UNIX Unbounded 5UNIX Unbounded 5thth Edition EditionAmir AfzalAmir Afzal

Chapter 1 Chapter 1First Things FirstFirst Things First

Chapter 1Chapter 1First Things FirstFirst Things First

This chapter briefly describes the fundamentals of computer hardware and software and explains basic computer terms and concepts. It discusses the types of software, explains the importance of the operating system, and explores its primary functions.

Computers can be grouped according to their sizes, capabilities, and speed into four classifications, as follows:

• Supercomputers

• Mainframe computers

• Minicomputers

• Microcomputers

These are rather arbitrary classifications: the low-end systems of one category can overlap the high-end systems of the other.

1.1 Computers: An Overview

Class Typical Specifications Approximate Speed

Microcomputer 64+ million main memory cells

4 billion disk storage cells single

user

10+ million instructions per second

Minicomputer 128+ million main memory cells

10 billion disk storage cells

1 tape drive

128 interactive users

30+ million instructions

per second

Mainframe 1+ billion main memory cells

100 billion disk storage cells

Multiple tape drives

100s interactive users

4+ central processing units or more

50+ million instructions

per second

Table 1.1: Computer Classifications

Figure 1-1: Four Functional Parts of a Computer SystemFigure 1-1: Four Functional Parts of a Computer System

Certain devices can be used for either input or output for example, magnetic disks and touch screen terminals.

1.3 Computer Hardware

The Central Processing Unit (CPU) consists of three basic sections:

• Arithmetic and Logic Unit (ALU)

• Registers

• Control Unit (CU)

The CPU is also called the brain, heart, or thinking part of the computer.

Computers usually have two types of main memory:

• Random access memory (RAM)

• Read only memory (ROM)

Main memory is short term and retains data only for the period that a program is running.

Processor Unit

Data Representation

Bit (Binary Digit) Each bit can hold either a 0 or a 1. A bit is the smallest unit of information a computer can understand

Byte A group of eight bits is called a byte (pronounced bite)

ASCII When you input data to a computer, the system must change it from what you recognize (letters, numbers, and symbols) into some format that the computer understands

Word Most computers are able to manipulate a group of bytes called a word. The word size is system dependent and could vary from 16 bits (2 bytes) to 32 bits (4 bytes) or even 64 bits (8 bytes)

The Memory Hierarchy

Memory Size K stands for kilobytes, which represent 1,024 bytes of storage (2 to the power of 10) For example: 32K of memory means 32,768 bytes (32 times 1,024) Other measurements referring to the size of the computer memory:

• Megabyte (MB) is approximately one million bytes• Gigabyte (GB) is approximately one billion bytes• Terabytes (TB) is approximately one trillion bytes

Storage Type Location Usage

Registers Within the CPU

Very high-speed devices Currently executing instructions;

instructions; part of the related data

Primary Storage Outside of the CPU

High-speed devices (RAM)

Entire programs or part of the part of the associated data

Secondary Storage Low-speed devices Electromagnetic or optical

Programs not currently being

executed; large amount of data

Table 1.2: Summary of the Different Storage Types

External Storage

• Secondary storage is an extension of main memory, not a replacement for it.

• A computer cannot execute a program or manipulate data stored on disk unless the data are first copied into main memory.

• Main memory holds the current programs and data, whereas secondary storage is for long-term storage.

Figure 1-4: Types of SoftwareFigure 1-4: Types of Software

WHAT IS SOFTWARE?In general, computer programs are called software. ProgramA program is a set of instructions that directs the activities of a computer system. It consists of instructions that are logically sequenced to perform a specific operation. Software Categories:

- system software - application software

Figure 1-5: User Interaction with Software LayersFigure 1-5: User Interaction with Software Layers

Who Is the Boss?

The operating system is the boss and is the most important system software component of a computer:

• It is a collection of programs that controls all hardware and software in a computer

• The necessary parts of the operating system are loaded into the main memory when you turn the computer on and remain there until you turn it off

• The operating system plays different roles as service provider, hardware manager, and facilitator of the user interface

The primary purposes and functions of an operating system:

• To provide an interface for users and application programs to low-level hardware functions

• To allocate hardware resources to users and their application programs

• To load and accept the application programs on behalf of the users

The necessary parts of the operating system are always resident in the main memory.

The operating system continually responds to the program’s resource requirements, resolves resource conflicts, and optimizes the allocation of resources

Operating System Model

Viewing the operating system as a layered set of software:

Kernel LayerThe kernel is the innermost layer of the operating system software

Service LayerThe service layer accepts service requests from the command layer, or the application programs, and translates them into detailed instructions to the kernel. The service layer provides the following types of services:

Access to I/O devices for example, the movement of data from an application to a printer or terminal

Access to storage devices for example, the movement of data from a tape drive or a magnetic disk to an application program

File manipulation for example, opening and closing files, reading from a file, and writing to a file

Other services such as window management, access to communication networks, and basic database services

File manipulation for example, opening and closing files, reading from a file, and writing to a fileOther services such as window management, access to communication networks, and basic database services

Command LayerThe command layer, also called the shell (because it is the outermost layer), provides the user interface and is the only part of the operating system with which users can interact directly

Figure 1-6: Operating System LayersFigure 1-6: Operating System Layers

Some basic concepts and terminology describing the different operating systems and their environments:

Single-Tasking A single-tasking (single-programming) operating system is designed to execute only one process at a time.

Multitasking A multitasking (multiprogramming) operating system is capable of executing more than one program at a time for a user.

Multiuser In a multiuser environment, more than one user (terminal) can use the same host computer. The multiuser operating system is complex software that provides services for all users concurrently.

Operating Systems Environment

Figure 1-7: A Multiuser Computer SystemFigure 1-7: A Multiuser Computer System

Chapter 1: First Things First

Figure 1-8: The Multiuser in a Network EnvironmentFigure 1-8: The Multiuser in a Network Environment

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