Operating System Principles And Multitasking

What is an Operating System? An Operating system is a program that controls the execution of application programs and acts as an interface between the user of a computer and the computer hardware. An Operating system is concerned with the allocation of resources and services, such as memory, processors, devices and information. The Operating System correspondingly includes programs to manage these resources, such as a traffic controller, a scheduler, memory management module, I/O programs, and a file system.

Computer System Components 1. Hardware – provides basic computing resources (CPU, memory, I/O devices). 2. Operating system – controls and coordinates the use of the hardware among the various application programs for the various users. 3. Applications programs – define the ways in which the system resources are used to solve the computing problems of the users (compilers, database systems, video games, business programs). 4. Users (people, machines, other computers).

Abstract View of System Components

Operating System Principles Memory Management Processor Management Device Management File Management Security Control over system performance Job accounting Error detecting aids Coordination between other software and users

Memory Management Memory management refers to management of Primary Memory or Main Memory. Main memory is a large array of words or bytes where each word or byte has its own address. Main memory provides a fast storage that can be access directly by the CPU. So for a program to be executed, it must in the main memory. Operating System does the following activities for memory management.  Keeps tracks of primary memory i.e. what part of it are in use by whom, what part are not in use.  In multiprogramming, OS decides which process will get memory when and how much.  Allocates the memory when the process requests it to do so.  De-allocates the memory when the process no longer needs it or has been terminated.

Processor Management In multiprogramming environment, OS decides which process gets the processor when and how much time. This function is called process scheduling. Operating System does the following activities for processor management.  Keeps tracks of processor and status of process. Program responsible for this task is known as traffic controller.  Allocates the processor (CPU) to a process.  De-allocates processor when processor is no longer required.

Cont….

Device Management OS manages device communication via their respective drivers. Operating System does the following activities for device management.  Keeps tracks of all devices. Program responsible for this task is known as the I/O controller.  Decides which process gets the device when and for how much time.  Allocates the device in the efficient way.  De-allocates devices.

File Management A file system is normally organized into directories for easy navigation and usage. These directories may contain files and other directions. Operating System does the following activities for file management.  Keeps track of information, location, uses, status etc. The collective facilities are often known as file system.  Decides who gets the resources.  Allocates the resources.  De-allocates the resources.

Some Other Activites  Security -- By means of password and similar other techniques, preventing unauthorized access to programs and data.  Control over system performance -- Recording delays between request for a service and response from the system.  Job accounting -- Keeping track of time and resources used by various jobs and users.  Error detecting aids -- Production of dumps, traces, error messages and other debugging and error detecting aids.  Coordination between other software and users -- Coordination and assignment of compilers, interpreters, assemblers and other software to the various users of the computer systems.

Multitasking Multitasking refers to term where multiple jobs are executed by the CPU simultaneously by switching between them. Switches occur so frequently that the users may interact with each program while it is running. Operating system does the following activities related to multitasking.  The user gives instructions to the operating system or to a program directly, and receives an immediate response.  Operating System handles multitasking in the way that it can handle multiple operations / executes multiple programs at a time.  Multitasking Operating Systems are also known as Time-sharing systems.  These Operating Systems were developed to provide interactive use of a computer system at a reasonable cost.  A time-shared operating system uses concept of CPU scheduling and multiprogramming to provide each user with a small portion of a time-shared CPU.  Each user has at least one separate program in memory.  A program that is loaded into memory and is executing is commonly referred to as a process.

Cont….  When a process executes, it typically executes for only a very short time before it either finishes or needs to perform I/O.  Since interactive I/O typically runs at people speeds, it may take a long time to complete. During this time a CPU can be utilized by another process.  Operating system allows the users to share the computer simultaneously. Since each action or command in a time-shared system tends to be short, only a little CPU time is needed for each user.  As the system switches CPU rapidly from one user/program to the next, each user is given the impression that he/she has his/her own CPU, whereas actually one CPU is being shared among many users.

Cont….