3

Computer Memory and Storage

1.  List various units that help in measuring the computer memory.

Ans.: All the quantities, physical or otherwise, are measured in some units. For example, length is measured in metres and mass in grams. Likewise, for measuring the computer memory, a standard unit is required. Various units that are used to measure the computer memory are as follows:

	Bit: It is the smallest unit of data on a machine and a single bit can hold only one of two values: 0 or 1. It is represented as b.
	Byte: A unit of 8 bits is known as a byte. Hence, it is able to contain any binary number between 00000000 and 11111111. It is represented as B.
	Kilobyte: In a decimal system, kilo stands for 1000, but in a binary system, kilo refers to 1024. Therefore, a kilobyte is equal to 1024 bytes. It is usually represented as KB.
	Megabyte: It comprises 1024 kilobytes, or 1,048,576 bytes. However, since this number is hard to remember, a megabyte can be thought of as a million bytes. It is the standard unit of measurement for RAM and is represented as MB.
	Gigabyte: It consists of 1024 megabytes (1,073,741,824 bytes). It is the standard unit of measurement for hard disks and is often represented as GB.
	Terabyte: It refers to 1024 gigabytes and is often represented as TB. Terabyte memory is usually associated with supercomputers only.

2. What are the types of memories available in the computer system? How are they organized in a hierarchy?

Ans.: We know that the processor is the brain of the computer where all the essential computing takes place. But unlike a human brain, the computer processor has very limited memory. Thus, it has to rely on other kinds of memories to hold data and instructions and to store results. The memory in a computer system is of the following three fundamental types:

1. Internal processor memory: This memory is placed within the CPU (processor) or is attached to a special fast bus. It usually includes cache memory and special registers, both of which can be directly accessed by the processor. It is used for temporary storage of data and instructions on which the CPU is currently working. It is the fastest among all the memories but is the most expensive also. Therefore, a very diminutive part of the internal processor memory is used in the computer system. It is generally used to compensate for the speed gap between the primary memory and the processor.
2. Primary memory: Random access memory (RAM) and read only memory (ROM) fall under the category of the primary memory. This memory is also known as main memory. Every computer comes with a small amount of ROM, which contains the boot firmware (called BIOS— basic input/output system). This holds enough information to enable the computer to check its hardware and load its operating system into RAM at the time of system booting. RAM is the place where the computer temporarily stores its operating system, application programs, and current data so that the computer's processor can reach them quickly and easily. It is volatile in nature, that is, when the power is switched off, the data in this memory are lost. Unlike RAM, ROM is non-volatile. Even when the computer is switched off, the contents of the ROM remain available.
3. Secondary memory: This memory (also known as auxiliary memory) provides back-up storage for instructions (computer programs) and data. The most commonly used secondary storage devices are magnetic disk and magnetic tapes. These are the least expensive as well as have much larger storage capacity than primary memory. The instructions and data stored on secondary storage devices are permanent in nature. They can only be removed if the user wants it so or the device is destroyed. Secondary memory can also be used as overflow memory (also known as virtual memory), when the capacity of main memory is surpassed. It is to be noted that unlike the processor memory and main memory, secondary memory is not directly accessible to the processor. Firstly, the data and instructions from secondary memory have to be shifted to the main memory and then to the processor.

Figure 3.1 illustrates the memory hierarchy. The CPU accesses memory according to a distinct hierarchy. When the data come from permanent storage (e.g., hard disk), firstly it goes in RAM. The reason behind it is that if the CPU has to access the hard disk constantly to retrieve every piece of required data, it would operate very slowly. When the data are kept in the primary memory, the CPU can access it more quickly. Subsequently, the CPU stores the required pieces of data and instructions in the processor memory (cache and registers) to process the data.

Figure 3.1 Memory Hierarchy

3.  Write a short note on RAM.

Ans.: RAM is like the computer's scratch card. It allows the computer to store data for immediate manipulation and to keep track of what is currently being processed. It is the place in a computer where the operating system, application programs, and data in current use are kept so that they can be accessed quickly by the computer's processor. It is much faster to read from and write to than the other kinds of storage in the computer like the hard disk or floppy disk. However, the data in RAM stay there only as long as the computer is running. When the computer is turned off, RAM loses all its contents. When the computer is turned on again, the operating system and other files are once again loaded into RAM. When an application program is started, the computer loads it into RAM and does all the processing there. This allows the computer to run the application faster. Any new information that is created is kept in RAM and since RAM is volatile in nature, one needs to continuously save the new information to the hard disk. RAM is of the following two types:

1. Static RAM (SRAM): The word ‘static’ indicates that the memory retains its contents as long as power is being supplied. However, as soon as the power goes down, the data are lost. This makes SRAM a volatile memory as opposed to ROM. It does not need to be refreshed (pulse of current through all the memory cells) periodically. It is very fast but much more expensive than DRAM. It is often used as cache memory due to its high speed.
2. Dynamic RAM (DRAM): It is named so because it is very unstable. The data continue to move in and out of the memory as long as power is available. Unlike SRAM, DRAM must be continually refreshed in order to maintain the data. This is done by placing the memory on a refresh circuit that rewrites the data several hundred times per second. It is used for most systems memory because it is cheap and small.

4.  What is the difference between SRAM and DRAM?

Ans.: The primary difference between SRAM and DRAM is the life of the data they store. SRAM retains its contents as long as electrical power is supplied to the chip. If the power is turned off, its contents are lost. On the other hand, DRAM must be continuously refreshed after about every 15 μs. This is true even when power is supplied constantly. SRAM chips are not as dense as DRAM chips, that is, the total number of cells in the SRAM chip is less than that on DRAM chip. SRAM is beneficial because it is fast, has low latency (the time lag between a request and the action being performed), and does not need to be refreshed. However, it is large and expensive, requires more power to operate, and produces a lot of heat. DRAM is simple, small, and space efficient. It may be slower and may have a longer latency than SRAM, but it is still very useful. Typical access time of SRAM is 25 ns while of DRAM 60 ns.

SRAM is useful for low amount of memory. Anything over 4 MB is very bulky. SRAM is good for internal memory in processors and cache, but DRAM is best for system's main memory. DRAM is used where its small size and power efficiency outweigh its slowness as compared to SRAM. SRAM is less dense than DRAM (fewer bits per unit area) and is, therefore, not suitable for high-capacity, low-cost-per-megabyte applications. The power consumption of SRAM varies widely depending on its speed. Fast SRAM is much more power-hungry than DRAM and some integrated circuits can consume power of the order of a watt at full speed. Slow SRAM can have very low power consumption in the region of a microwatt. Currently, the technology does not exist to produce small SRAMs so that they can replace DRAMs. Thus, DRAM is still used in computers.

5. What is ROM and why it is necessary?

Ans.: The special chip where the start-up instructions is stored is called ROM. It is non-volatile in nature, that is, its contents are not lost when the power is switched off. The data and instructions stored in ROM can only be read and used but cannot be altered thereby making ROM much safer and secure than RAM. ROM chips are used not only in the computer but also in other electronic items like washing machine and microwave oven. Generally, designers program ROM chips at the time of manufacturing circuits. Burning appropriate electronic fuses to form patterns of binary information does programming.

Just as a human being needs instructions from the brain to perform actions in certain event, a computer also needs special instructions every time it is started. This is required because during the start-up operation, the main memory of the computer is empty due to its volatile property. So there has to be some instructions (special boot programs) stored in a special chip that could enable the computer system to perform start-up operations and transfer the control to the operating system.

ROM can have data and instructions written into it only one time. Once a ROM chip is programmed, it cannot be reprogrammed or rewritten. If it is erroneous, or the data need to be reorganized, one has to replace it with the new chip. Thus, the programming of ROM chips should be perfect having all the required data at the time of its manufacturing. Note that in some instances, it can be changed using certain tools. For example, flash ROM (a type of ROM) is a non-volatile memory that can be changed occasionally such as when a BIOS chip must be updated. The ROM chips can consume very little power, are extremely reliable, and in case of most small electronic devices, contain all the necessary programming to control the device.

6. Explain in brief various types of ROM.

Ans.: Memories in the ROM family are distinguished by the methods used to write data on them and the number of times they can be rewritten. This classification reflects the evolution of ROM devices from hard-wired to programmable to erasable-and-programmable. One common feature of all these devices is their ability to retain data and programs even during a power failure. Some different types of ROMs are as follows:

	Masked ROM: The very first ROM, known as masked ROM, was hard-wired device that contained a pre-programmed set of data or instructions. The contents of such ROM had to be specified before chip production so the actual data could be used to arrange the transistors inside the chip.
	Programmable ROM (PROM): Creating a ROM chip from scratch is a time-consuming and an expensive process. For this reason, developers created a type of ROM known as programmable read-only memory (PROM), which can be programmed. Blank PROM chips can be bought economically and coded by the users with the help of a special device known as PROM-programmer. However, once a PROM has been programmed, its contents can never be changed. As a result, PROM is also known as one-time programmable (OTP) device. Like other ROMs, PROM is also non-volatile. However, it is more fragile than other ROMs as a jolt of static electricity can easily cause fuses in the PROM to burn out, thus changing the bit pattern from 1 to 0. Nevertheless, blank PROMs are economical and are great for prototyping the data for a ROM before committing to the costly ROM fabrication process.
	Erasable programmable ROM (EPROM): This type of ROM is programmed in exactly the same manner as a PROM. However, unlike PROM, an EPROM can be erased and reprogrammed repeatedly. It can be erased by simply exposing the device to a strong source of ultraviolet light for a certain amount of time. Note that an EPROM eraser is not selective; it will erase the entire EPROM. Although EPROM is more expensive than PROM, its ability to be reprogrammed makes it more useful.
	Electrically erasable programmable ROM (EEPROM): This type of ROM can be erased by an electrical charge and then written by using slightly higher-than-normal voltage. EEPROM can be erased one byte at a time, rather than erasing the entire chip with ultraviolet light. Hence, the process of re-programming is flexible, but slow. Also, changing the contents does not require any additional committed equipment. As these chips can be changed without opening a casing, they are often used to store programmable instructions in devices like printers.
	Flash ROM: This type of ROM is also called flash BIOS or flash memory. It is a type of constantly powered non-volatile memory that can be erased and re-programmed in blocks. It is a variation of EEPROM, which, unlike flash memory, is erased and rewritten at the byte level. It is often used to hold the control code such as the BIOS in a personal computer. When BIOS needs to be changed or rewritten, the flash memory can be written in block (rather than byte) sizes, thus making it easier to update. Flash memory gets its name because the microchip is organized so that a section of memory cells are erased in a single action or ‘flash’. It is used in digital cellular phones, digital cameras, and other devices.

7.  With the help of diagrams, explain how RAM, ROM, and CPU interact with each other.

Ans.: One way to think of ROM is that it is similar to the hard-copy notes placed under the glass of a bulletin board. At the end of the day, they remain unchanged. The next day, the notes are exactly the way they were the day before. Students are able to only read them. RAM, on the other hand, can be thought of as a blackboard, which starts out blank and during the day, information is written on it, read from it, and even erased from it. When something is erased, new data can then be written on the same place on the board. When students go home at the end of the day, the blackboard is washed clean and whatever data was on it goes away forever. This is what happens when a computer's power is turned off; RAM no longer has the electrical current available to sustain the data in its memory cells. ROM is more like your long-term memory, the things you remember from your past. When you wake up in the morning, you get ready for school/office and know the address of your destination. Similarly, when the computer wakes up, it searches for start-up routines from ROM BIOS and then hands over the control to the operating system to function properly.

The most essential part of computer processing is the memory. From the moment the computer is turned ON and until it is shut down, the CPU constantly uses the memory. The steps involved in the interaction of RAM, ROM, and CPU are as follows (Figure 3.2):

1. The computer is switched ON.
2. CPU loads data and instructions from ROM and checks whether all the major components like processor and hard disk are functioning properly.
3. CPU loads BIOS from ROM to determine the machine's fundamental configuration and environment. The information stored in ROM BIOS chip determines what peripherals the system can support.
4. CPU loads the operating system from the secondary storage (hard disk) into RAM. This allows the CPU to have immediate access to the operating system, which enhances the performance and functionality of the overall system.
5. When an application is opened, it is loaded into RAM and any file that is opened for use in that application is also loaded into RAM.
6. After processing, when the user saves the file and closes the respective application, the file is written to the specified location on the secondary storage device. After that, the file(s) and the application are flushed out from RAM.

Figure 3.2 RAM, ROM, and CPU Interaction

Every time something is loaded or opened, it is placed into RAM so that the CPU can access that information more easily and promptly. The CPU requests the required data from RAM, processes it, and writes new data back to RAM in a continuous cycle. In most computers, this shuffling of data between the CPU and RAM happens millions of times every second. When the application is closed, the application and any other accompanying files are usually erased from RAM to make space for the new data.

8.  How can data be accessed from secondary storage devices?

Ans.: Secondary storage devices facilitate storing of data and instructions permanently. The data stored on a secondary storage device can be accessed, depending upon how it is stored on the device. Primarily, there are two methods of accessing data from the secondary storage devices. They are as follows:

1. Sequential access: It means the computer system must search the storage device from the beginning until the desired data are found. The most common sequential access storage device is the magnetic tape where data are stored and processed sequentially. Suppose, a tape contains information regarding employees of an organization. Now, to look for employee number 100’s information, the computer will have to start with employee number 1 and then go past 2, 3, and so on, until it finally comes to 100. This method is quite simple than other methods but searching for data is slow.
2. Direct access: It means that the computer can go directly to the location, where the data that the user wants are stored. This method is also called random access. The most common direct access storage devices are magnetic disk and optical disk. In these devices, the data are stored as a numbered sequence of blocks. Thus, one can access block 12, then access block 78, and then block 2. The direct access method is ideal for applications like airline reservation systems or computer-based directory assistance operations. In these cases, there is no fixed pattern of requests for data.

9.  Explain the working of a magnetic tape.

Ans.: It is a plastic tape with magnetic coating on it. The data are stored in the form of tiny segments of magnetized and demagnetized portions on the surface of the material. Magnetized portion of the surface refers to the bit value 1 whereas the demagnetized portion refers to the bit value 0. It is available in different sizes, but the major difference between different magnetic tape units is the speed at which the tape is moved past the read/write head and the tape's recording density.

It is divided into vertical columns (frames) and horizontal rows (channels or tracks), as shown in Figure 3.3. The data are stored in a string of frames with one character per frame and each frame spans multiple tracks (usually 7 or 9 tracks). Thus, a single bit is stored in each track, that is, one byte per frame. The remaining track (7th or 9th) stores the parity bit. When a byte is written to the tape, the number of 1’s in the byte is counted, the parity bit is then used to make the number of 1’s even (even parity) or odd (odd parity). When the tape is read again, the parity bit is checked to see if any bit has been lost. In case of odd parity, there must be an odd number of 1’s represented for each character and an even number of 1’s in case of even parity.

Figure 3.3 Representing Data in a Magnetic Tape

Magnetic tape drive uses two reels: supply reel and take-up reel. Both reels are mounted on the hubs and the tape moves from the supply reel to the take-up reel. Figure 3.4 shows the basic tape drive mechanism. The magnetic oxide coated side of the tape passes directly over the read/write head assembly, thus making contact with the heads. As the tape passes under the read/write head, the data can be either read and transferred to the primary memory or read from the primary memory and written onto the tape.

Figure 3.4 Basic Tape Drive Mechanism

A magnetic tape (Figure 3.5) is physically marked to indicate the location from where reading and writing on tape is to begin (BOT or beginning of tape) and end (EOT or end of tape). The length of tape between BOT and EOT is referred to as the usable recording (reading/writing) surface. BOT/EOT markers are usually made up of short silver strips of reflective type. These markers are sensed by an arrangement of lamps and/or photodiode sensors to indicate the location from where reading and writing is to begin and end. On a magnetic tape, data are recorded in form of blocks where each block consists of a grouping of data (known as records) that are written or read in a continual manner. Between these blocks, the computer automatically reserves some blank space called inter-block gap (IBG). One block may contain one or more records that are again separated by blank space (usually 0.5 inch) known as inter-record gap (IRG). In case of reading data from a moving tape, whenever an IRG is reached, the moving tape is stopped. It remains in immobile motion until the record is processed.

Figure 3.5 Information Format of a Magnetic Tape

10.  What are the advantages and disadvantages of a magnetic tape?

Ans.: Magnetic tape appears similar to the tape used in music cassettes, hence are very durable. The important advantages of magnetic tape are as follows:

	It holds high data recording density thereby resulting in low cost per bit of storage.
	It has virtually unlimited storage capacity because as many tapes as required can be used to store a large amount of data.
	It is portable because it is compact in size, lightweight, and removable. Due to these properties, it is also easy to handle and store.
	It represents a very cheap mode of offline data storage and a large amount of data can be stored in a small storage space.

But on the other hand, magnetic tape has got few disadvantages also. They are as follows:

	Since magnetic tape is sequential in nature, it is not suitable in situations where data access is required in a random order. Moreover, data transmission in magnetic tape is also slow as compared to the magnetic disk.
	It should be kept in a dust-free environment and away from corrosive gases and chemicals as it can cause tape-reading errors.
	Since it uses parity bit to check the data, the data on such devices are difficult to recover even if a minor bit error occurs.
	It is not flexible as compared to other media types when file updating requires record insertion or deletion.
	One more drawback of magnetic tape is that it wears out.

11.  Explain how data are stored on a magnetic disk.

Ans.: Magnetic disk is the widely used and popular medium for direct access secondary storage. It offers high storage capacity and reliability and has capability to access the stored data directly. It consists of a plastic/metal circular plate/platter, which is coated with magnetic oxide layer. Data are represented as magnetized spots on a disk. A magnetized spot represents 1 and the absence of a magnetized spot represents 0. To read the data, the magnetized spots on the disk are converted into electrical impulses, which are then transferred to the processor. Writing data onto the disk is accomplished by converting the electrical impulses received from the processor into magnetized spots on the disk. The data in a magnetic disk can be erased and reused virtually infinitely. It is designed to reside in a protective case or cartridge to shield it from dust and other external interference.

The surface of a disk is divided into imaginary tracks and sectors. Tracks are concentric circles where the data are stored, and are numbered from the outermost to the innermost ring, starting with zero. Sectors refer to the number of fixed-size areas (imaginary pie slices) that can be accessed by one of the disk drive's read/write heads, in one rotation of the disk, without the head having to change its position. An intersection of a track and a disk sector is known as track sector. Generally, a disk has eight or more disk sectors per track. However, different types of magnetic disk may have different number of tracks. Today disks are marked (tracks and sectors) on both surfaces, hence they are also known as double-sided disks. Each sector is uniquely assigned a disk address before a disk drive can access a piece of data. The disk address comprises sector number, track number, and surface number (if double-sided disks are used). The track sectors are grouped into a collection known as cluster. It refers to the basic allocation unit for storage on a disk, consisting of one or more track sectors. It is also the minimum amount of disk space used by a single file.

Figure 3.6 Disk Pack and Cylinder

Frequently, multiple disks (platters) are maintained and used together to create a large disk storage system. Typically, two or more platters are stacked on top of each other with a common spindle, which rotates them. There is a gap between the platters, making room for magnetic read/write head. There is a read/write head for each side of each platter and all the heads are attached to a single assembly called a disk arm assembly, which can move them towards the central spindle or towards the edge (Figure 3.6). All the read/write heads are on the equal diameter track on different platters at one time. The tracks of equal diameter on different platters form a cylinder.

12.  Illustrate the steps involved in accessing data from the magnetic disk.

Ans.: Data in a magnetic disk are recorded on the surface of the circular tracks with the help of read/write head, which is mounted on the access arm assembly. These heads can be in multiple numbers to access the adjacent tracks simultaneously and making a disk faster. The access arm assembly can be positioned in both inward and outward directions so that the read/write can move on the horizontal surfaces of the disk. In case of multiple disk packs, each disk surface has its own read/write head that works in harmony with other heads to record the data. Therefore, information is stored on the tracks, constituting a cylindrical shape through a disk pack. The process of accessing data comprises of the following three steps:

1. Seek: As soon as the disk unit receives the read/write command, the read/write heads are positioned on the specific track on the disk platter. The time taken in doing so is known as seek time. It is the average time required to move the heads to the desired track on the disk. Seek times of modern disks may range between 2 and 15 ms, but the seek time of most common disks is 9 ms.
2. Rotate: Once the heads are positioned on the desired track, the head of the specific platter is activated. Since the disk is rotated constantly, the head has to wait for the required sector or cluster (desired data) to come under it. This waiting time is known as rotational delay time or latency of the disk. The rotational latency of a disk with 7200 rpm (rotations per minute) is 4.17 ms.
3. Data transfer: Once the read/write head is positioned over the desired sector, the data can be transferred to or from the disk to the primary memory. The rate at which the data are read from or written to the disk is known as data transfer rate. It is measured in kilobits per second (KB/s). The data transfer rate depends upon the rotational speed of the disk. If the disk has a rotational speed of 6000 rpm, having 125 sectors and 512 bytes/sector, the data transfer rate per rotation will be 125 × 512 = 64000 bytes. Hence, the total transfer rate per second will be 64000 × 6000 / 60 = 6,400,000 bytes/s or 6.4 MB/s.

13.  Discuss the advantages and disadvantages of a floppy disk?

Ans.: A floppy disk is a round, flat piece of Mylar plastic coated with ferric oxide (a rust like substance containing tiny particles capable of holding a magnetic field) and encased in a protective plastic cover (disk jacket). It is a removable disk and is read and written by a floppy disk drive (FDD), which is a device that performs the basic operation on a disk, including rotating the disk and reading and writing data onto it. Traditionally, floppy disk was used on personal computers to distribute software, transfer data between computers, and create small back-ups. However, some other advantages of floppy disk are as follows:

	It is small and inexpensive.
	It is portable and can be easily carried from one place to another.
	The data on the floppy disk can easily be accessed randomly.
	It possesses the write-protect feature, which allows the users to protect a disk from being written on.

But on the other side, floppy disk has some disadvantages also. They are as follows:

	It is relatively slow.
	Due to its limited capacity, some heavy graphics files do not fit on disks.
	It can easily be destroyed due to dirt or dust.

14.  How one can write protect a 5¼ in. floppy disk?

Ans.: To write protect data onto a 5¼ floppy disk, floppy disk drive (FDD) is used. The drive is made up of a box with a slot (having a drive gate) into which user inserts the disk. When user inserts a disk into the FDD, the drive grabs the disk and spins it inside its plastic jacket. Also the drive has multiple levers that get attached to the disk. One lever opens the metal plate, or shutter, to expose the data access area. Other levers and gears move two read/write heads until they almost touch the diskette on both sides. The drive's circuit board receives instructions for reading/writing the data from/to the disk through the FDD controller. If the data are to be written onto the disk, the circuit board first verifies that no light is visible through a small window in the floppy disk. If the photosensor on the opposite side of the FDD detects a beam of light, floppy drive detects disk to be write-protected and does not allow recording of data.

15.  What is meant by floppy formatting?

Ans.: Although some blank floppy disks do come pre-formatted from the company itself, still many a times, they usually need to be formatted before one can use them. Formatting is a useful way to erase files from a floppy disk and to scan the disk for errors before it can be reused to store files on it. There are various steps that need to be followed to format a floppy disk:

	Insert the disk in the appropriate drive of the computer that needs to be formatted.
	Double-click My Computer icon on your desktop. Notice that the icon representing the 3½ Floppy (A:) drive will be highlighted. Right click on the drive icon and then click Format on the shortcut menu. A Format 3½ Floppy (A:) dialog box will appear as shown in Figure 3.7.
	From the Format options, select Quick Format checkbox if one want to simply format a disk by removing all files from it. However, if one does not select this option, then all files will be removed in addition the disk will be scanned for bad sectors also.
	Click on Start button and then OK in the message box.
	After clicking OK, one can see a progress bar that informs the user about the progress of formatting a disk.
	After format is complete, the Format complete message box appears. Then click OK.
	The floppy disk is now formatted and can be used to store new files.

Figure 3.7 Dialog Box of Format 3½ Floppy (A:)

16.  Explain the role of hard disk in a computer system. Draw the internal design of a hard disk?

Ans.: The hard disk, also called the hard drive or fixed disk, is the primary storage unit of the computer. It consists of a stack of disk platters that are made up of aluminium alloy or glass substrate coated with a magnetic material and protective layers. It is tightly sealed to prevent any dust particles, which cause head crash, from getting inside. It can be external (removable) or internal (fixed) and can hold a large amount of data. The capacity that is the amount of information that a hard disk can store is measured in bytes. A typical computer today comes with 80–320 GB of hard disk. The storage capacity of the hard disk has increased dramatically since the day it was introduced. The hard disk speed is measured in terms of access time (typically in milliseconds). A hard disk with lower access time is faster than a hard disk with higher access time; the lower the access time, the faster the hard disk. The internal design of a hard disk is shown in Figure 3.8.

Figure 3.8 Internal Design of a Hard Disk

17.  What is an optical disk? Name three forms of optical disks. What are its advantages and disadvantages?

Ans.: An optical disk is a flat, circular, plastic disk coated with material on which bits may be stored in the form of highly reflective areas and significantly less reflective areas, from which the stored data may be read when illuminated with a narrow-beam source, such as a laser diode. It is capable of storing enormously high amount of data in a limited amount of space. Its storage system consists of a rotating disk coated with a thin layer of metal (aluminium, gold, or silver) that acts as a reflective surface and a laser beam, which is used as a read/write head for recording data onto the disk.

It comes in various sizes and capacities that are made in somewhat different ways for different purposes. Compact disk (CD), digital versatile disk (DVD), and blu-ray disk (BD) are the three forms of optical disks. Each requires a different type of disk drive and disk. Even within one category, there are many competing formats. Various advantages of optical disk are as follows:

	It possesses large capacity to store data/information in the form of multimedia, graphics, and video files. It can store more data in less amount of space as compared to magnetic tapes and floppy or zip disks.
	The life span for data storage in optical disk is considered to be more, about 10–20 years as compared to magnetic disks, which have a comparatively lesser life span.
	It holds more data recording density as compared to other storage media; therefore, it has low cost per bit of storage.
	It is not affected by magnetic field.
	It is tougher than magnetic tapes or floppy disks. It is physically harder to break or melt.
	Due to its small size and lightweight, it is easily portable and stored.

But on the other hand, optical disk has got few disadvantages also. They are as follows:

	It is not easy to write as a floppy disk. One needs to use both software and hardware for writing optical disks.
	It possesses slow data access speed as compared to magnetic disks.
	The drive mechanism of optical disk is more complicated than the magnetic and floppy disks.

18.  Give the difference between CD-ROM, CD-R, and CD-RW.

Ans.: CD is the most popular and the least expensive type of optical disk. It was originally intended only for storing music (in the form of digital audio) and can record about 80 min of uninterrupted playing time. It is capable of being used as a data storage device along with storing digital audio. It is available in various formats such as CD-ROM (compact disk read only memory), CD-R (compact disk-recordable), and CD-RW (compact disk-rewritable) disks.

A CD-ROM disk comes with prerecorded data by the manufactures and can be read but cannot be altered. CD-R is a type of WORM (write once read many) disk that allows you to record your own data. Once written, the data on the CD-R can be read but cannot be altered. A CD-RW disk is the rewritable version of CD-R that means it allows writing, erasing, and rewriting of data several times. The data recorded on all CD formats can be read using the CD-ROM drive; however, to write data on CD-R and CD-RW disks, one needs a special peripheral device known as CD writer (or CD burner).

19.  What is a DVD? What is the difference between DVD and CD?

Ans.: DVD (digital video disk) is high-capacity data storage medium. At first glance, it can easily be mistaken for a CD as both are plastic disks 120 mm in diameter and 1.2 mm thick and both rely on lasers to read data. However, the DVD's seven-fold increase in data capacity over the CD has been largely achieved by tightening up the tolerances throughout the predecessor system.

In DVD, the tracks are placed closer together, thereby allowing more tracks per disk. Its track pitch (the distance between each) is reduced to 0.74 μ, less than half of CD's 1.6 μ. The pits, in which the data are stored, are also a lot smaller, thus allowing more pits per track. The minimum pit length of a single layer DVD is 0.4 μ as compared to 0.834 μ for a CD. With the number of pits having a direct bearing on capacity levels, DVD's reduced track pitch and pit size alone give DVDs four times the storage capacity of CDs. The packing of as many pits as possible onto a disk is however the simple part, but the real technological breakthrough of DVDs was with its laser. Smaller pits mean that the laser has to produce a smaller spot, and DVD achieves this by reducing the laser's wavelength from the 780 nm infrared light of a standard CD to 635 or 650 nm.

An interesting feature of DVD is that the disk's second data layer can be read from the inside of the disk out, as well as from the outside in. In standard density CDs, the information is always stored first near the hub of the disk. The same is true for single-layer and dual-layer DVD, but the second layer of each disk can contain data recorded ‘backwards’, or in a reverse spiral track. With this feature, it takes only an instant to refocus a lens from one reflective layer to another. On the other hand, a single-layer CD that stores all data in a single spiral track takes longer to relocate the optical pickup to another location or file on the same surface.

20.  Write down the steps to use a pen drive.

Ans.: A pen drive (also called flash drive) is a removable storage device that is frequently used nowadays to transfer audio, video, and data files from one computer to another. It consists of a small printed circuit board, which is fitted inside a plastic, metal, or rubber casing to protect it. The USB connector that is present at one end of the pen drive is protected by either a removable cap or pulling it back in the casing. The steps involved in using a pen drive are as follows:

1. Insert the pen drive in the USB port of the computer. The computer detects the device, configures the necessary software, and displays a drive corresponding to the pen drive in My Computer.
2. Perform any of the following actions:
   • Double-click the drive icon in My Computer to access the files stored in the pen drive.
   • Right-click the file or folder you want to send to pen drive, which displays a menu. Select Send To and select the drive corresponding to the pen drive from the submenu that appears.
3. After performing all your actions, left-click the Safely Remove Hardware icon in the system tray to display a menu containing Safely Remove USB Mass Storage Device option.
4. Select this option to display a notification icon indicating that it is now safe to remove the hardware and plug-out the pen drive.

21.  What is memory stick? Write in brief on various memory stick formats.

Ans.: Memory stick (also known as memory card) is a digital storage device, which is designed to be used with portable electronic devices such as mobile phone, digital camera, PDA and iPod. It was launched in 1998 by Sony and immediately gained popularity due to its support for fast data transfer speed and large storage capacity. Though original memory stick provided storage capacity of up to 128 MB, nowadays memory stick with storage capacity up to 32 GB is available in the market. Even the memory stick with the smallest storage capacity (4 MB) can store up to 80 images, which is much more than a standard 3.5 inch floppy disk. Memory stick can be removed from the portable device and accessed by a personal computer using memory stick-capable memory card reader. Over the years and with advancement in technology, the memory stick is getting smaller in physical size and larger in logical size. Nowadays, several different standards or formats of memory stick are available in the market. They are as follows:

	Memory stick PRO: It was introduced in the year 2003 jointly by two big companies Sony and SanDisk. It supports marginally higher data transfer speed than the original one and provides theoretical storage capacity of up to 32 GB. It provides a 4-bit parallel interface with theoretical transfer rates up to 480 MB/s. It is widely used in high megapixel digital cameras and camcorders.
	Memory stick duo: It is a small size memory stick for small, pocket-sized devices such as mobiles, music players, and digital cameras. It reduced the use of large size memory stick that had been very popular over long time. It is equipped with MagicGate technology that is used to encrypt the data stored on the card. It provides all the features of large standard memory stick and is smaller in size, but it costs more.
	Memory stick PRO duo: Though memory stick duo fulfils the need of smaller memory card for pocket-sized devices, it is superseded by memory stick PRO duo because of its slow transfer rate and limited storage capacity of 128 MB. It provides larger memory space (up to 32 GB) and high speed of data transfer to/from the card.
	Memory stick PRO-HG duo: It was introduced by Sony and SanDisk in 2006. Unlike, memory stick PRO that has parallel interface of 4-bit, memory stick PRO-HG duo has 8-bit parallel interface. In addition, the clock frequency has increased from 40 MHz to 60 MHz in the memory stick PRO-HG duo. With these improvements, it provides high transfer speed than the memory stick PRO duo.
	Memory stick micro (M2): It is a light and compact storage media that comes in the dimension of 15 mm × 12.5 mm × 1.2 mm. It has mainly been developed to meet the demands of the mobile devices market. It offers large storage capacity ranging from 16 MB to 32 GB and transfer speed of 160 MB/s.

22.  What are mass storage devices?

Ans.: In order to get vast amount of storage capacity of several bytes (trillions and more) in a computer system, a different kind of storage system is used. In such type of system, multiple units of similar kinds of storage media are associated together to form a chain of mass storage devices. These storage media may include multiple magnetic tape reels or cartridges, multiple arrays of magnetic disks, or multiple CD-ROMs as a storage device. They have relatively slow access time, generally in the order of seconds, instead of milliseconds. Due to this, their use is limited in number of applications. The slow access time of mass storage devices is because of the time taken by the transport mechanism to first move onto the particular storage media such as disks and then time taken in few milliseconds to transfer the desired data to the main memory in the computer system. However, they have huge amount of storage capacity and possess minimum cost per bit storage. Mass storage devices are categorized into the following three types:

1. Redundant array of inexpensive disks (RAID): The basic idea of RAID is to combine multiple hard disks into an array of disk drives to obtain high performance, large capacity, and reliability. These drives appear to the host computer as a single logical drive. The disk arrays can be made fault-tolerant by redundantly storing information in various ways. Seven types of array architectures, RAID 0 through RAID 6, were defined; each provides disk fault-tolerance with different compromises in features and performance.
2. Automated tape library: It comprises numerous set of magnetic tapes along with their drives and controllers mounted in a single unit. The unit comprises one or more tape drives to perform read/write operations on the tapes in the tape library. In the multiple tape drive environment, these tapes can be simultaneously read or write, thus resulting in the speedy rate of data transfer. Multiple drives lead to the reliability of the storage unit because if one of the drives fails, then the unit can continue to operate with the other tape drives. The unit with the help of a robotic arm retrieves appropriate tape from the tape library, mounting on the tape drives for the processing, and then returning to the library after the job has been finished. These tape libraries can store up to several terabytes of data in it so they can be used for archiving data for offline storage and used as a data back-up device during online storage.
3. CD-ROM jukebox: It comprises numerous sets of CD-ROM disks along with their drives and controllers mounted in a single unit. The unit comprises one or more CD-ROM drives to perform read/write operations on the CD-ROM in the jukebox. In the multiple CD-ROM drive environment, these CD-ROMs can be simultaneously read or write, resulting in the speedy rate of data transfer. Multiple drives lead to the reliability of the storage unit because if one of the drives fails, then the unit can continue to operate with other CD-ROM drives. These jukeboxes can store up to several terabytes of data in it so they can be used for archiving of read-only data for offline storage and used as a data back-up device during online storage.

23.  How many bytes will be there in a 20 GB hard disk?

Ans.: The computer mainly uses binary math, which is based on the power of 2. Similarly, memory chips are also made in quantities that are measured in the power of 2. Therefore, the number of bytes in a 20 GB hard disk is calculated as follows:

24.  How many bits are there in 1 KB?

Ans.: As we know,

Multiple-choice Questions

1.  Which of the following memory provides back-up storage for instructions and data?

(a) Internal processor memory

(b) Primary memory

(c) Secondary memory

(d) None of these

2.  The special chip where the start-up instructions are stored is called ___________.

(a) RAM

(b) ROM

(c) SRAM

(d) DRAM

3.  EEPROM stands for ___________.

(a) Electrically encoded programmable ROM

(b) Electrically erasable programmable ROM

(c) Electronically erasable programmable ROM

(d) Elective encoded programmable ROM

4.  The type of memory that can be erased by simply exposing the device to a strong source of ultraviolet light for a certain amount of time is ___________.

(a) PROM

(b) Flash memory

(c) EPROM

(d) EEPROM

5.  The secondary storage device that follows the sequential mode of access ___________.

(a) Optical disk

(b) Magnetic disk

(c) Magnetic tape

(d) None of these

6.  ___________ are concentric circles where the data are stored and are numbered from the outermost to the innermost ring.

(a) Tracks

(b) Sectors

(c) Cluster

(d) None of these

7.  In which kind of disk does the read/write head physically touches the surface?

(a) Hard disk

(b) Compact disk

(c) Floppy disk

(d) None of these

8.  Which disk consists of a stack of disk platters that are made up of aluminium alloy?

(a) Hard disk

(b) Magnetic disk

(c) Compact disk

(d) None of these

9.  DVD stands for ___________.

(a) Decoded video disk

(b) Digital versatile disk

(c) Digital virtual disk

(d) None of these

10.  RAID stands for ___________.

(a) Reproduce array of intelligent disks

(b) Reproduce array of inexpensive disks

(c) Redundant array of inexpensive drives

(d) Redundant array of inexpensive disks

Answers

1. (c)

2. (b)

3. (b)

4. (c)

5. (c)

6. (a)

7. (c)

8. (a)

9. (b)

10. (d)