This relates to the Operating System’s activity of managing the processor in the system, i.e. allocating and de-allocating of the processor to the process.  The Operating System decides the same based on the priority of the process depending if there exists any and certain pre-defined algorithms.  The following section explains some related terminologies.

Process

A process may be defined as the running instance of a program characterized by a change of state and attributes.  Each and every process maintains a Process Control Block or PCB of its own.

Process Control Block

A process control block (PCB) is a data structure that contains information related to a running process.  These include:

·         Process ID

·         Processor Information

·         Process State

·         Process Priority

·         Stack information

·         Memory information at which the process has been loaded

Process States

A process changes its state from one state to another depending on its required activity.  The process can be in one of the following states.

·         Ready

·         Running

·         Suspended

Thread

A thread is a light weight process.  It is the smallest unit of CPU utilization and is also the path of execution within a process.  A task is an enhanced form of a thread.  Each and every process should have at least one thread; this is the primary or the main thread of the application.  All other user created threads run in the background and are also called worker threads.  When the application’s main thread terminates so does the application.

User Level Threads

The User Level threads are those that are executed in the User’s memory context and do not make use of System calls.  Therefore, Kernel switching does not occur in User Level threads.

Kernel Level Threads

The Kernel Level threads are those that are executed in the Operating System’s Kernel memory context.  The threads are managed and controlled by the Kernel using System calls.

Inter Process Communication

This refers to the communication between two or more processes for the purpose of data and message transfer.

Multithreading

Multithreading is the ability of the operating system to have at the same of time multiple threads in memory that switch between the tasks.  This is to provide a pseudo parallelism, as if all the tasks are running simultaneously.  This illusion of concurrency is ensured by the Operating System by providing a specific time slice to each and every thread and then switching between the threads very fast once their slice is over.

Multithreading can be of the following types.

·         Serial

·         Cooperative

·         Preemptive

In the serial mode of multithreading a thread is executed by the Operating System and until the execution of that thread is over, any other thread cannot be executed.  Hence, a thread has to wait until the earlier thread's turnaround time is over.  In other words, the currently executing thread monopolizes the processor.  In the Cooperative mode of multithreading, a thread can have the control of the processor as long as it needs without the need to necessarily preempt them.  In order words, in this type of multithreading the control of the processor lies with the executing thread.  In the preemptive mode of operation however, the operating system has control over the processor and decides the time slice for each thread for which it would execute and preempt threads if and when required.  Serial Multithreading is supported by the MS DOS operating system, cooperative multithreading is supported by Windows 3.11, while preemptive mode is supported by Windows 98, NT.

Thread Priorities

Thread priorities are values that can be set to distinguish one thread from another depending on their priorities.  A thread with a higher priority would execute more frequently than a thread having a comparatively less priority.

Multitasking

A task is an enhanced form of a thread.  Multitasking is the logical extension of multiprogramming.  The following list shows the names of some commonly used Operating Systems and the type of multitasking that they support.

·         DOS                           : Serial Multitasking

·         Win 3.11                      : Co-operative Multitasking

·         Win 9x/NT                    : Preemptive Multitasking

·         UNIX                           : Time Sliced Priority Based Multitasking

Multiprocessing

Multiprocessing or parallel processing refers to multitasking on multiple processors.  It is of the following two types.

·         Symmetric

·         Asymmetric