What is a Process

🧠 What is a Process?

A process is a program in execution.
It’s the running instance of your code — complete with its own memory, state, and system resources.

Think of a process as a living, breathing version of your static code.

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🧩 Key Characteristics of a Process

Feature	Explanation
🧠 Code (Text) Segment	The actual program code (machine instructions)
📦 Data Segment	Global and static variables
📚 Heap	Dynamically allocated memory (via malloc, new)
📄 Stack	Function calls, local variables, return addresses
🪪 PID (Process ID)	Unique identifier assigned by OS
⏳ State	Running, Ready, Blocked, Terminated
⏱️ Program Counter (PC)	Address of next instruction
📊 CPU Registers	Store temporary data
🔐 Isolation	Processes don’t share memory by default

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🧱 Process vs Program

Program	Process
Passive entity (just code)	Active entity (in execution)
Stored on disk (.exe, .class)	Loaded in RAM
Doesn’t do anything	Consumes CPU, memory, I/O
Can have multiple instances	Each instance is a separate process

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🔁 Process Lifecycle States

          ┌────────┐
          │ New    │ ← Created by OS
          └──┬─────┘
             ↓
         ┌────────┐
         │ Ready  │ ← Waiting to be scheduled
         └──┬─────┘
             ↓
         ┌────────┐
         │Running │ ← On the CPU now
         └──┬─────┘
      ┌─────┴─────┐
      ↓           ↓
┌──────────┐  ┌───────────┐
│ Blocked  │  │ Terminated│
└──────────┘  └───────────┘

• New: Process is created (e.g., fork() in Linux)

• Ready: Waiting in the queue for CPU

• Running: Actively executing instructions

• Blocked: Waiting for I/O (e.g., file read)

• Terminated: Finished or killed

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🛠️ Process Control Block (PCB)

The OS uses a PCB to keep track of each process.

Contains
PID
CPU registers
Program counter
Scheduling info
Memory limits
I/O info
Process state

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🧪 Real-World Example

$ firefox

• The OS:

  • Loads the Firefox binary

  • Allocates memory for code, heap, stack

  • Assigns PID (e.g., 10234)

  • Tracks it in the scheduler

Now Firefox is a process.

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🧠 Interview-Ready Definition:

A process is a program in execution. It includes the program code, current activity (registers, program counter), memory segments (heap, stack, data), and resources assigned by the OS. Processes are isolated from each other and managed via Process Control Blocks (PCBs).