COMPUTER SYSTEM

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CHAPTER 1:

Computer System

A computer is an electronic device that can be programmed to accept input, process it and generate result.

A computer along with hardware and software together is called a computer system.

Introduction to Computer System

Block diagram of a computer system

Input Unit

The input unit consists of input devices that are attached to the computer.

These devices take input and convert it into binary language that the computer understands. E.g : keyboard, mouse, joystick, scanner etc.

E.g. of Input Devices

• A computer system primarily comprises a central processing unit (CPU), memory, input/output devices and storage devices.

• All these components function together as a single unit to deliver the desired output.

The directed lines shown in diagram shows the flow of data and signal between the components. Central Processing Unit (CPU)

• It carries out the actual processing and usually referred as the brain of the computer.

• It is commonly called processor also. Physically, a CPU can be placed on one or more microchips called integrated circuits (IC). The Ics comprise semiconductor materials.

• The CPU is given instructions and data through programs. The CPU then fetches the program and data from the memory and performs arithmetic and logic operations as per the given instructions and stores the result back to memory.

• While processing, the CPU stores the data as well as instructions in its local memory called registers.

• Registers are part of the CPU chip and they are limited in size and number. Different registers are used for storing data, instructions or intermediate results.

CPU has two main components — Arithmetic Logic Unit (ALU) and Control Unit (CU).

ALU performs all the arithmetic and logic operations that need to be done as per the instruction in a program.

CU controls sequential instruction execution, interprets instructions and guides data flow through the computer’s memory, ALU and input or output devices. CPU is also popularly known as microprocessor

ALU

Arithmetic and Logic Unit (ALU)

• The ALU, as its name suggests performs mathematical calculations and takes logical decisions.

• Arithmetic calculations include addition, subtraction, multiplication and division.

• Logical decisions involve comparison of two data items to see which one is larger or smaller or equal (Operators used : <,><=,>=,!=)

Control unit

• The Control unit coordinates and controls the data flow in and out of CPU and also controls all the operations of ALU,

memory registers and also input/output units.

• It is also responsible for carrying out all the instructions stored in the program. It decodes the fetched instruction, understands it and sends control signals to input/output devices until the required operation is done properly by ALU and memory.

Memory

• Memory attached to the CPU is used for storage of data and instructions and is called internal memory. During processing, it is the internal memory that holds the data.

• The internal memory is divided into many storage locations, each of which can store data or instructions. The internal memory is also called the Primary memory or Main memory.

The time of access of data is independent of its location in memory, therefore this memory is also called Random Access memory (RAM). Primary memory is volatile in nature. That means when the power is switched off, the data stored in this memory is permanently erased.

That is why secondary memory is needed to store data and information permanently for later use. Some of the examples of secondary storage devices are hard disk, compact disks, pen drives etc.

Units of Memory

• A computer system uses binary numbers to store and process data. The binary digits 0 and 1, which are the basic units of memory, are called bits.

• Further, these bits are grouped together to form words.

• A 4-bit word is called a Nibble.

• Examples of nibble are 1001, 1010, 0010, etc.

• A two nibble word, i.e., 8-bit word is called a byte, for example, 01000110, 01111100, 10000001, etc.

Measurement units for digital data

Unit Description

1 Unit Description KB (Kilobyte) 1 KB = 1024 Bytes PB (petabyte) 1 PB = 1024 TB MB (Megabyte) 1 MB = 1024 KB EB (Exabyte) 1 EB = 1024 PB GB Gigabyte) 1 GB = 1024 MB ZB (Zettabyte) 1 ZB = 1024 EB TB (Terabyte) 1 TB = 1024 GB YB (Yottabyte) 1 YB = 1024 ZB

Types of Memory

Computers have two types of memory — primary and secondary.

(A)Primary Memory

Primary memory is an essential component of a computer system. Program and data are loaded into the primary memory before processing. The CPU interacts directly with the primary memory to perform read or write operation. It is of two types viz. (i) Random Access Memory (RAM) and (ii) Read Only Memory (ROM).

RAM

RAM is volatile, i.e., as long as the power is supplied to the computer, it retains the data in it. But as soon as the power supply is turned off, all the contents of RAM are wiped out. It is used to store data temporarily while the computer is working. RAM is usually referred to as main memory and it is faster than the secondary memory or storage devices.

ROM

ROM is non-volatile, which means its contents are not lost even when the power is turned off. It is used as a small but faster permanent storage for the contents which are rarely changed. For example, the startup program (boot loader) that loads the operating system into primary memory, is stored in ROM

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