What is a Program?

A program is an executable file residing on the disk (secondary storage) in a directory. It is also termed as a set of instructions stored in the secondary storage device that are intended to carry out a specific job. It is read into the primary memory and executed by the kernel.

What is a process?

An executing instance of a program is called a process. Some operating systems use the term ‘task‘ to refer to a program that is being executed.

A process is termed as an ‘active entity’ since it is always stored in the main memory and disappears if the machine is power cycled. Several process may be associated with a same program.

On a multiprocessor system, multiple processes can be executed in parallel. On a uni-processor system, though true parallelism is not achieved, a process scheduling algorithm is applied and the processor is scheduled to execute each process one at a time producing an illusion of concurrency.

Example: Executing multiple instances of the ‘Calculator’ program. Each of the instances are termed as a process.

What is a thread?

A thread is called a ‘lightweight process’. It is similar to a real process but executes within the context of a process and shares the same resources allotted to the process by the kernel.

A process has only one thread of control – one set of machine instructions executing at a time. A process may also be made up of multiple threads of execution that execute instructions concurrently. Multiple threads of control can exploit the true parallelism possible on multiprocessor systems. On a uni-processor system, a thread scheduling algorithm is applied and the processor is scheduled to run each thread one at a time.

All the threads running within a process share the same address space, file descriptor, stack and other process related attributes. Since the threads of a process share the same memory, synchronizing the access to the shared data within the process gains unprecedented importance.
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