Process - Process Management in Operating Systems

🧠 Process Management — States, PCB, Context Switching, and Memory Layout

A structured guide to how operating systems manage processes,
including foreground/background processes, PCB, scheduling, memory layout, and process creation.

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1️⃣ What Is a Process?

A process is a program in execution, requiring CPU time, memory, and system resources.

All processes share the CPU over time — execution is divided using timer interrupts.

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2️⃣ Foreground vs Background Processes

🔹 Foreground Process

• Runs in a visible user space
• Interacts directly with the user

🔹 Background Process

• Runs without direct UI visibility

Types

✔ Interactive background processes (services, daemons)
✔ Non‑interactive background processes

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3️⃣ CPU Time Sharing & Timer Interrupt

• CPU gives each process a limited time slice
• When time expires, a timer interrupt forces a switch

This enables multitasking.

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4️⃣ Process Control Block (PCB)

The PCB is a kernel‑resident data structure that stores all process state.

PCB Stores

Category	Description
PID	Unique Process ID
Register State	PC, SP, CPU registers
Process State	Ready / Running / Waiting
Scheduling Info	Priority, time slice
Memory Info	Page table, address space
I/O Info	Open files, devices
Accounting Info	CPU usage statistics

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5️⃣ Process States & Transitions

State	Meaning
New	Process created
Ready	Waiting for CPU
Running	Executing
Waiting (Blocked)	Waiting for I/O
Terminated	Finished

State Flow

New → Ready → Running → Waiting → Ready → Terminated

Key Transitions

• Dispatch: Ready → Running
• Timer Interrupt: Running → Ready
• I/O Request: Running → Waiting
• I/O Complete: Waiting → Ready

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6️⃣ Context Switch

A context switch saves the current process state and restores another process’s state.

Steps

1. Save Process A context (registers, PC, SP, memory map)
2. Load Process B context from PCB
3. Resume execution

Enables CPU to rapidly alternate between processes.

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7️⃣ Process Memory Layout (User Space)

Each process has a private virtual address space:

Segment	Purpose
Text (Code)	Executable instructions (read‑only)
Data	Global/static variables
Heap	Dynamic memory allocation
Stack	Function calls, local variables

Allocation Type

• Text + Data → Static region
• Heap + Stack → Dynamic region

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8️⃣ Process Hierarchy (Parent–Child)

• A process can create another process via system calls
• Creator = Parent process
• Created = Child process
• Child has a unique PID
• Some OS track PPID (Parent PID)

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9️⃣ Process Creation Mechanism — fork() & exec()

🔹 fork()

Creates a child process as a copy of the parent

• Inherits memory, file descriptors, environment

pid_t pid = fork();

🔹 exec()

Replaces process memory with a new program

execve("/bin/program", args, env);

Common pattern:

fork() → exec()

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🔟 Developer Takeaways

✔ Processes share CPU via time slicing
✔ PCB is the OS memory of each process
✔ Context switches enable multitasking
✔ Each process has isolated memory
✔ fork + exec power Unix process creation