Disk Drives

We have identified the need for an external storage media and made a comparison between two popular forms of storage media—the hard disk and the floppy disk. However, the microprocessor does not perform the task of writing on or reading from this media. A device specially designed to perform these functions is the disk drive.

Data is fed into the PC and written on the hard disk or the diskette by the disk drive. When the data is to be processed, it has to be read from the disk before processing actually takes place. The disk drive also performs this function of reading the data. Since the disk drive performs both the tasks of writing and reading the data, in other words, the input and output functions, it is referred to as an \nput-output device.

A simple analogy of how a disk drive works is the cassette tape recorder. It encodes music and stores it on a cassette tape as a series of signals. It also reads the stored signals on the cassette tape and converts them back to music. The cassette player records as well as plays back the music. Therefore, it performs both reading and writing. A disk drive works in much the same way, reading and writing whenever required.

The disk drive is contained within the system unit. The drive for a diskette is called a diskette drive while the drive for a hard disk is called the hard disk drive.

It is important to differentiate between the storage media and storage devices. While the diskette and the hard disk on which data is stored are the storage media, the disk drives are the devices that do the reading and writing. Going back to the analogy of the cassette tape recorder mentioned earlier, the cassette tape would be called the storage medium while the cassette player is the storage device.

External Storage

Since the internal storage capacity of PCs is limited, it places a restriction on how much data can be stored at a time for processing. However, this is not the only drawback. Once the PC is switched off, or in case of a power failure, all the data stored in the internal storage is lost. This means that every time you want to work on the PC, you would have to input the data required for processing.

For permanent storage of data, external storage media can be used with a PC system. External storage is also referred to as secondary storage.

There are two kinds of external storage media used with a PC, they are:

• Floppy Disk
• Hard Disk

Another media for external storage is the cartridge tape. It is particularly suitable for storage of large volumes of data. Now, CD-ROMs and Magneto-Optical (MO) disks have also become an important media to store large volumes of data.

Floppy Disk	Hard Disk
Also referred to as diskettes or floppies.	Also referred to as the fixed disk.
Removable. Suitable for moving data from one computer to another.	Non-removable. It is attached within the system unit.
Made of flexible vinyl material. Less resistant to damage by heat, dust and accidental twists.	Less prone to damage since it is within the system unit and is packed airtight.
The diskettes currently in use have a diameter of 3.5 inches with a storage capacity of 720KB or 1.44MB.	Can store data in the range of 20 MB to a few GB.
Diskettes having a diameter of 5.25 Inches and a typical storage capacity of 360KB is also used, but is slowly being phased out.

Comparison between a Diskette and a Hard Disk

Internal Storage

Besides the microprocessor, the system unit also contains a storage area where the data is stored before being actually processed. This storage area is called the internal storage. It is also referred to as primary storage, main memory ox Random Access Memory (RAM).

Internal storage capacities may differ in different PCs. Typically; a PC will have an internal storage capacity of 640,000 characters or more.

In computer terminology, the storage capacity of a PC is measured in terms of bytes, where one byte can store one character. Character here refers to any alphabet, number or other symbol. Therefore, to store the word COMPUTER, 8 bytes would be required. Just as there is a basic unit, gram, and another unit, kilogram to measure weight, there is also byte and kilobyte (KB) to measure storage capacity. One KB is approximately equal to 1000 bytes. Therefore, 1 KB can store approximately 1000 characters. Another common unit of measurement of storage capacity is the megabyte (MB), which is equal to approximately 1000 KB. Very large storage capacities are measured in terms of gigabytes (GB). One GB is approximately equal to 1000 MB.

The System Unit and Its Components

The system unit is a box-like unit filled with a number of useful components, each performing a discrete function. These components work together to accomplish the main function of the computer, viz. accept and process input and deliver output. This section will elaborate on these components one by one.

• Internal Power Supply
• Exhaust Fan
• Speaker
• Motherboard
• Microprocessor
• Clock chip
• Memory
• Random Access Memory (RAM)
• Read Only Memory (ROM)
• Bus
• Ports
• Representation of Characters
• Expansion Slots
• Storage System
• Floppy Disk System
• Hard Disk System
• Display Adaptors
• Enhanced Graphics Adaptor adaptors (EGA)
• Video Graphics Array adaptors (VGA)
• Super Video Graphics Array adaptors (SVGA)
• Add-on Cards

Internal Power Supply

The system unit draws power from the AC mains through a power protection device. This power is not directly supplied to the internal components. Instead, one of the components, called the internal power supply, converts the AC input into DC output of 5 and 12 volts. Normally, the internal power supply is referred to as Switched Mode Power Supply (SMPS). The SMPS provides cable connectors to supply the required voltage to the other internal components like the floppy drives, the hard disk drive, the motherboard and external device such as the keyboard. The ON/OFF switch of the system unit is actually a part of the SMPS. Thus, when you switch the system unit on or off, you actually switch the SMPS on or off. Figure shows the various components of the system unit.

Components of System Unit

Exhaust Fan

The SMPS has a small fan, called the exhaust fan, attached to it. This fan rotates as long as the computer is switched on. Its function is to cool the SMPS unit.

Speaker

The system unit also has a small audio speaker attached to it. This speaker is connected to the motherboard and produces a sound whenever instructed by software programs. For example, when the machine starts a self-test program is executed that uses beep to indicate that everything is working satisfactorily. It is also used by entertainment programs to produce sound effects.

Motherboard

When you open the system unit, a large board containing a number of tiny electronic circuits and other components are visible. This is called the motherboard. All peripheral devices are connected to the motherboard. In an IBM PC, the motherboard is called the system board. The components of the motherboard are:

• Microprocessor
• Clock chip
• Memory
• Random Access Memory (RAM)
• Read Only Memory (ROM)
• Bus
• Ports

Representation of Characters

Before proceeding with the understanding of a microprocessor chip, it is necessary to understand as to how characters are represented in the computer. The term byte refers to a single character of storage. A byte is essentially a collection of 1s and Os representing a character.

One is familiar with the ten distinct decimal digits 0 to 9 commonly used in the decimal arithmetic system. The computers being electronic machines work better with just two signals (on/off) thus giving them only two distinct digits 1 (for on) and 0 (for off) to work with.

This inherent ease of working with two digits makes the computer amenable to binary arithmetic (as opposed to the decimal arithmetic that one is familiar with).

Hence, in the binary system used by computers, each number or character is represented in terms of the binary digits 1 and 0. Each of these binary digits is called a bit. A collection of 8 bits or a byte is used to represent a character in the computer.

The number of bits used to represent a character will determine the number of unique characters that can be represented.

If only 3 bits were used, there would be 8 (2³) possible combinations of 1s and 0s. These are:

000,
001,
010,
011,
100,
101,
110,
111.

Each could represent a unique character in the computer. This would mean that 8 different characters can be represented using 3 bits. In a similar way, by using 8 bits, the PC can represent a larger set of characters (2⁸).

As to which combination of 1s and 0s will represent which number (0-9) or character (A-Z, *, /... etc.) is decided by a representation scheme used by the computer. The most widely-used representation scheme is called ASCII.

A sample of the ASCII representation for some of the characters is shown in Table.

Character	ASCII Representation
6	0011                  0110
7	0011                  0111
A	0100                  0001
B	0100                  0010

ASCII Representation

Microprocessor

The entire design of the PC is based on the microprocessor chip. The microprocessor can take in data, perform arithmetic operations like addition, subtraction, division or multiplication on the data and send out the result. It also performs comparison operations on data to check if they are equal, or if one is greater than the other. It also has the ability to perform logical operations on data. The microprocessor controls the activities of the various components of the computer and also responds to requests from the peripheral devices; for example, printer signals indicating that it has run out of paper.

The microprocessor consists of the Arithmetic-logic unit (ALU), the Control Unit and some special purpose storage areas called Registers. The ALU section is where the actual arithmetic and logic instructions are executed. During its ALU operations, the microprocessor holds its intermediate results in special purpose registers. These registers are storage areas which are physically a part of the microprocessor chip. These are not accessible to the programmer.

The control section makes available to the ALU, the requisite data that it needs to work upon. It does this by keeping track of the next instruction to be executed and the address of the data referred to by this instruction. The control also uses some of the special purpose registers of the processor to hold the next instruction and the data addresses.

There are many types of microprocessors available in the market that are manufactured by different companies like Intel, Advanced Micro Devices, Motorola, etc. The most widely-used are the range of Intel microprocessors. The microprocessors in the family of Intel processors are the Intel 8004, 8085, 8086, 8088, 80286, 80386, 80486 and the Pentium. The Pentium is the most powerful microprocessor from Intel.

The capacity of a microprocessor is measured in terms of the number of bits it can send or receive and the number of bits it can process internally. The 8088 is an 8/16 bit processor, indicating that it can send or receive 8 bits of data and internally process 16 bits of data at a time. The 80286 is a 16/16 bit processor. Thus, it is a true 16 bit processor and is faster than the Intel 8088 chip. Also, the 80286 can work in two different modes: the real and the protected. When working in the real mode, the 80286 works just like the 8088. In the protected mode, the 80286 makes available many facilities that are not available on the 8088. For example, it provides features that allow more than one program or task to be executed simultaneously. The protection feature prevents one program from tampering with any part of the memory that does not belong to it.

The 80386 chip has a 32-bit processing capability with a 32-bit data path and, as a result, is much faster than either the 8088 or the 80286 chip. Like the 80286, it can also work in the protected mode.

The 80486 chip was introduced by Intel in late 1989 and for the next four years, enjoyed the distinction of being the best performing microprocessor in the Intel family. The Intel 80486, like the 80386, is a 32-bit processor. It has architectural enhancements, which makes it perform better than the 80386 chip.

The Pentium is a 32-bit processor with a 32-bit data path, and was introduced in 1993. It is three times faster than the 80486, and is considered the best in the Intel family.

Note: DOS is a 16-bit operating system and most applications that run under it are 16-bit applications, including Windows 3.1. Windows NT however, is a 32-bit operating system that runs 32-bit applications. To maintain a degree of compatibility, older 16-bit applications are also allowed to run on the 32-bit platform. But what is required today is not just better hardware that runs applications faster, but also better software that matches the changing hardware.

Interrupts

Although the processor executes the instructions in a specified sequence, it should be able to respond to adhoc requests for its attention, such as pressing keys on the keyboard, or the printer signaling that it has run out of paper, and take the appropriate action required.

This is done through interrupts. An interrupt signal makes the microprocessor respond to such adhoc requests, even though it is busy working on something else. However, the microprocessor stores the status of the current work being processed before it diverts itself to handle the interrupt request. The microprocessor stores this status, i.e. the next instruction that was to be executed and all the intermediate results generated etc. into an area of the memory called the stack and retrieves this status to continue processing after it has completed the handling of the interrupt request.

Clock Chip

The components in a computer are designed to operate in perfect synchronization. To do this, they need a time keeper. The clock chip provides the timing signal in the form of electronic pulses that are used by the computer components to set up a working pace. The chip generates a regular beat (like the ticking of a clock) and the operations of a computer are timed to this beat. It is like the baton held by the conductor in an orchestra, the rhythmic movements of which set the pace for the individual instruments of the orchestra to follow. The faster the rhythmic movements, the faster the speed at which the orchestra plays. Thus, a computer, that operates at higher clock speeds is faster. For its timing, the clock chip uses a quartz crystal like the one used in quartz watches.

Clock Chip

Speed of Operation

The speed of a clock is measured in terms of frequency of pulses generated. The unit used for measuring this frequency is MHz (Mega Hertz). If the clock speed is 1 MHz, it means that the clock produces 1 million pulses per second. The clock speed of PCs ranges from a low of 16 MHz to a high of 200 MHz (used by the Pentium microprocessor). The clock speed is one way of measuring the speed of a computer.

Another unit for measuring the speed of a computer is MIPS (Million Instructions Per Second). It gives the number of instructions executed per second. The standard PC is rated to have a speed of 0.4 MIPS.

Memory

The internal memory of a PC is present on the Motherboard in the form of chips. The computer uses two types of internal memory:

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

Random Access Memory (RAM)

This is the memory that the computer uses for storing the programs and their data while working on them. So, if you are drafting a letter using a wordprocessor, say MS-Word, you are working with a document loaded into the RAM. To store your letter for later use, it has to be saved on the hard disk. Later, you can retrieve it from the disk and continue working on the document. Thus, RAM functions as a scratch pad for the computer and is sometimes called the scratch pad memory.

RAM has the following characteristics:

• Data within the RAM can be read or modified, i.e. you can either read from the RAM or write onto it. Hence it is called read/write memory.
• The contents of the RAM are lost when the computer is switched off. Hence, the RAM is said to be volatile.

The capacity of RAM can vary from 640 KB to 64 MB and more, depending on the number of memory chips installed, which in turn depends on the capacity the microprocessor can handle. The RAM size is an important parameter in determining the size and complexity of problems that a computer can handle. When people refer to the amount of memory that a computer has, they are talking about the amount of RAM available.

Read Only Memory (ROM)

Another portion of the internal memory found on the motherboard is called ROM. The ROM contains permanently recorded instructions that are vital for starting up a computer. One set of instructions found in ROM is called the ROM-BIOS, that stands for Read-Only Memory Basic Input-Output Services. These programs perform the most basic control and supervisory operations for the computer. For example, they check whether the I/O devices have been connected properly to the system unit. They also handle the basic needs of the hardware involved, which include all input and output devices. Any set of programs residing in ROM is called firmware.

Instructions in ROM can be executed but cannot be changed and hence the name Read-Only Memory. Further, these instructions are not erased when the power goes off. Therefore, ROM is said to be nonvolatile.

Note: In recent years, PCs have become increasingly memory-hungry, and the minimum desirable memory seems to go up every year. Now, 8 MB of memory is no longer considered large; it is the minimum required in most situations. Windows 3.1, for example, can function with 4 MB of RAM. However, it will spend a vast amount of time swapping data to disk unless you have a minimum of 8 MB of RAM.

Bus

As stated earlier, the microprocessor works on the data stored in RAM. Further, to execute the instructions stored in ROM, it has to fetch these instructions into RAM. Communication between the microprocessor and the memory chips as well as other chips found on the motherboard is accomplished through a set of wires running between them. These are called the bus. The bus comprising a set of 8 wires for carrying data is called the data bus. When data is sent from one unit to another, the address (location in memory at which the data is stored) is also sent with the data.

The bus has a set of 20 wires (in an 8088-based system) for carrying these addresses. This type of bus is called the address bus.

Ports

All input and output devices like the keyboard, the printer and the mouse are connected to the system unit through ports. These ports are actually the inlets and outlets of the microprocessor. The microprocessor communicates with the outside world through them. These ports can be either serial or parallel. To transfer a byte through a serial port, eight bits are queued and sent bit by bit. However, in a parallel port, all the eight bits are transferred simultaneously.

If all PC components were made by the same manufacturer, there would be no problems in connecting the input and output devices to the system unit. But this is not always the case, and each manufacturer follows a different standard for transferring data. Thus, you may not be able to connect a brand X monitor to a brand W computer. The solution to this problem lies in standardizing the way data is transferred.

Serial Port Standard

The industry standard for serial transfer of data is the RS-232C port. This standard defines the various parameters for data transfer, one of them being the speed of transfer. A mouse, for example, uses RS-232C port for communicating with the CPU.

Parallel Port Standard

An industry acknowledged parallel port standard is the Centronics printer interface developed by the makers of the Centronics printers. Printers normally use this standard.

Expansion Slots

Your PC comes with a limited number of serial and parallel ports for connecting input and output devices. However, you may find it necessary to attach an extra input or output device to the PC. The motherboard has a set of sockets called expansion slots, that help you attach extra input and output devices, in case you run out of ports. These slots can be used to add extra memory and additional devices.

Attaching a device to a PC is an easy task. You need to buy an appropriate add-on card or an option board. This card contains the electronic components, which interface the input and output device with the motherboard. In addition, it provides a port to which you can connect your device. Next, you open the system unit, plug the card into an empty expansion slot and attach the device to the card's port. Now the device is connected to the motherboard.

Storage System

The storage system inside a PC, be it a diskette, a tape storage or a hard disk, consists of three logical parts. First part is the medium itself, that is the diskette, the cartridge tape or the hard disk. The second part is the mechanism that operates this medium—spins the disk or spools the tape. This is called the drive. The various kinds of drive are the floppy disk drive, the tape drive and the hard disk drive. The third part is the electronics, that controls the drive through commands given by the computer system. This is called the controller. In this section, you will learn about the floppy disk and hard disk systems.

Floppy Disk System

Floppy disks come in two sizes: 5¼ inch and 3½ inch. The 5¼ inch diskette is circular in shape with a hole in the center, called the clamping hole. It is covered with a square protective jacket. The jacket has an oval slot, called the head slot, which exposes a portion of the diskette surface. There is also a small hole near the center, called the index hole, which is the starting point of the diskette.

5¼ inch Floppy Disk

The diskette is inserted into the floppy disk drive, which is a sealed enclosure. It is clamped to the drive through the hub. The drive contains a spindle mechanism for rotating the diskette. It also contains another delicate piece of hardware called the read-write head that accesses the disk surface through the head slot and retrieves or stores data. The drive uses the index hole to sense the beginning of a track. The drive controller card supplies the drive with the necessary signals for its operations. For example, the controller tells the drive when to move the read-write head, when to start rotating the disk, and so on.

A 3½ inch diskette, also called as the microdisk, is also circular in shape and is covered with a square protective jacket. The media cover is pushed to expose the diskette surface. The operations are similar to those of 5¼ inch diskettes. A 3½ inch diskette is shown in Figure.

A 3½ Inch Diskette

A diskette is divided into a series of concentric circles called tracks. The tracks are identified by numbers, with the outermost track as track zero. The tracks do not spread across the whole width of the recording surface, but take up a small area. The track is divided into sectors. Each sector is identified by a number that is assigned to it, starting with zero. Each type of floppy has a sector of a fixed size. A disk has two sides. Data can be recorded on either one or both sides of the disk.

The storage capacity of a disk is determined by factors like the number of tracks, sectors and the number of sides on which data is recorded. Depending upon the capacity of storage, the 5¼ inch diskettes are divided into double-density (360 KB) and high-density (1.2 MB).

The microdisk may be double-density (720 KB) or high-density (1.44 MB). Accordingly, disk drives can be double-density or high-density drives. A high-density drive can read and write on to a double-density diskette but the reverse is not possible.