonable cost.

Storage Mechanism: Magnetic disk drives contain metal disks that are coated on both sides with an iron oxide recording material. Several disks are mounted together on a vertical shaft which typically rotates the disks at speeds of 3600 to 7600 revolutions per minute (rpm). Electromagnetic read/write heads are positioned by access arms between the slightly separated disks to read and write data on concentric, circular tracks. Data are recorded on tracks in the form of tiny magnetized spots to form the binary digits of common computer codes. Thousands of bytes can be recorded on each track, and there are several hundred data tracks on each disk surface, which provides billions of storage positions for your software and data.

There are basically two types of magnetic disk arrangements, one having a removable disk cartridge and other having a fixed disk unit. Removable disk devices are popular because they are transportable and can be used as backup copies of your data.

Data Organizations: A magnetic disk is a surface device, which stores data on its surface. Its surface is divided into circular concentric tracks. The number of tracks on 

a disk range up to 800. Each track is divided into sectors (normally 10-100). These sectors can be either fixed or variable length sectors. The division of track into equal 

sized blocks or pages is set by the Operating system during disk formatting. The number of bytes stored in each sector is kept the same.

The numbers vary but there are often 200 or more ranging up to 800 is sectors per track. Magnetic disks are semi-random devices. A track on a disk is selected in a random fashion, but data is written to or read from a sector in serial fashion.

Hard-Disk Drives (HDD)

·           Hard disks are on-line storage devices.

·         The term online means that the device (hard-disk) is permanently connected to the computer system and when the computer is on, the device (hard-disk) is available to       store information or to retrieve information.

·          HDD  stores  programs,  data,  operating system,  compiler,  assemblers,  application

      programs etc.

Storage Organization in HDD

·         HDD contains magnetic disks, access arms and read/write heads into a sealed, air filtered enclosure. This technique is known as Winchester technique.

·         Winchester disk is another name for "hard disk drive". There are two stories behind the name Winchester disks; one is that the disk was developed at IBM‟s             facility at Winchester, New York State; that had 30MB of fixed storage and 30MB of removable storage; the other is that the first model number was given as 3030,       which is also the model number of the well-known Winchester Rifle popular in the Wild West. Although modern disk drives are faster and hold more data, the basic         technology is the same, so Winchester has become synonymous with hard disk.

·       Thus Winchester disk is a sealed "hard disk" having rotation speed typically

    7200 rpm. A disk has 5000 to 10,000 concentric tracks per centimeter and about 100,000 bits per centimeter around circumference. Figure 3.12 illustrates a portion       of Winchester disk.

·         The read/write head reads data from the disk and writes data to the disk. A disk is mounted (or stacked) on the disk drive, which has the motor that rotates it. Hard-      disks together with read/write heads, access mechanism and driving motor constitute a unit called hard-disk-drive (HDD) unit. The whole unit is fixed.

               ·         Hard disk is also known as platter. It can not be removed or inserted into a HDD unit. Some disks have a single platter e.g. floppy disk.

             

               

              ·         To increase the storage capacity several hard-disks (platters) are mounted (stacked) vertically, normally at a distance of an inch. This is known as disk pack or 

                multi-platter configuration.

              ·         A set of corresponding tracks in all surfaces of a disk pack (i.e. the tracks with the same diameter on the various surfaces) is called a cylinder (see Figure). 

                Here the concept of cylinder is very important because data stored on the same cylinder can be retrieved much faster than if it were distributed among different                        

               

                cylinders.

                                                               

Figure 3.13: A disk having n platters (0 to n-1 plates). A set of corresponding tracks on all the (n-1) Surfaces, at a given radial distance, is called a cylinder

Relationship among Capacity, density and speed

Suppose a HDD (or disk pack) having n plates, has: m=2n= total number of recording surfaces

t= tracks per surface p= Sectors per track s=bytes per sector,

π=3.14 then

 Storage capacity of the disk=(m*t*p*s) bytes

 If d is the diameter of the disk, the density of the recording is:

Density=(s*p)/(π*d) byte/inches

Example 1: A 2.5 inch diameter disk pack has 6 plates (12 recording surfaces), 256 sectors per track, 5250 tracks per surface, 512 byes per sector. Thus disk capacity = 12×5250×256×512=8,257,536,000 bytes = 7.69 GB and recording density= (512×256)/(3.14×2.5)=16697 bytes/inch.

Example 2: What will be the storage capacity of a 2.5 inch diameter disk pack having 8 plates, 400 sectors per track, 2820 tracks per surface where 512 bytes of data can be stored per sector.

Solution:    Total number of recording surface (m) = 2n = 2*8=16

Storage Capacity=16*2820*400*512 = 9240576000 bytes = 8.6 GB

There are several disk drives (C,D,F etc.) in a computer, which are connected to a disk controller. The controller converts instructions received form the computer (software) to electrical signals to operate disks. The Disk controller accepts commands from the computer and positions the read/write head of the specified disk for reading or writing.

For reading or writing operations on a disk pack, the computer must specify the drive number, cylinder number, surface number, and sector number. The drive number must be specified, because a controller normally controls more than one drive. Table 3.6 shows a disk address format for a disk controller of 8 drives, each disk pack having 250 cylinders, 12 surface and 256 sectors.

Access time on a magnetic disk

Magnetic disks are semi-random devices. A track on a disk is selected in random fashion, but data is written to or read from a sector in serial fashion. In order to access information from a disk, the disk address of the desired data has to be specified. The disk address is specified in terms of track number, surface number and the sector number. Information is always written from the beginning of a sector and can be read only from the track beginning.

As soon as the read/write command is received by the disk controller, the read/write heads are first positioned onto the specified track number (or cylinder) by moving the arm assembly in the proper direction. The time required to position the read/write head over proper track is called the seek time.

Seek time (Ts): The time required to move the read/write head on a specific (address) track.

·         Seek time varies depending on the position of the arm assembly when a read/write command is received.

·         Seek time will be maximum, if the arm assembly is positioned on the outer most track and the track to be reached is the inner most one and it will be         zero if the arm assembly is already on the desired track.

·         The average seek time is thus specified for most systems which is generally between few milliseconds to fractions of a second.

Note that seek time is associated only with movable-head system. For a fixed-head system, it is always 0 because there is a head for each track and no head movement is required for accessing a particular track.

Once the heads are positioned on the desired track, the head on the specified surface is activated. Since the disk is continuously rotating, this head should wait for the desired data (specified sector) to come under this head. This rotational waiting time i.e. time required to bring the needed data (i.e. starting position of the addressed sector) under the read/write head is called the latency time.

Latency Time (t_L) or Search time: Time required to bring the needed data under the R/W head. Latency time is also a variable and depends on the following two parameters:

·         Distance of the desired data from the initial position of the head on the specified track.

·         Rotational speed of the disk.

The average seek time is thus normally specified for most systems which is generally of the order of 10 to 15 milliseconds.

The total access time for a disk is equal to the seek time plus the latency time.

The average access time for most disk systems is usually between 10 to 100 milliseconds.

Pen Drive

Now-a-days a Pen Drive is available as a very convenient and flexible data storage medium which can store up to 256 GB data. It can be used for the same purposes as floppy-disks or CD-ROMs. Pen Drives are a smaller, faster, durable and more reliable storage medium. Compared to floppy disks or CD-ROMs it has thousands of times more 

data storage capacity. It is a portable USB flash memory device. It is integrated with a
USB (Universal Serial Bus) interface. It can be used to quickly transfer data from one
system to another. The pen drive derives its name from the fact that many of these
devices resemble a small pen or pencil in shape and size. Flash drives implement the
USB mass storage device class so it is possible for modern operating systems to read and
write from them without installing the device driver software. Some computers can even
boot up from flash drives.

Magnetic Tapes

A Magnetic tape is a sequential access type secondary storage device. It is used for backups in servers, workstations, and large computers. The main advantages of magnetic tapes are that they are cheaper and since these are removable from the drive, they provide unlimited storage capacity (20 GB to 150 GB).

The read/write heads of magnetic tape drives record data in the form of magnetized spots on the iron oxide coating of the plastic tape. Magnetic tape devices include tape reels and cartridges in mainframes and midrange systems, and small cassettes or cartridges for PCs.

The main drawback of magnetic tapes is that they store information sequentially. A file or some particular information stored on a magnetic tape cannot be accessed directly on random basis as is possible in the case of hard-disks or floppy disks. These devices are slower, but due to their low cost, they are still widely used for massive data warehouse and other business storage requirements.

The storage capacity of a tape is measured by multiplying its length and data recording density. Data recording density is the amount of data that can be stored on a given length of tape. That is,

Storage Capacity = data recording density * length

Example 1 : If a tape length is 3400 feet long and has a data recording density of 900 bpi (bytes per inch) its storage capacity will be 3400*12 inches *900 bpi =36720000 bytes.

Check Your Progress 2

1.  State True or False :

a)

ROM is a volatile type of memory.

True

False

b)

1 Terabyte (1 TB) equals 230 bytes.

True

False

c)

Magnetic disk has higher storage capacity

than Magnetic tape.

True

False

d)

The

basic

difference

between

PROM

and

a

ROM

is

manufactured

as

blank

that

a

PROM

is

memor,

whereas a ROM is programmed during the

manufacturing process.

True

False

e)

When

we

load

software

from

afloppy

disk,

hard

disk

or CD-ROM, it is stored in the main memory.

True

False

f)

Tracks with the same diameter on the various surfaces

are known as a cylinder.

True

False

g)

Time required to bring the needed data under R/W.

head is known as seek time.

True

False

h)

Access time is the sum of seek time and latency time.    True

False

Multiple Choice Questions

2)      The different types of memory units are:

a)      RAM

b)      ROM

c)      PROM

d)      All of the above

3)      Which of the following memory loses it contents when the computer is turned off?

a)      RAM

b)      ROM

c)      PROM

d)      All of the above

4)      Which of the following memory chips is programmed during the manufacturing process?

a)      RAM

b)      ROM

c)      PROM

d)      EEPROM

5)      An EEPROM can be erased by exposing it to:

a)      Sunlight

b)      Ultraviolet Radiation

c)      Magnetic field

d)      Electric Charge

6.	Match the following:
	i)	Semiconductor memory	a)	Hard-disk
	ii)	Magnetic memory	b)	CD-ROM
	iii)	Optical memory	c)	Floppy disk
	iv)	Double side double density	d)	RAM
1(d), 2(a), 3(b), 4(c) 7.	Suppose in your library the following types of memory are present:
	RAM, ROM, PROM, EPROM, and EEPROM.

       Volatile memory means the stored data are lost, if the power goes off.. For example RAM is a volatile memory. A non-volatile memory means the information stored in it is not lost even if the power supply goes off. A Read-Only memory (ROM) is a non-volatile memory.

Differentiate between these memories on the basis of their volatility.

8.      A 2.5 inch diameter disk pack has 6 plates (12 recording surfaces), 256 sectors per track, 5268 tracks per surface, and 512 byes per sector. Find the capacity and 

     recording density of the disk pack.

      Disk capacity= 12x5268x256x512=7.716 GB and Recoding density= (512x256)/(3.14x2.5)=16688 bytes/inch.

9.     Explain the following terms with respect to. magnetic memory:

a)      Seek time and Latency time

b)      Track, sectors and cylinder

a) Seek time and Latency time : The time required to position the read/write head over proper track is called the seek time. The time required to bring the needed data      (i.e., starting position of the addressed sector) under the read/write head is called the latency time.

Latency Time (t_L) or Search time: Time required to bring the needed data under R/W head. Latency time is also a variable and depends on the following two things:

·         Distance of the desired data from the initial position of the head on the specified track

·         Rotational speed of the disk.

Access time= Seek time + Latency time

b)   Track, sectors and cylinder : A magnetic disk is a surface device, which stores data on its surface. Its surface is divided into circular concentric tracks. The number of tracks on a disk range up to 800. Each track is divided into sectors (normally 10-100). These sectors should be either fixed or variable length sectors. The division of track into equal sized blocks or pages is set by the Operating system during disk formatting.

A set of corresponding tracks in all surfaces of a disk pack (i.e. the tracks with the same diameter on the various surfaces) is called a cylinder.

What is Optical Memories

Optical memories or Optical disks are alternate mass storage devices with huge capacity (up to 20 GB). Information is written to or read from an optical disk using a laser beam. Only one surface of an optical disk is used to stored data. An optical disk is relatively inexpensive, and has a long life of at least 15 years. Since the read/write head does not touch the disk surface, there is no problem of disk wear or head crash. The main draw back of the optical disk system is its slow average access time. Here, we will discuss 3 types of optical disks:

1.      CD-ROM (Compact-Disk Read Only Memory)

2.      WORM (Write Once Read many) or CD-R (CD-Recordable).

3.      Erasable Optical Disk

4.      DVD-ROM, DVD-R and DVD-RAM

(1) CD-ROM

CD-ROM technology uses 12-centimeter (4.7-inch) compact disks (CDs) similar to those used in stereo music systems. Each disk can store more than 600 MB. That is approximately equivalent to 400 1.44 MB floppy disks or 300,000 double-spaced pages of text.

First of all a master disk is prepared. On a master disk, a laser records data by burning permanent microscopic pits in a spiral track to represent 1. From a master disk, CD-ROMs are produced on mass scale. Then CD-ROM disk drives use a laser device to read the binary codes formed by those pits.

For reading the data a laser beam of lower intensity is employed. A laser system needs 25mW for writing whereas only 5mW are needed for reading.

CD-ROMs use long spiral tracks to store data serially, as shown in Figure. The track is divided into blocks of same size as shown in the figure. A CD-ROM disk rotates at a variable speed so that the pits are read by the laser at a constant linear speed. The speed of the disk is adjusted in such a way that the track passes under the read/write head at a constant linear velocity.

Advantages

·         High storage capacity.

·         Cost per bit of storage is cheaper than the other types of memory devices.

·         Removable from the computer, so suitable for archival storage. 5.25 inch disks store 650 MB data.

Disadvantages

·         Longer access time as compared to that of a magnetic hard disk (because locating a desired address involves first moving the head to the specific area then adjusting       the rotating speed and then reading the address, and then to find and access the specific sector).

·         Information can not be updated because it is a read-only (permanent) memory.

(2) WORM or CD-R (CD-Recordable)

CD-R (compact-disk recordable) is another optical disk technology. The user can record (write) their own data once on a CD with a CD-R disk drive unit. After this recording user can read the data as many times as desired.

CD-R is suitable for data and files which are not to be changed. The user can store permanent data, information, and files for maintaining records.

Advantages and Limitations

·         High storage capacity.

·         Better reliability and long life.

·         Greater access time as compared to a hard-disk.

(3) Erasable Optical disk or CD-RW (CD-rewritable)

The major limitation of CD-ROM and CD-R disks is that recorded data can not be erased. However, CD-RW (CD-rewritable) optical disk systems have now become available which record and erase data by using a laser to heat a microscopic point on the disk‟s surface.

Advantages and limitations

·         Very high storage capacity. A 5.25 inch optical disk can store about 650 MB data

·         It is more reliable and has a long life.

·         Longer access time as compared to that of a hard-disk.

(4)   DVD-ROM, DVD-R and DVD-RAM

DVD stands for Digital Video Disks or Digital Versatile Disks. A DVD stores much more data than a CD-ROM. Its capacities are 4.7GB, 8.5GB, and 20GB etc. The capacity depends on whether it is a single layer, double layer; single sided or double sided disk. DVD uses laser beam of shorter wavelength than CD-ROM uses and therefore more tracks are available. Working principles of DVD disks are same as those of a CD-ROM, CD-R or CD-RW.

The Speed of CD-ROM or DVD-ROM is given in terms of nX, where n is an integer. For example 32X. In case of CD, X=150 KB/s, so 32X=32x150=4.8 MB/s. In case of DVD, X=1.38 MB/s.

DVD-R: It is a recordable DVD, same as a CD-R disk. The user can write data once on a DVD-R, then read the data as many times as required.

DVD-RAM: It is a rewritable DVD, same as a CD-RW disk. DVD-RAM uses a phase change technology to write, read and erase data.

                        Table summarizes the different types of secondary (auxiliary) memory devices.

Medium

Capacity

Advantages

Disadvantages

Primary Uses

Storage

mechanism

Hard

Variable

Usually

·

Slower

· To store

data

Magnetic

Disk

integrated

into

computer

·

and files

the PC

performance

To

store

• very robust

when

disk

is

software

full

Pen Drive

1GB-

Portable

Most  USB

flash

256 GB

Large

storage

drives

do

not

•

To

store

data

capacity

include

a

write-

and files.

Smaller,

faster

protect

For

transferring

and reliable

mechanism.

data

and

files

Due

to

its

small

between

size

they

can

computers

easily

be

misplaced or lost.

CD-

650-700

Portable

&

Some

older

To

store

files

Optical

ROM/

MB

Medium

storage

computers

cannot

and software

CD-R/CD-

capacity

read

CD-RW

·

To

store

RW

·

Inexpensive

media

archive

·

Some

types

·

CD-R

discs

material

from

(CD-RW)

can

are

„write

·

hard disks

be

reused

i.e.

once‟,

which

To

store

·

rewritable disk.

means

once

scanned

files

Can be used in

data is copied

such  as

exam

certain

models

to  it,

new

or

·

papers

of DVD players

additional data

To

store

cannot

be

applications

added

from

the

Internet

DVD-

· Large

storage

·

Not

all

Same as..

Optical

ROM

4.7GB to

·

capacity

computers can

CD-ROM/

DVD±R

8.5GB

Some

types

read  DVD±R

CD-R/CD-RW

DVD±R

(DVD±RW)

or  DVD±RW

W

can

be

reused

disks.

·

or rewritable.

DVD±R discs are

Can be used in

„write

once‟,

certain

models

which means once

of

DVD

data

is  copied

to

players.

it,

new

or

additional

data

cannot be added

Magnetic

20GB to

·

Very

Large

·

Data

cannot

·

To

store

Magnetic

tape

2TB +

storage

be

accessed

·

files

·

capacity

·

immediately

Ideal

for

Disks

are

Requires tape

large

scale

durable,

drive

and

daily

and

robust

and

third

party

weekly

·

rewriteable

·

software

backup

Inexpensive

Tape

drives

operations,

for

large

particularly

capacity

for servers.

tapes  can

be

very

expensive

WHAT IS  MEMORY HIERARCHY AND THEIR NEEDS

Every programmer wishes to have a large and fast memory. However, the two requirements are conflicting. Fast memories are expensive and small; and slow memories are cheaper and large. To give a user the illusion of both fast and large, the memory system of modern computers is organized in a hierarchical way. The very top of the hierarchy is CPU registers, between the CPU and main memory, a fast cache memory is added. The hard disk is used by the technique of virtual memory to expand the capacity of main memory. Most computer systems make use of a hierarchy of memory technologies as a single type of memory is not sufficient. This hierarchy is known as the memory hierarchy.

In the previous section, we have discussed the various types of memory systems such as semiconductor (main) memory, Magnetic memory and Optical memory. In this section, we will discuss the hierarchy of these memory systems.

As you have seen in the previous section faster memory technology (such as semiconductor memory) is more expensive. In addition fast memory requires power supply till the information needs to be stored. Furthermore, the memory with less cost (such as Optical memory) have very high access time, that is the time taken by CPU to access the memory location is high, which result in a slower operation of CPU. Thus the cost versus access time leads to a memory hierarchy. The overall goal of Memory Hierarchy is to obtain the highest possible access speed while minimizing the total cost of the memory system. Figure (a) illustrates the components of a typical memory system.

A computer system uses a variety of devices for storing the instructions and data. A storage devices (or units) may vary according to the access time, storage capacity, and cost-per-bit of storage, as discussed in the previous section. Based on these criteria, a memory system can be considered to consist of three groups of memories.

1.      Processor's internal (CPU) memories: consisting of the small set of high speed registers which are internal to a processor and are             used as temporary locations where actual processing is done.

2.     Primary (main) memory: It is a fast and large memory but slower than 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 size of the main
memory is kept small because of its high cost.

3.      Secondary (or auxiliary) memory: The secondary memory is mainly used for bulk storage (mass storage) of programs, data and other information. It has much larger capacity than main memory but slower than main memory. It basically stores system software, compiler, assembler and useful packages, large data files etc.

A typical storage hierarchy is shown in Figure (b).

A block diagram of storage hierarchy, as shown in Figure-3.16(b) includes:

·         CPU (register)

·         Cache memory

·         Main memory

·         Secondary storage,  and

·         Mass storage

As we move up the storage hierarchy, we encountered memory elements having faster access time, less capacity and higher cost per bit stored. When we move down, we have a larger storage capacity, slower access time and lower cost per bit stored. Thus, CPU storage components generally have the fastest access time, the smallest storage capacity and the highest cost per bit stored. The cache memory which is placed in between the CPU and the main memory is a very high speed semiconductor memory used to enhance the speed of main memory. The main (primary) memory falls next in the memory hierarchy list. Secondary storage media such as hard-disk/magnetic disk memories make up the level of hierarchy just below the main memory. Secondary storage devices are at the bottom of the memory hierarchy. Secondary storage devices such as magnetic tapes are used for archival storage. They are very cost effective and so are used for mass storage of data, when fast access time is not required.

Check Your Progress 3
1.  State True or False :
a)	Cache memory is faster than the Register (CPU) memory.	True	False
b)	Cache memory has a smaller capacity than the main memory.True		False
c)	Secondary memory has a faster access-time than
	primary memory	True	False
d)	The main draw back of the optical disk system
	is its slow average access time.	True	False
e)	CD-ROM has longer access time as compared to
	that of a magnetic hard disk.	True	False
f)	In CD-R data can be read once and write indefinitely	True	False

g)      The overall goal of Memory Hierarchy is to obtain the highest possible access speed while minimizing

	the total cost of the memory system.			True	False
h)  Magnetic tape has a higher capacity than magnetic
	disk but slow access time than magnetic disk.			True	False
2.  Match the following:
1)	CD-ROM	a)	Write once read many times
2)	CD-R	b)	Read and write indefinitely
3)	CD-RW	c)	Read only
4)	DVD-ROM	d)	Write once, read   indefinitely and having capacity up   to
	20GB      1-c, 2-a,   3-b, 4-d.

3.  Differentiate between the following:

a)      CD-ROM, CD-R and CD-RW

            CD-ROM technology uses 12-centimeter (4.7-inch) compact disk (CDs). CDROM are the typical random access storage devices. Any data record stored on a                     magnetic or optical disk can be accessed directly in approximately the same time period.

         Advantages

·         High storing capacity.

·         Mass copy of information stored, which is very cheaper.

·         Removable disk from the computer, so suitable for archival storage. 5.25 inch disks store 650 MB data.

b)      DVD-ROM versus DVD-RW

                CD-R (compact-disk recordable) is optical disk technology. The user can record (write) on it. It follows write once and read many times CD-R is suitable for                       data and files which are not to be changed. The user can store permanent data, information, and files for maintaining records.

         Advantages

     High storing capacity.

   Better reliability and long life.

      Erasable Optical disk or CD-RW (CD-rewritable)-  As the name indicates the data

    

     written on it can be erased by using a laser to heat a microscopic point on the disk‟s

     surface.

       Advantages

Very high storing capacity. A 5.25 inch optical disk can store about 650.

     It is more reliable and having long life.

4.      What is the overall purpose of the memory hierarchy? Name the general classes of storage media that might make up a memory hierarchy.

A memory system is a hierarchy of storage devices with different capacities, costs, and access times. CPU registers hold the most frequently used data. Small, fast cache memories nearby the CPU act as staging areas for a subset of the data and instructions stored in the relatively slow main memory. The main memory stages data stored on large, slow disks, which in turn often serve as staging areas for data stored on the disks or tapes of other machines connected by networks.

The overall effect is a large pool of memory that costs as much as the cheap storage near the bottom of the hierarchy, but that serves data to programs at the rate of the fast storage near the top of the hierarchy.

The overall purpose of Memory Hierarchy is to obtain the highest possible access speed while minimizing the total cost of the memory system.

                                 
 

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