# Study On The Basic Computer System Computer Science Essay

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Logic gates process signals which represent true or false. Normally the positive supply voltage +Vs represents true and 0V represents false. Other terms which are used for the true and false states are shown in the table on the right. It is best to be familiar with them all. Gates are identified by their function such as OR, AND, NOT, NAND, NOR, XOR and XNOR.

## AND gate

The output Q is true if input A AND input B are both true: QÂ =Â AÂ ANDÂ B

An AND gate can have two or more inputs, its output is true if all inputs are true.

IEC Symbol

## &

Input AInput BOutput Q000010100111Truth Table

## OR gate

IEC Symbol

> 1

Input AInput BOutput Q000011101111Truth Table

The output Q is true if input A OR input B is true (or both of them are true): QÂ =Â AÂ ORÂ B An OR gate can have two or more inputs, its output is true if at least one input is true.

## NOT gate (inverter)

IEC Symbol

= 1

Input AOutput Q0110Truth TableThe output Q is true when the input A is NOT true, the output is the inverse of the input: QÂ =Â NOTÂ A A NOT gate can only have one input. A NOT gate is also called an inverter.

IEC Symbol

## &

Input AInput BOutput Q001011101110Truth Table

This is an AND gate with the output inverted, as shown by the 'o' on the output.

The output is true if input A AND input B are NOT both true: QÂ =Â NOTÂ (AÂ ANDÂ B)

A NAND gate can have two or more inputs, its output is true if NOT all inputs are true.

## NOR gate (NOR = Not OR)

This is an OR gate with the output inverted, as shown by the 'o' on the output.

The output Q is true if NOT inputs A OR B are true: QÂ =Â NOTÂ (AÂ ORÂ B)

A NOR gate can have two or more inputs, its output is true if no inputs are true.

## EX-OR (EXclusive-OR) gate

The output Q is true if either input A is true OR input B is true, but not when both of them are true: QÂ =Â (AÂ ANDÂ NOTÂ B)Â ORÂ (BÂ ANDÂ NOTÂ A)

This is like an OR gate but excluding both inputs being true.

The output is true if inputs A and B are DIFFERENT.

EX-OR gates can only have 2 inputs.

IEC Symbol

= 1

Input AInput BOutput Q000011101110Truth Table

## EX-NOR (EXclusive-NOR) gate

This is an EX-OR gate with the output inverted, as shown by the 'o' on the output.

The output Q is true if inputs A and B are the SAME (both true or both false): QÂ =Â (AÂ ANDÂ B)Â ORÂ (NOTÂ AÂ ANDÂ NOTÂ B) EX-NOR gates can only have 2 inputs.

IEC Symbol

= 1

Input AInput BOutput Q000011101110Truth Table

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## Digital Logic States

A computer is made up of many digital circuits that pass information between components in the form of digital signals. These signals may represent either instructions or data to be processed. A digital signal can be considered a logic variable that has only one of two possible values at any moment in time. These values are called logic states.

The binary (Base 2) number system is often used to represent the states of a group of logic variables at specific instant in time. Each digit of a binary number is called a bit.

Only two possible values can exist for each binary digit, either "1" or "0". Note that it takes four bits to represent the decimal numbers 8 and 9. A thorough discussion of the binary number system, including conversion methods, is discussed in the next section.

The study of number systems is useful to compute due to the fact that number systems other than the familiar decimal (base 10) number system are used in the computer field.

Digital computers internally use the binary (base 2) number system to represent data and perform arithmetic calculations. The binary number system is very efficient for computers, but not for humans. Representing even relatively small numbers with the binary system requires working with long strings of ones and zeroes.

The hexadecimal (base 16) number system (often called "hex" for short) provides us with a shorthand method of working with binary numbers. One digit in hex corresponds to four binary digits (bits), so the internal representation of one byte can be represented either by eight binary digits or two hexadecimal digits.

Less commonly used is the octal (base 8) number system, where one digit in octal corresponds to three binary digits (bits).

## The Binary Number System

Computers store everything, both instructions and data, by using many, many transistors, each of which can be in one of two states: off or on. We represent the former transistor state with a 0 and the latter state with a 1. The number system that uses only these two digits is called binary (base 2) and each binary digit (0 or 1) is called a bit. Every instruction we give a computer and every number, character, and object is ultimately represented in binary notation.

## Octal Number System

TheÂ octalÂ numeral system, orÂ octÂ for short, is theÂ base-8 number system, and uses the digits 0 to 7. Numerals can be made fromÂ binaryÂ numerals by grouping consecutive binary digits into groups of three (starting from the right). For example, the binary representation for decimal 74 is 1001010, which can be grouped into (00)1 001 010 - so the octal representation is 112.

In decimal systems each decimal place is a base of 10. For example:

In octal numerals each place is a power with base 8. For example:

By performing the calculation above in the familiar decimal system we see why 112 in octal is equal to 64+8+2 = 74 in decimal.

The problem with the binary number system is that relatively small numbers use a large number of bits, which human beings have trouble reading. For example,

819210 = 10000000000000 6= 1000000000000 = 409610, but it is hard for us to tell that by just looking. The hexadecimal number system (base 16) has two things going for it:

1. Numbers represented in hexadecimal are far easier for humans to compare (and write) than

are binary numbers.

2. The conversion between binary and hexadecimal is very easy, much easier than between

binary and decimal.

Therefore, hexadecimal is often used as a shorthand for binary strings. Since hexadecimal is

base 16, we need 16 digits. The hexadecimal digits are the decimal digits, plus the digits A, B, C, D, E, and F which denote the values 10, 11, 12, 13, 14, and 15, respectively.

## Input Device

Any devices that can provide an input of information, data or commands into the computer system.

In the case of a primitive and simple computer system the most common input devices are the keyboard and the mouse. There are some other input devices we can use to input data such as scanners, Barcode reader, Optical mark reader, Optical character recognition, Magnetic ink character recognition, Sensors, Digital camera, speech recognizer, Joy stick, Tracker ball, Touch pad, Digitizer, Light pen, Touch screen, microphone, video input, etc.

## Output devices

The computer uses output devices to output information to the user. That is the computer uses output devices to transfer information to the environment and the user. There are two types of output such as softcopy and hardcopy. Output devices are Monitor, Printer, Plotter, Speaker, Multimedia Projector, Headset, etc.

## Hardcopy

Eg:-

Monitor

Speaker

Multimedia projector

Eg:-

Printer

Plotter

## Performance Metrics of Main Memory:

Latency: Affects cache miss penalty

Access Time: time between the request and when the desired word arrives

Cycle Time: time between requests

Bandwidth: Affects I/O performance & cache miss penalty (especially when a large block is used in the L2 cache)

## Main Memory uses DRAM (dynamic RAM)

Dynamic because of the need to be refreshed periodically (but requires only 1 transistor/bit)

Addresses are divided into 2 parts:

RAS or Row Access Strobe

CAS or Column Access Strobe

## Cache uses SRAM (static RAM)

No refresh (but requires 6 transistors/bit)

Address not divided for fast access

## Comparison

Capacity: DRAM is 4-8 times that of SRAM

Cycle time: SRAM is 8-16 times faster than SRAM

Cost: SRAM is 8-16 times more expensive than SRAM

## Key features

RAM is packaged as a chip

Basic storage unit is a cell (one bit per cell)

Multiple RAM chips form a memory

## Static RAM (SRAM)

Each cell stores bit with a six-transistor circuit

Retains value indefinitely, as long as it is kept powered

Relatively insensitive to disturbances such as electrical noise

Faster and more expensive than DRAM

## Cache Memory

The memory is about eight times slower than the speed of the modern day processors. Therefore if the processor had to read data from the main memory all the time, it would dramatically reduce the performance of the processor. To avoid this problem, computer manufacturers have developed the cache memory. The cache memory is manufactured using SRAM technology and functions at speeds compatible with the processor. The cache has only a small storage capacity relative to the main memory. Cache comes as Level 1 (L1), Level 2 (L2) and Level 3 (L3). Previously only L1 cache memory was on the processor die (the die is a part of the silicon wafer the processor is manufactured from) and the L2 cache was external. Hence L1 was referred to as the internal cache and the L2 cache was referred to as the external cache. In modern day computers the L2 cache is also embedded on the processor die. The L3 cache is normally found on high end machine such as servers.

## Central Processing Unit

The Central Processing Unit (CPU) performs the actual processing of data. The data it processes is obtained, via the system bus, from the main memory. Results from the CPU are then sent back to main memory via the system bus. In addition to computation the CPU controls and co-ordinates the operation of the other major components. The CPU has two main components, namely:

The Control UnitÂ -- controls the fetching of instructions from the main memory and the subsequent execution of these instructions. Among other tasks carried out are the control of input and output devices and the passing of data to the Arithmetic/Logical Unit for computation.

The Arithmetic/Logical Unit (ALU)Â -- carries out arithmetic operations on integer (whole number) and real (with a decimal point) operands. It can also perform simple logical tests for equality and greater than and less than between operands.

It is worth noting here that the only operations that the CPU can carry out are simple arithmetic operations, comparisons between the result of a calculation and other values, and the selection of the next instruction for processing. All the rest of the apparently limitless things a computer can do are built on this very primitive base by programming!

Modern CPUs are very fast. At the time of writing, the CPU of a typical PC is

## -Some fundamental and enduring properties of hardware and software:

Fast storage technologies cost more per byte and have less capacity

Gap between CPU and main memory speed is widening

Well-written programs tend to exhibit good locality

## Introduction to How Operating Systems Work

When you turn on your computer, it's nice to think that you're in control. There's the trusty computer mouse, which you can move anywhere on the screen, summoning up your music library or Internet browser at the slightest whim. Although it's easy to feel like a director in front of your desktop or laptop, there's a lot going on inside, and the real man behind the curtain handling the necessary tasks is the operating system.

Â­Most desktop or laptop PCs come pre-loaded with Microsoft Windows. Macintosh computers come pre-loaded with Mac OS X. Many corporate servers use the Linux or UNIX operating systems. The operating system (OS) is the first thing loaded onto the computer -- without the operating system, a computer is useless.

Â­Â­Â­More recently, operating systems have started to pop up in smaller computers as well. If you like to tinker with electronic devices, you're probably pleased that operating systems can now be found on many of the devices we use every day, from cell phones to wireless access points. The computers used in these little devices have gotten so powerful that they can now actually run an operating system and applications. The computer in a typical modern cell phone is now more powerful than a desktop computer from 20 years ago, so this progression makes sense and is a natural development.

The purpose of an operating system is to organize and control hardware and software so that the device it lives in behaves in a flexible but predictable way. In this article, we'll tell you what a piece of software must do to be called an operating system, show you how the operating system in your desktop computer works and give you some examples of how to take control of the other operating systems around you.

An operating system creates the ability to:

serve a variety of purposes

interact with users in more complicated ways

keep up with needs that change over time

All desktop computers have operating systems. The most common are the Windows family of operating systems developed by Microsoft, the Macintosh operating systems developed by Apple and the UNIX family of operating systems (which have been developed by a whole history of individuals, corporations and collaborators). There are hundreds of other operating systems available for special-purpose applications, including specializations for mainframes, robotics, manufacturing, real-time control systems and so on.

In any device that has an operating system, there's usually a way to make changes to how the device works. This is far from a happy accident; one of the reasons operating systems are made out of portable code rather than permanent physical circuits is so that they can be changed or modified without having to scrap the whole device.

## Types of Operating Systems

Within the broad family of operating systems, there are generally four types, categorized based on the types of computers they control and the sort of applications they support. The categories are:

Real-time operating system (RTOS) - Real-time operating systems are used to control machinery, scientific instruments and industrial systems. An RTOS typically has very little user-interface capability, and no end-user utilities, since the system will be a "sealed box" when delivered for use. A very important part of an RTOS is managing the resources of the computer so that a particular operation executes in precisely the same amount of time, every time it occurs. In a complex machine, having a part move more quickly just because system resources are available may be just as catastrophic as having it not move at all because the system is busy.

Single-user, single task - As the name implies, this operating system is designed to manage the computer so that one user can effectively do one thing at a time. The Palm OS for Palm handheld computers is a good example of a modern single-user, single-task operating system.

Single-user, multi-tasking - This is the type of operating system most people use on their desktop and laptop computers today. Microsoft's Windows and Apple's MacOS platforms are both examples of operating systems that will let a single user have several programs in operation at the same time. For example, it's entirely possible for a Windows user to be writing a note in a word processor while downloading a file from the Internet while printing the text of an e-mail message.

Multi-user - A multi-user operating system allows many different users to take advantage of the computer's resources simultaneously. The operating system must make sure that the requirements of the various users are balanced, and that each of the programs they are using has sufficient and separate resources so that a problem with one user doesn't affect the entire community of users. Unix, VMS and mainframe operating systems, such as MVS, are examples of multi-user operating systems.

It's important to differentiate between multi-user operating systems and single-user operating systems that support networking. Windows 2000 and Novell Netware can each support hundreds or thousands of networked users, but the operating systems themselves aren't true multi-user operating systems. The system administrator is the only "user" for Windows 2000 or Netware. The network support and all of the remote user logins the network enables are, in the overall plan of the operating system, a program being run by the administrative user.

The operating system's tasks, in the most general sense, fall into six categories:

Processor management

Memory management

Device management

Storage management

Application interface

User interface

## Operating system software and Graphical user Interface

The operating systems controls the computer, and without it you could not make use of the computer. The OS is software that runs between the hardware and applications software. It enables applications software to use the computer's hardware and other resources. The OS has many functions, which include

Input and Output control

Error Handling

Resource allocation

Providing a user interface

Allowing users to give commands to the computer

File Handling

Providing a User Interface

The OS enables you to give commands to the computer. You could give the computer commands in different ways.

Type in commands on a command line

Select an option from a menu-driven system

Use a graphical User Interface (GUI), and navigate and select using a mouse.

Command Line user Interface

It can be difficult to the user because:

User has to know the exact instruction to type in

Need to type in exactly the right instruction otherwise the OS will reject it

To make full use of the OS. You need to know all the instructions available.

IF you don't know what instruction to use, you have to look it up in the documentation and this can be very time-consuming.