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UNIX Unbounded 5th Edition
Amir Afzal
Chapter 1
First Things First
Chapter 1
First 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.
1.1 Computers: An Overview
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.
Table 1.1: Computer Classifications
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
1.3 Computer Hardware
Figure 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.
Processor Unit
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.
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
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.
Table 1.2: Summary of the Different Storage Types
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
WHAT IS SOFTWARE?
In general, computer programs are called software.
Program
A 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 1-4: Types of Software
Figure 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 Layer
The kernel is the innermost layer of the operating system software
Service Layer
The 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 file
Other services
such as window management, access to communication networks, and basic
database services
Command Layer
The 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 Layers
Operating Systems Environment
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.
Figure 1-7: A Multiuser Computer System
Chapter 1: First Things First
Figure 1-8: The Multiuser in a Network Environment
Thank you