Process and Process Creation – A Simple Overview

1. What Is a Process?

A process is one of the most fundamental abstractions provided by an operating system.

👉 Informal definition:

A process is a running program.

• A program by itself is passive—it is just a set of instructions and data stored on disk.

• A process is active—it is what the program becomes when the operating system loads it into memory and starts executing it.

For example:

• A web browser stored on disk is a program

• When you double-click it and it starts running, it becomes a process

Modern systems usually run many processes at the same time, such as:

• Web browsers

• Music players

• Email clients

• Games

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2. The Illusion of Many CPUs: CPU Virtualization

Even if a system has only one CPU (or a few CPUs), it appears as if many programs run simultaneously.

Key Question (The Crux of the Problem)

How does the OS provide the illusion of many CPUs when only a few exist?

Answer: Time Sharing

The OS virtualizes the CPU using time sharing:

• Run one process for a short time

• Stop it

• Run another process

• Repeat very quickly

Because this switching happens fast, users perceive that all programs run in parallel.

⚠️ Trade-off:

More processes → each process gets less CPU time → performance may decrease.

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3. Mechanisms and Policies in Process Management

Operating systems use two important concepts:

1. Mechanisms (How things are done)

Low-level operations that make functionality possible.

• Example: Context switch

  • Stops one process

  • Saves its state

  • Restores another process’s state

2. Policies (Which decision to make)

High-level rules that decide what to do.

• Example: CPU scheduling policy

  • Which process should run next?

  • Based on priority, fairness, responsiveness, etc.

💡 Design Principle:

Separating policy from mechanism improves modularity and flexibility.

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4. What Makes Up a Process? (Process State)

A process is defined by its machine state, which includes:

1. Address Space (Memory)

• Instructions (code)

• Static data

• Heap (dynamic memory)

• Stack (function calls, local variables)

2. CPU Registers

• General registers

• Program Counter (PC): points to the next instruction

• Stack Pointer: manages function calls

3. I/O Information

• List of open files

• Input/output resources

Together, these elements fully describe the current execution of a process.

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5. Process API: What Can the OS Do With Processes?

Every modern OS provides basic process operations:

1. Create

   • Start a new process (e.g., running a command)

2. Destroy

   • Forcefully terminate a process

3. Wait

   • Pause until a process finishes

4. Control

   • Suspend and resume processes

5. Status

   • Query information (state, execution time, etc.)

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6. How Is a Process Created?

Process creation transforms a program on disk into a running process.

Step 1: Load Program into Memory

• OS reads the executable from disk

• Loads:

  • Code

  • Static (initialized) data

• Modern OSes use lazy loading:

  • Load parts only when needed

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Step 2: Set Up the Stack

• OS allocates memory for the stack

• Used for:

  • Function calls

  • Local variables

  • Return addresses

• Initializes parameters for main() (argc, argv)

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Step 3: Set Up the Heap

• Heap is used for dynamic memory allocation (malloc, free)

• Initially small

• Grows as the program requests more memory

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Step 4: Initialize I/O

• OS opens default file descriptors:

  • Standard input

  • Standard output

  • Standard error

• Enables interaction with keyboard and screen

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Step 5: Start Execution

• OS transfers control to the program’s entry point (main())

• The process begins executing

• CPU is now running the process

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7. Program vs Process (Key Difference)

Program	Process
Stored on disk	Loaded into memory
Passive	Active
Static	Dynamic
No execution state	Has registers, memory, I/O state

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8. Summary

• A process is a running program managed by the OS.

• The OS creates the illusion of multiple CPUs using time sharing.

• A process consists of memory, registers, and I/O state.

• Process creation involves:

  1. Loading code and data

  2. Initializing stack and heap

  3. Setting up I/O

  4. Starting execution at main()

Understanding processes is essential for learning:

• CPU scheduling

• Context switching

• Multitasking

• Performance optimization