CMSC 104, Lecture 031 Operating Systems I An Introduction to Operating System Concepts.

CMSC 104, Lecture 03 1

Operating Systems I

An Introductionto

Operating SystemConcepts

CMSC 104, Lecture 03 2

What is an Operating System (OS)?

• A computer program

• Performs many operations, such as:

• Allows you to communicate with the computer (tell it what to do)

• Controls access (login) to the computer

• Keeps track of all processes currently running

CMSC 104, Lecture 03 3

How Do I Communicate With the Computer Using the OS?

• You communicate using the particular OS’s user interface.

• Graphical User Interface (GUI) - Windows• Command-driven interface - DOS, UNIX, Linux

• We will be using the Linux operating system, which is very similar to UNIX.

CMSC 104, Lecture 03 4

How Do I Communicate With the Computer Using the OS? (con’t)

• When you log in to the Linux system here, a user prompt will be displayed: linux#[1]% _

where # is the number of the Linux server that you have connected to. You may use any of the Linux servers.

• The number in the brackets will change as you work It is the “number” of the command that you are about to type.

• If this prompt is not on the screen at any time, you are not communicating with the OS.

CMSC 104, Lecture 03 5

OS Introduction

• Computer applications today require a single machine to perform many operations and the applications may compete for the resources of the machine.

• This demands a high degree of coordination

• This coordination is handled by system software known as the operating system

CMSC 104, Lecture 03 6

Evolution of Operating System

• OS for batch jobso Program execution required significant

preparation of equipmento Program execution (job)o OS was a system to simplify program setup

and simplify transition between jobso Physical separation of users and

equipment led to computer operators

CMSC 104, Lecture 03 7

OS Evolution (cont’d)

o Users left jobs with the operator and came back the next day (batch jobs)

o Users had no interaction with computer during program execution. Maybe okay for some applications, but not for all.

CMSC 104, Lecture 03 8

OS Evolution

• OS for Interactive Processingo Allowed programs to carry on dialogue with

user via remote terminals (workstations)o Real-time processingo Users demand timely responseo Machines too expensive to serve only one

usero Common for several users to want

interactive services at the same time

CMSC 104, Lecture 03 9

OS Evolution (cont’d)

• OS for time-sharingo To accommodate multiple real-time users,

the OS rotates its various jobs in and out of execution via time-sharing

o Each job gets a predetermined “time slice”o At end of time slice current job is set aside

and a new one startso By rapidly shuffling jobs, illusion of several

jobs executing simultaneously is created

CMSC 104, Lecture 03 10

OS Evolution (cont’d)

o Without time slicing, a computer spends most of its time waiting for peripheral devices or users

o A collection of tasks can be completed in less time with time-sharing than when completed sequentially

CMSC 104, Lecture 03 11

Different Operating Systems on the Same Machine ?

• It is possible to have more than one operating system available to be used on a machine.

• Only one operating system is run at a time, though.

• Examples: o VAX -- VMS or Ultrixo PCs -- DOS, Windows, or Linux

CMSC 104, Lecture 03 12

Types of software

• Applications softwareo Performs tasks specific to the machine’s

utilization.• System Software

o Performs tasks common to computer systems in general

o Operating systems vary based on the hardware they’re used on

CMSC 104, Lecture 03 13

Types of software (cont’d)

• Utility softwareo Provides fundamental activities, yet not

included with OSo “Extends” the OSo Distinction between applications and

utilities is often vagueo Distinction between OS and utilities is also

vague

CMSC 104, Lecture 03 14

The OS Shell

• Defines interface between OS and userso Windows GUIo UNIX command lineo UNIX users can choose among a variety of

shells� csh is the “C shell”� tcsh is an enhanced “C shell”

CMSC 104, Lecture 03 15

OS Shell interface

O / S

shell

Users

UsersUsers

CMSC 104, Lecture 03 16

The OS Kernel

• The internal part of the OS is often called the kernel

• Kernel Componentso File Managero Device Driverso Memory Managero Schedulero Dispatcher

CMSC 104, Lecture 03 17

OS File Manager

• Maintains information about the files that are available on the system

• Where files are located in mass storage, their size and type and their protections, what part of mass storage is available

• Files usually allowed to be grouped in directories or folders. Allows hierarchical organization.

CMSC 104, Lecture 03 18

OS Device Drivers

• Software to communicate with peripheral devices or controllers

• Each driver is unique• Translates general requests into

specific steps for that device

CMSC 104, Lecture 03 19

OS Memory Manager

• Responsible for coordinating the use of the machine’s main memory

• Decides what area of memory is to be allocated for a program and its data

• Allocates and deallocates memory for different programs and always knows what areas are free

CMSC 104, Lecture 03 20

OS Scheduler

• Maintains a record of processes that are present, adds new processes, removes completed processeso memory area(s) assignedo priorityo state of readiness to execute (ready/wait)

CMSC 104, Lecture 03 21

OS Dispatcher

• Ensures that processes that are ready to run are actually executed

• Time is divided into small (50 ms) segments called a time slice

• When the time slice is over, the dispatcher allows scheduler to update process state for each process, then selects the next process to run

CMSC 104, Lecture 03 22

OS Summary

• Shell -- interface to user• File Manager -- manages mass memory• Device Drivers -- communicate with

peripherals• Memory Manager -- manages main

memory• Scheduler & Dispatcher -- manage

processes

CMSC 104, Lecture 03 23

Utilities

• Operating systems usually come with some associated utility programs

• UNIX usually has the text editors xemacs/emacs and vi

(and sometimes pico)• UNIX has its own sort utility• UNIX has its own mail utility