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Introduction to PC
UNIT 1 INTRODUCTION TO PC
Structure Page Nos.
1.0 Introduction 5 1.1 Objectives 5 1.2 Introduction to PC 6 1.3 Central Processing Unit (CPU) 10 1.3.1 Control Unit 1.3.2 Arithmetic Logic Unit 1.4 Types of Main Memory 14 1.4.1 RAM 1.4.2 ROM 1.4.3 PROM 1.4.4 EPROM 1.5 Secondary (auxiliary) Memory 19 1.6 Input/Output devices 22 1.7 Peripherals 23 1.8 Summary 26 1.9 Solutions / Answers 28 1.10 Further Reading and References 31
1.0 INTRODUCTION
Computer is the most powerful tool man has ever created. Computers are used in almost every working place in the modern times that is school, colleges, offices, railways, banks, industries, hospitals etc. The word “computer” comes from the word “compute” which means to calculate. So a computer can be considered as a calculating device that can perform arithmetic operations at a high speed but more accurately.
Computer is a digital electronic storage device, which accepts data and set of instructions, manipulates it, and retrieves the information at very high speed. In other wards we can say that a computer is a data processing device which following specific instructions, perform a sequence of operations to achieve a desired result. In this unit we will discuss about personal computer (PC) and its components. From here onward, the words PC and computer are used interchangeably.
1.1 OBJECTIVES
After going through this unit, you will be able to:
- describe the Main components of Personal Computer (PC);
- discuss the role of CPU, CU and ALU;
- describe the various types of memory such as RAM, ROM, PROM and EPROM etc.;
Hardware and Software: An
Introduction •^ define and explain the various types of I/O (Input and Output) devices; and
- define the various types of peripheral devices.
1.2 INTRODUCTION TO PC
Specifically, a computer receives input (data or/and instructions) through the input devices (such as keyboard or mouse) process the data according to the specified instructions and then presents the output (results) in a readable form on its output device, i.e. either on its screen/monitor or on paper through the printer attached to it. A simple analogy can be drawn between the sequence of operations performed by a computer and man to process data.
Suppose a student asks a teacher “what happens when 8 is multiplied by 6”. He receives the answer 40 from teacher. In the case of computer, this process can be described as follows: The teacher’s brain receives the question through his ears (analogous to a computer’s input device), processes the question with the help of his brain’s information processing and analytical ability (analogous to computer’s Central Processing Unit) and give the answer through the mouth (analogous to a computer’s output device). Further, just as the teacher can write down the answer on a sheet of paper or blackboard, the computer can also print the answer on a paper through a printer attached to it.
From the outside, a computer looks like:
System Unit Monitor Keyboard Mouse
Figure 1: Computer System (or Desktop PC)
The CPU resides inside a box known as the System unit, along with various support devices and tools for storing information. You can think of the system unit as a container for the CPU. The system unit case can either be short and wide usually underneath the screen (as in Figure-1) or it can be taller and narrow (Figure 2) and generally sits below of your desk.
CPU Container
Hardware and Software: An
Introduction 1.^ Input Device^ (i.e., Keyboard, mouse, scanner etc.)
2. Processor (or CPU):
- Control Unit (CU)
- Arithmetic-Logic Unit (ALU)
- Memory Unit
3. Output Device (Visual Display Unit (Monitor/Screen), Printer etc.)
Figure 4: Block Diagram of a Computer
The basic parts of a computer are shown in Figure 4.
- Input Devices such as Keyboard, Mouse or Scanner are used to enter input (data or/and instructions), directly into the computer.
- The CPU is like the human brain; it has a memory and just like there is a faculty in the brain that regulates the functioning of all parts of the body; it has a control-unit (CU), which controls the entire operation of a computer including its input and output devices.
- Processing of data is done in the arithmetic and logic unit (ALU). It performs Arithmetic and logic operations such as addition, subtraction, multiplication, and division. Here, the word “Logic” is used because a computer (unlike a calculator) has a capacity to do logical operations also, such as comparing two numbers etc.
The function of a computer can be described as follows:
- The input is released under the direction of control unit (CU) from the input device (say key board) to the memory unit.
- The control unit directs the requisite part of input to be sent to the ALU for being processed.
- After the processing is over, control unit directs the ALU to release the processed output to the memory.
- Then, finally, the output is released from the memory to the output device (say
Central Processing Unit
Memory (main)
Control Unit
Arithmetic-Logic Unit
Input Devices (^) Output-Devices
monitor/screen). Introduction to PC
Thus, a computer does mainly the following four functions:
- Receiving Input : Accept information from outside through various input devices such as Keyboard, Mouse, etc.
- Process Information : Perform arithmetic and logical operations on the information
- Produce output : Communicate information to the outside world through output devices like monitor, printer etc.
- Store information: Store the information in storage devices like, hard disk, pen drive etc.
These four functions are responsible for everything that computers do. Components of a computer system are given in Figure 5.
(Auxiliary Storage Device-Floppy/Pen drive/CD-ROM)
(Input Device) (Output Device)
(Primary Memory)
Figure 5: Components of a Computer System
☞ Check Your Progress 1
- State True/False against each of the following statements:
a) The CPU resides inside a box known as the System unit.
b) Every modern computer system consists of three basic sections-input devices, Processor (CPU) and Output devices.
c) Hardware in the mechanical and electrical devices, which we can see and touch.
d) ALU stands for Automatic Logic Unit.
e) Peripheral devices allow people to interact with the CPU.
f) Primary memory is the part of the computer that executes
program instructions.
CPU
function as directed by the brain. Similarly, in order to work a computer, also need some Introduction to PC sort of “brain” or “calculation capacity”. In a computer system, there is a device roughly the size of a large postage stamp, known as the central processing unit (CPU). In a computer system, all major calculations and comparisons are made inside the CPU. It is also responsible for activating and controlling the operations of other units of a computer system.
The CPU resides inside a box known as the system unit, along with various support devices and tools for storing information. Just think of the system unit as a container for the CPU. The system unit case, that is the metal case itself, usually sits on top of your desk, often underneath the screen (as shown in Figure-1), or it can be taller and narrow, in which case it generally sits underneath your desk and is referred to as a tower case (as shown in Figure-2).
CPU reads and executes program instructions, performs calculations, and makes decisions. The CPU is mainly responsible for storing and retrieving information on disks and other media. It also handles information from one part of the computer to another and directs the flow of traffic throughout the computer system. Below you can see meaning of each word of a Control Processing Unit.
Central Processin g
Unit
It is a Unit because it is a chip, which consists of transistors It is called a processor because it does arithmetic and logical operation
It is central because it controls all activities of a system.
A block diagram of a modern computer is shown in Figure 6:
Figure 6: A Block Diagram of a Basic Computer Organization.
Basically the Control Unit (CU) and the Arithmetic-Logic-Unit (ALU) of a computer system are jointly known as the Central Processing Unit (CPU). Logically we can say:
CPU=CU+ALU
Input Devices
Control Unit Arithmetic & Logic Unit
Primary Storage
Output
Devices
Secondary Storage Communication
Devices
Hardware and Software: An Introduction
Thus the CPU has mainly two parts:
- Control Unit (CU) ; and
- Arithmetic-Logic Unit (ALU).
The Control Unit (CU) controls the entire operation of a computer. It is responsible of carrying out program instructions and telling the rest of the computer system what to do. It manages and coordinates the entire computer system.
It obtains instructions from the program stored in the main memory, interprets the instructions, and issues signals that cause other units of the system to execute them. Although, it does not perform any actual processing on the data, the control unit acts as a central nervous system for the other components of the computer. It manages and coordinates the entire computer system.
The Arithmetic-Logic-Unit (ALU) of a computer system is the place where the actual execution of the program instructions takes place during the processing operation.
The internal architectural design of computers differs from one system to another. However, the basic organization remains the same for all computer system. A block diagram of the basic computer organization is shown in Figure 6. Now let us look at each part of CPU in detail.
1.3.1 Control Unit (CU)
How does the input device know that it is the time for it to feed data into the storage unit (or memory)? How does the ALU know what should be done with the data once those are received?
And how the final results are sent to output device (monitor/screen or printer etc.), not the intermediate result? All this is possible because of the control unit (CU) of the computer system. By selecting, interpreting and seeing to the execution of the program instructions, the control unit is able to maintain order and direct the operation of the entire system. The control unit (CU) basically does the following things:
- It manages and coordinates the entire computer system.
- The CU is responsible of carrying out program instructions and telling the rest of the computer system what to do.
- It obtains the instructions from the program stored in the main memory, interprets the instructions and issues signals that cause other units of the system to execute them.
- The Instruction Register contains a current instruction once it has been fetched from the primary memory. The control unit uses the instruction contained in the Instruction Register to decide which circuits need to be activated (see Figure 5). Program counter contains the address of the next instruction to be fetched for execution (Figure 5).
Hardware and Software: An Introduction •^ No processing is done in the primary storage unit (or memory).
- Intermediate results generated in the ALU are temporarily transferred back to the primary storage unit until needed at later time.
- Data may thus move from primary storage to ALU and back again to storage many times before the processing is over.
- After the completion of processing, the final results, which are stored in the storage unit, are released to an output device (monitor/screen or printer).
- The accumulator is used to accumulate results. It is the place where the answers of many operation are stored temporarily before being sent to the computer's memory (see Figure 6). Only the final result is transmitted to the memory unit for storage.
To understand the operation of CU, ALU etc., let us execute the statement (or command) R=n (^) 1+n 2. This statement adds two numbers n 1 and n 2 and places the result in a location X (refer the Figure 8).
Let R = n1+n
ADD INSTRUCTION
NUMBER 1
NUMBER 2
RESULT
Numbers 1(n1)
CENTRAL PROCESSING UNIT
CONTROL UNIT
MEMORY UNIT
ARITHMETIC AND LOGIC UNIT (ALU)
Memory Locations Numbers 2(n2) Result R
(ACCUMULATOR)
Step 3 The control unit signals the ALU perform “addition operation” on the two numbers and to store the result in the Step 2^ accumulator The two numbers are brought from memory and placed in the ALU
Step 1 Move instruction to the control unit
Step 4 Send result in memory
Figure 8: CPU Operation to find data and instructions (Adding two numbers)
Introduction to PC
1.4 TYPES OF MAIN MEMORY
The data and instructions which are entered into the computer system through input devices (keyboard, mouse etc.) have to be stored inside the computer before the actual processing starts. Similarly, after processing, the results produced by the computer must be kept somewhere inside the computer system before being passed onto the output devices (monitor/screen or printer). Thus, the storage unit (primary/main memory) of a computer system is designed to take care of all these needs.
The memory unit is the electronic device that holds data and instructions for processing. It consists of binary cells (i.e., 0 or 1 signals) to store the information. Thus,
- the memory unit that directly communicate with the CPU is called the primary storage or primary memory or main storage or main memory.
- it basically stores the necessary programs of system software, which are required to execute the user’s program.
- when we load software from floppy disk, pen drive, hard disk or CD-ROM, it is stored in the main memory.
- there are two types of computer (main) memory inside the computer (as shown in Figure 9): 1. RAM (Random Access Memory) 2. ROM (Read Only Memory)
Figure 9: Main Memory : ROM (Non-volatile) and RAM (Volatile)
1.4.1 Random Access Memory (RAM)
- Random Access Memory (RAM) is really the main store and is the place where the program and software we load gets stored. When CPU runs a program, it
Introduction to PC
Figure 11: Relationship between the CPU and main memory (RAM and ROM).
1.4.3 Programmable Read Only Memory (PROM)
- 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.
1.4.4 Erasable Programmable Read Only Memory (EPROM)
- Once the information is stored in a ROM chip or a PROM chip, it cannot be changed. However, there is another type of memory chip called erasable programmable read-only memory (EPROM) that overcomes this problem.
- As the name implies, it is possible to erase information stored in an EPROM chip and the chip can be reprogrammed to store new information by using a special device called a PROM programmer or a PROM burner.
- The information stored in an EPROM chip is erased by exposing the chip for some time to ultraviolet light.
- When an EPROM chip is in use, information can only be READ and the information remains on the chip until it is erased.
- An EPROM differs from a PROM in that a PROM can be written to only once and can not be erased.
- EPROMs are widely used in personal computer they enable the manufacturer to
CPU
RAM
ROM
Hardware and Software: An Introduction change the contents of the PROM before the computer is actually delivered. This means that bugs can be removed and new versions installed shortly before delivery.
☞ Check Your Progress 2
- State True/False against each of the following statements:
a) Computer can communicate with other computers and can receive and send data in various forms like text, sound, video, graphics etc.
b) A digital computer operates on Hexadecimal digits 0,1,….9,A,B,…,F.
c) The Control Unit communicates with both ALU and Main memory.
d) The Arithmetic & Logic unit (ALU) controls the entire operation of a computer. It
manages and coordinates the entire computer system.
e) The process of reading an instruction is often referred to as the fetch-
execute process.
f) ALU is not a part of the CPU.
g) ALU can do only arithmetic & logical operations.
h) ALU can perform performs logical tests only for integers operands only.
i) The accumulator is the place where the answers from many
operations are stored temporarily before being put out to the computer's memory.
j) Primary memory is directly communicated with the CPU.
k) When we load software from floppy disk, hard disk or CD-ROM, it is stored in the main memory.
l) RAM & ROM are the two types of computer main memory inside of a computer.
m) Non-Volatile memory means the stored data are lost, if the power goes off.
- Multiple Choices Questions
i) The different types of memory units are:
a) RAM
b) ROM
c) PROM
d) All of the above
Hardware and Software: An Introduction …………………………………………………………….. ……………………………………..
1.5 SECONDARY (AUXILIARY) MEMORY
DEVICES
External Storage/Auxiliary Storage (i.e. Hard Disk, Floppy Disk, CD-ROM, DVD, Zip-Drive, magnetic tape) – provides a place where data can be permanently stored and transferred to and from main memory (RAM).
Secondary memory is also known as auxiliary memory or secondary storage. This is a Non-volatile type memory; it means the stored data and programs do not get lost, even after the computer is switched off. Unlike RAM which looses the contents when the computer is turned off, and ROM, to which it is not possible to add anything new, the auxiliary storage devices allows the computer to record information semi-permanently, so it can be used later by the same computer or another computer. Auxiliary storage devices are also useful in transferring data and programs from one computer to another computer. They can also be used as a back-up storage, which allows back up the valuable information that you are working on. So even if by some accident if your computer crashes and the data in it is unrecoverable, you can restore it from back-ups.
The most important auxiliary storage devices are Hard Disk, Floppy Disk, CD-ROMs, DVD, magnetic tapes, magnetic disks etc.
Portable. Usually the A drive on your computer.
Portable disks means that you can carry with you or can be remove them from disk drive A.
A standard floppy disk can hold 1.44 MB of data.
Slower to access then hard-disks and has less storage capacity, but they are less expensive and portable.
Non-Portable. The hard drive is the primary device that a computer uses
to store information.
In most of the computers partitioning of hard drive is done and each portion is given a separate drive name. For example a 80GB Hard Disk may be partitioned into four drives C, D, E and G each of 20 GB.
Most computers have one hard drive located inside the computer case. If a computer has one hard drive, it is called drive C. If a computer has additional hard drives, they are called drives D, E, F, and so on.
The hard drive stores the programs on your computer. When you buy a new program, you must first install in onto your hard drive before you can
run it. The hard drive is also called the hard disk, hard disk drive or fixed Introduction to PC disk drive.
The size of a Hard Drive is measured in Gigabytes. A 20+ GB hard drive is common for new computers, and the larger the hard drive, the more programs you can store on your computer.
A Hard Disk/Hard Drive is shown in Figure 12.
Figure 12: Hard Disk
CD-ROM stands for Compact Disk Read Only Memory. A CD-ROM drive reads information from a compact disk. CD-ROMs are high capacity storage, and like, floppy disks are portable (removable). You cannot write information to a CD-ROM (because it is read only), you can only retrieve information. However, CD-RW (CD Rewritable) drives are becoming more common, these drives allow you to record data such as music files etc. onto CD-RW disks. You can change the data on CD-RW disks.
When you type a letter in MS Word, as you type it is being stored in RAM, when you save the file (either to your floppy disk or your hard disk) you are saving to auxiliary storage. The next time you open your letter, you are retrieving it from the auxiliary storage into memory. Auxiliary storage is considered both an input and an output device.
Magnetic tape is a magnetically coated strip of plastic on which data can be encoded. Tapes for computers are very similar to the tapes used to store the music. Storing data on tapes is considerably cheaper than storing data on disks. Tapes also have larger storage capacities, ranging from a few hundred kilobytes to gigabytes. Magnetic tapes are used for taking backup of the large date from computer system.
The table on next page summarizes the different types of secondary (auxiliary) memory devices:
R/W Head
Platter
Introduction to PC
DVD-ROM DVD±R DVD±RW
4.7GB to 8.5GB
Large storage capacity
- Some types (DVD±RW) can be reused or rewritable.
- Can be used in certain models of DVD player.
- Not all computers can read DVD±R or DVD±RW disks.
- DVD±R discs are ‘write once’, which means once data is copied to it, new or additional data cannot be added
Same as CD-ROM/ CD-R/CD- RW
Optical
Hardware and Software: An Introduction
1.6 INPUT-OUTPUT DEVICES
In this section you will get introduced with the various types of I/O devices available for computer systems. The topics discussed here briefly will be taken up in detail in the Unit 3 of this block.
The input-output (also known as I/O) devices provide the means of communication between the computer and the outside world. They are also known as peripheral devices because they surround the CPU.
Without any input devices a computer would simply be a display device and not allow users to interact with it, much like a TV.
When using a computer, you must send commands to the computer, and data to be processed. Also, data must be returned from the computer to the user. In order to do this, the computer makes use of input and output devices.
- Input devices (such as keyboard, mouse, joystick etc.) are used to enter data into primary storage (or primary memory) or a hardware device that sends information into the CPU. The Table 3 highlights the categories of which the input devices.
Table 3: Categories of Input Devices
Category Device Name Keying device Keyboard Punch card reader Pointing Devices Mouse Joystick Touch Screen Light Pen Pen based computing Speech recognizers Optical character Recognizer
Optical scanner Bar Code Reader Magnetic Ink Character Recognition (MICR) Other devices Digital Camera Web cam Smart Cards Telephone
Output Devices
The output generated by a computer can be transmitted to the user via some devices or media. These devices are called output devices. The following are some examples of
