Question

What is a program? What is a process? What is a thread? Explain the differences/similarities in details between them.

Answer 1

1. What is a Program: A program is a group of instructions which is created to perform a specific task on the computer. The program itself does not perform any action and it stored on any file or memory device.

Example: below is a program which will add a and b variables and print the sum which is stored in c variable:

Main(){

Int a =10, b= 20;

Int c;

Cout<<“Sum of a and b is “<

}

2. What is a Process: A process is an activity going on in the computer. The program to be executed needs a process or group of processes for its execution. When a process is started required data is moved from secondary storage to processor’s registers. A process needs Processor, Input/Output devices, memory to accomplish the goal. A process can be:

• An executing program
• Any unit of work scheduled by the operating system.
• Any activity going on the system

Execution of the program means, that program is using the processor, accessing the other resources required, changing its variable values etc. Process understands what is being required in the program to be executed and perform the same actions. A program can have multiple processes in it.

Example: The process to execute the example program will have below characteristics:

• Process ID: Unique ID of the process e.g.
• Process State: State of the process to execute the program. It can be waiting, running, stopped, terminated.
• Program Counter: The address of the next instruction in the process to be executed
• Registers: The registers list which is to be used for the process to execute the program
• Memory: Process will hold memory of processor which can be maim memory OR cache memory
• Opened Files: The process will refer multiple files while execution

3. What is a Thread: Threads are the light weight processes OR sub-part of processes, which can be independently managed by the scheduler. The threads are part of processes. Each thread has its parent process and it belongs to exactly one process during its life time.

Example: To execute the above program in the processor, it will be divided into small threads to perform the independent task. E.g. Thread #1 will move the variables to the main memory, Thread #2 will make a space in memory to store variable c whereas another variable will calculate sum of a and b. The use of threads can be user defined as well as processor defined.

Below diagram shows the purpose and definition of Program, Purpose and Threads:

Similarities between Program, Process and Thread

From above definitions, it can be clearly defined that program, process and thread has same purpose, which is to perform the action required on the user’s input. The program tells what to do in a passive way; the process is created by the process to DO the actions in program. The process is divided into small threads to manage the processes in a better way.

The following figure depicts that how three aspects are used together to make the computer functioning correctly and provide the results for the user input.

Difference between Program, Process and Thread

Although the program, process and threads work with each other to achieve the result for the end user yet all terms has certain differences among functionality, lifetime and behaviour. Please find below the differences between the same:

Feature	Program	Process	Thread
Mode	It a ordered instuctions in passive mode.	Program instance in active mode is called process	Threads have the active mode
Dependency	It does not depends on process or thread.	Process depends on the instuctions in program	Threads depends on the parent process
Lifespan	It has longer lifespan as compared to process and thread. The program needs to be deleted by the user manually.	Process has longer lifespan than thread but smaller lifespan than program	Threads have smallest lifespan
Memory	It needs only a file space in memory. The file stores the instructions in it	Needs independent CPU memory and dedicated registers.	Doesn't has own memory, share the process's memory
Purpose	Tell the computer what to do.	Execute the action defined in 'What to do'	Help process to perform action in faster and better way
Input Output devices	The program does not need input output devices for execution.	Process needs dedicated input and output devices for executing the process.	Threads use parent process's input output devices
System Calls	System calls are not needed.	System calls are needed in various stages of the process.	System calls are not needed.
Memory sharing	It does not shared the memory	Processes do not share the memory.	Threads shares the memory with each other.
Existence	A program can exist individually	Process can exist individually for a program	Threads can not exist individually, these are created and destroyed within a process
Instances	Program can have different instances of processes. Whenever s program is executed a separated process is created.	Process has its life cycle, where it starts, executes and ends as per instructions.	Threads are the small instances of process which starts and ends within the life of parent process.