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

CMSC 104, Lecture 032 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 033 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 034 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 035 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 036 Evolution of Operating System OS for batch jobs oProgram execution required significant preparation of equipment oProgram execution (job) oOS was a system to simplify program setup and simplify transition between jobs oPhysical separation of users and equipment led to computer operators

CMSC 104, Lecture 037 OS Evolution (cont’d) oUsers left jobs with the operator and came back the next day (batch jobs) oUsers had no interaction with computer during program execution. Maybe okay for some applications, but not for all.

CMSC 104, Lecture 038 OS Evolution OS for Interactive Processing oAllowed programs to carry on dialogue with user via remote terminals (workstations) oReal-time processing oUsers demand timely response oMachines too expensive to serve only one user oCommon for several users to want interactive services at the same time

CMSC 104, Lecture 039 OS Evolution (cont’d) OS for time-sharing oTo accommodate multiple real-time users, the OS rotates its various jobs in and out of execution via time-sharing oEach job gets a predetermined “time slice” oAt end of time slice current job is set aside and a new one starts oBy rapidly shuffling jobs, illusion of several jobs executing simultaneously is created

CMSC 104, Lecture 0310 OS Evolution (cont’d) oWithout time slicing, a computer spends most of its time waiting for peripheral devices or users oA collection of tasks can be completed in less time with time-sharing than when completed sequentially

CMSC 104, Lecture 0311 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: oVAX -- VMS or Ultrix oPCs -- DOS, Windows, or Linux

CMSC 104, Lecture 0312 Types of software Applications software o 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 0313 Types of software (cont’d) Utility software oProvides fundamental activities, yet not included with OS o“Extends” the OS oDistinction between applications and utilities is often vague oDistinction between OS and utilities is also vague

CMSC 104, Lecture 0314 The OS Shell Defines interface between OS and users oWindows GUI oUNIX command line oUNIX users can choose among a variety of shells csh is the “C shell” tcsh is an enhanced “C shell”

CMSC 104, Lecture 0315 OS Shell interface O / S shell Users

CMSC 104, Lecture 0316 The OS Kernel The internal part of the OS is often called the kernel Kernel Components oFile Manager oDevice Drivers oMemory Manager oScheduler oDispatcher

CMSC 104, Lecture 0317 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 0318 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 0319 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 0320 OS Scheduler Maintains a record of processes that are present, adds new processes, removes completed processes omemory area(s) assigned opriority ostate of readiness to execute (ready/wait)

CMSC 104, Lecture 0321 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 0322 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 0323 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