Define a process? What are sequential and concurrent processes?

- Each and every task which we order our computer to carry out is accomplished by a set of processes. 

- It is these processes that actually run the program. 

- A process can be defined as an instance of the program that is currently being executed. 

- A program’s current activity and the code being executed are stored in the process itself. 

- However, it depends on the operating system that the process is to constitute of multiple threads for a concurrent execution or just one thread for sequential execution. 

This gives rise to two different types of processes namely:

Sequential processes 

Sequential processes can be executed on the same processor but the concurrent processors however may sometimes require more than one processor.

Concurrent processes

Concurrent processes are executed in parallel to each other and at the same whereas the sequential processes go step by step executing one instruction at a time. 

Concepts of Process

- A computer program can be defined as a set of passive instructions. When these instructions are actually executed, they form a process. 

- The same program may have a number of processes associated with it. 

- Multiple processes can be executed by sharing the processors and the resources. 

- If this is done, it is called multitasking. 

- Each processor takes up a single task. 

- With multitasking, switching between the different tasks becomes possible for the processor and so the processes won’t have to wait for long. 

- However, it depends entirely on the operating system when the switch has to be performed:

1. When the task is performing I/O operations or
2. When the task itself indicates that it can now be switched or
3. On hardware interrupts.

- Time sharing is a common type of multitasking and allows interactive user applications to response quickly. 

- In systems using the time sharing systems, switching is done quite rapidly. 

- This gives an illusion of the simultaneous execution of the multiple processes by the same processor. 

- Such type of execution is termed as concurrency.

- Direct communication that may take place between independent processes is avoided by many of the modern operating systems for the reasons of maintaining reliability and security. 

- The inter-process communication functionality is kept under strict control and mediation. 

- In general, the following resources are said to constitute a process:

Ø  Executable machine code’s image associated with the task.

Ø  Some part of virtual memory that is inclusive of the process specific data, executable code, heap and a call stack. Heap is for holding the immediate data generated during the execution and the call stack is for keeping a track of the subroutines.

Ø OS descriptors belonging to the resources that were allocated to the processes. These descriptors may be data sources, sinks and file descriptors etc.

Ø  Security attributes including the set of permissions for the process and the owner of the process.

Ø  Processor state like physical memory addressing or register contents etc. registers store the state during the execution of the process or otherwise it is stored in memory.

- Most of this information is held in the process control blocks regarding the active processes. 

- The operating system makes it a point to maintain a separation between its resources and allocate them the requested resources so that they won’t interfere with the working of each other and thus won’t cause any system failures such as thrashing or deadlocks. 

- But processes do require communicating with each other. 

- For such cases to make interaction safe, operating system has mechanisms especially for the inter-process communication.