SERVICES AND PROPERTIES OF OPERATING SYSTEM

An Operating System provides services to both the users and to the programs.

• It provides programs, an environment to execute.
• It provides users, services to execute the programs in a convenient manner.

Following are few common services provided by operating systems.

• Program execution
• I/O operations
• File System manipulation
• Communication
• Error Detection
• Resource Allocation
• Protection

Program execution

Operating system handles many kinds of activities from user programs to system programs like printer spooler, name servers, file server etc. Each of these activities is encapsulated as a process.

A process includes the complete execution context (code to execute, data to manipulate, registers, OS resources in use). Following are the major activities of an operating system with respect to program management.

• Loads a program into memory.
• Executes the program.
• Handles program's execution.
• Provides a mechanism for process synchronization.
• Provides a mechanism for process communication.
• Provides a mechanism for deadlock handling.

I/O Operation

I/O subsystem comprised of I/O devices and their corresponding driver software. Drivers hides the peculiarities of specific hardware devices from the user as the device driver knows the peculiarities of the specific device.

Operating System manages the communication between user and device drivers. Following are the major activities of an operating system with respect to I/O Operation.

• I/O operation means read or write operation with any file or any specific I/O device.
• Program may require any I/O device while running.
• Operating system provides the access to the required I/O device when required.

File system manipulation

A file represents a collection of related information. Computer can store files on the disk (secondary storage), for long term storage purpose. Few examples of storage media are magnetic tape, magnetic disk and optical disk drives like CD, DVD. Each of these media has its own properties like speed, capacity, data transfer rate and data access methods.

A file system is normally organized into directories for easy navigation and usage. These directories may contain files and other directions. Following are the major activities of an operating system with respect to file management.

• Program needs to read a file or write a file.
• The operating system gives the permission to the program for operation on file.
• Permission varies from read-only, read-write, denied and so on.
• Operating System provides an interface to the user to create/delete files.
• Operating System provides an interface to the user to create/delete directories.
• Operating System provides an interface to create the backup of file system.

Communication

In case of distributed systems which are a collection of processors that do not share memory, peripheral devices, or a clock, operating system manages communications between processes. Multiple processes with one another through communication lines in the network.

OS handles routing and connection strategies, and the problems of contention and security. Following are the major activities of an operating system with respect to communication.

• Two processes often require data to be transferred between them.
• The both processes can be on the one computer or on different computer but are connected through computer network.
• Communication may be implemented by two methods either by Shared Memory or by Message Passing.

Error handling

Error can occur anytime and anywhere. Error may occur in CPU, in I/O devices or in the memory hardware. Following are the major activities of an operating system with respect to error handling.

• OS constantly remains aware of possible errors.
• OS takes the appropriate action to ensure correct and consistent computing.

Resource Management

In case of multi-user or multi-tasking environment, resources such as main memory, CPU cycles and files storage are to be allocated to each user or job. Following are the major activities of an operating system with respect to resource management.

• OS manages all kind of resources using schedulers.
• CPU scheduling algorithms are used for better utilization of CPU.

Protection

Considering a computer systems having multiple users the concurrent execution of multiple processes, then the various processes must be protected from each another's activities.

Protection refers to mechanism or a way to control the access of programs, processes, or users to the resources defined by a computer systems. Following are the major activities of an operating system with respect to protection.

• OS ensures that all access to system resources is controlled.
• OS ensures that external I/O devices are protected from invalid access attempts.
• OS provides authentication feature for each user by means of a password.

An Operating System has many properties that are following

 Batch processing

Batch processing is a technique in which Operating System collects one programs and data together in a batch before processing starts. Operating system does the following activities related to batch processing.

• OS defines a job which has predefined sequence of commands, programs and data as a single unit.
• OS keeps a number a jobs in memory and executes them without any manual information.
• Jobs are processed in the order of submission i.e first come first served fashion.
• When job completes its execution, its memory is released and the output for the job gets copied into an output spool for later printing or processing.

Advantages of Batch Processing

• Batch processing takes much of the work of the operator to the computer.
• Increased performance as a new job get started as soon as the previous job finished without any manual intervention.
• Disadvantages
• Difficult to debug program.
• A job could enter an infinite loop.
• Due to lack of protection scheme, one batch job can affect pending jobs.

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.

Multitasking

• A program that is loaded into memory and is executing is commonly referred to as a process.
• 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 completed. 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.

Multiprogramming

When two or more programs are residing in memory at the same time, then sharing the processor is referred to the multiprogramming. Multiprogramming assumes a single shared processor. Multiprogramming increases CPU utilization by organizing jobs so that the CPU always has one to execute.

Following figure shows the memory layout for a multiprogramming system.

Memory layout

• Operating system does the following activities related to multiprogramming.
• The operating system keeps several jobs in memory at a time.
• This set of jobs is a subset of the jobs kept in the job pool.
• The operating system picks and begins to execute one of the job in the memory.
• Multiprogramming operating system monitors the state of all active programs and system resources using memory management programs to ensures that the CPU is never idle unless there are no jobs

Advantages

• High and efficient CPU utilization.
• User feels that many programs are allotted CPU almost simultaneously.
• Disadvantages
• CPU scheduling is required.
• To accommodate many jobs in memory, memory management is required.
• Interactivity
• Interactivity refers that a User is capable to interact with computer system. Operating system does the following activities related to interactivity.
• OS provides user an interface to interact with system.
• OS managers input devices to take inputs from the user. For example, keyboard.
• OS manages output devices to show outputs to the user. For example, Monitor.
• OS Response time needs to be short since the user submits and waits for the result.

Real Time System

Real time systems represents are usually dedicated, embedded systems. Operating system does the following activities related to real time system activity.

In such systems, Operating Systems typically read from and react to sensor data.

The Operating system must guarantee response to events within fixed periods of time to ensure correct performance.

Distributed Environment

Distributed environment refers to multiple independent CPUs or processors in a computer system. Operating system does the following activities related to distributed environment.

• OS Distributes computation logics among several physical processors.
• The processors do not share memory or a clock.
• Instead, each processor has its own local memory.

OS manages the communications between the processors. They communicate with each other through various communication lines.

Spooling

Spooling is an acronym for simultaneous peripheral operations on line. Spooling refers to putting data of various I/O jobs in a buffer. This buffer is a special area in memory or hard disk which is accessible to I/O devices. Operating system does the following activites related to distributed environment.

• OS handles I/O device data spooling as devices have different data access rates.
• OS maintains the spooling buffer which provides a waiting station where data can rest while the slower device catches up.

OS maintains parallel computation because of spooling process as a computer can perform I/O in parallel fashion. It becomes possible to have the computer read data from a tape, write data to disk and to write out to a tape printer while it is doing its computing task.

Spooling Advantages

• The spooling operation uses a disk as a very large buffer.
• Spooling is capable of overlapping I/O operation for one job with processor operations for another job.