5:22 AM
CR Zaman
Topics covered:
What is an Operating System?
An operating system is a program that manages the computer hardware.
An operating system may be viewed as an organized collection of software extension of hardware, consisting of control routines for operating a computer and for providing an environment for execution of program.
An operating system is an important part of almost every computer system.
A computer system can be divided roughly into four components:
The hardware,
The operating system,
The application programs, and
The users
Operating system performs two basically unrelated functions
Extending the machine and
Managing resources
What is UNIX?
UNIX is the multiuser multitasking Operating system. UNIX is an operating system which was first developed in the 1960s, and has been under constant development ever since.
By operating system, we mean the suite of programs which make the computer work. It is a stable, multiuser, multitasking system for servers, desktops and laptops.
UNIX systems also have a graphical user interface (GUI) similar to Microsoft Windows, which provides an easy to use environment.
However, knowledge of UNIX is required for operations, which aren't covered by a graphical program, or for when there are no windows interface available, for example, in a telnet session.
Hardware Requirement
Before beginning installation, you must verify that your computer meets the resource requirements.
The following resources are recommended for a UNIX operating system.
A server running a supported UNIX operating system
At least 400 MB free hard drive space in the installation location, depending upon the installation options selected
At least 4 GB RAM
One CD-ROM drive unless installing from downloaded files
Why UNIX?
One of the biggest reasons for using UNIX is networking capability.
With other operating systems, additional software must be purchased for networking.
With UNIX, networking capability is simply part of the operating system.
UNIX is ideal for such things as worldwide email and connecting to the Internet.
UNIX was written in a machine independent language. So UNIX and UNIX-like operating systems can run on a variety of hardware.
These systems are available from many different sources, some of them at no cost.
Because of this diversity and the ability to utilize the same "user-interface" on many different systems, UNIX is said to be an open system.
Salient Feature
The UNIX OS offers several salient features, the important of which are discussed below:
Multiuser capability
This means that inherent to UNIX is the idea that there are different users of the system, and that different users may have different sorts of privileges and types of access to different parts of the system. It allows for the idea that some users may want to protect some of their data from being accessed by other users on the system.
UNIX uses a system of login names to identify users and passwords to authenticate that a user is, in fact, who she/he claims to be.
Communication
UNIX has excellent provision for communicating with fellow users. The communication may be within the network for a single main computer or between two or more such computer networks.
The user can easily exchange mail, data, and programs through such networks.
Security
UNIX allow sharing of data but not indiscriminately. UNIX has three inherent provision for protecting data.
The first is provided by assigning passwords and login names to individual users ensuring that nobody can come and have access to your work.
At file level there is a read, write and execution permission to each file which decide who can access particular file.
Lastly, there is a file encryption. This utility encodes your file into an unreadable format, so that even if someone succeeds in opening it, he/she can’t read the content.
Portability
One of the main reason for the universal popularity of UNIX is that it can be ported to almost any computer system, with only the bare minimum of adaptions to suit the given computer architecture. UNIX is almost entirely written in C.
UNIX System Structure
The UNIX system consists of the following components:
The kernel
Commands and utilities
Shell
Applications
The hardware at the center of the diagram provides the operating system with basic services. The layer above physical hardware is the “kernel” . It interacts with the actual hardware in machine language.
The stream-lining of these (application & kernel) two modes of communication is done by middle layer, called the “Shell” .
The programs such as shell (sh) and editors (vi and ed)shown in the outer layers interacts with the kernel by invoking a well-defined set of systems calls.
The system calls instruct the kernel to do various operations for calling a program and exchange data between the kernel and program.
In the same layer several programs exist. They are commands (who, we, grep, comp), system libraries and private user programs.
The functioning of UNIX is manned in three levels. On the outer resides the “application programs” and standard C compiler, cc, which invokes a C preprocessor, two-pass compiler, assembler and loader.
UNIX application programs are built upon the kernel and kernel’s system calls. The kernel provides the service upon which all application programs rely and it defines those services.
Kernel and Shell
The shell, or the command interpreter is the mediator, which interprets the commands that we give and then conveys them to the kernel which ultimately executes them.
The kernel has various functions. It manages files, carries out all the data transfer between the file system and the hardware, and also manages memory.
The onus of scheduling of various programs running in memory or allocation of CPU time to all running programs also lies with the kernel.
It also handles any interrupts issued as it is the entity that has direct dealing with the hardware.
The kernel program is usually stored in a file called ‘UNIX’ whereas the shell program is in a file called ‘sh’ .
At a particular point in time there may be several shells running in the memory but only one kernel. This is because at any instance UNIX is capable of executing only one program as the other program one kernel is sufficient.
