Software-Engineering

Processes and Threads

1. What is a Process?

A process is an instance of a program in execution.

Key points:

• A process has its own isolated memory space
• Processes are independent of each other
• One process cannot directly access another process’s memory
• Each process has a unique Process ID (PID)

A process contains:

• Virtual memory (code, heap, stack, data)
• OS resources (PID, scheduling info)
• File descriptors
• Environment variables
• One or more threads

Example:

Chrome Browser
 ├── Process 1: Tab A
 ├── Process 2: Tab B
 └── Process 3: GPU Process

If one process crashes, other processes usually continue running.

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2. What is a Thread?

A thread is the smallest unit of execution inside a process.

Key points:

• Threads live inside a process
• Threads share the process’s memory and resources
• Threads actually execute the program logic

Threads share:

• Heap memory
• Global variables
• Open files

Threads have their own:

• Stack
• Program counter
• CPU registers

Threads are not independent like processes.

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3. Who Runs on the CPU?

The CPU runs threads, not processes.

• The OS scheduler schedules threads
• A process itself never executes instructions
• Threads are the entities that get CPU time

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4. Relationship Between Process and Thread

Think of it like this:

• Process → Container / execution environment
• Thread → Worker that executes code

Better definition:

A process provides memory and resources, while threads execute the program logic.

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5. How Many Threads Can Run at the Same Time?

This depends on the number of CPU cores.

Single-core CPU:

• Only one thread runs at a time
• Multiple threads use context switching
• Appears parallel, but is actually time-sharing

Multi-core CPU:

If a system has:

4 CPU cores

Then:

4 threads can run truly in parallel

Example:

• 8-core CPU
• 100 threads

Only 8 threads execute simultaneously. The rest are waiting or scheduled.

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6. Threads vs CPU Cores

Term	Meaning
Thread	Unit of execution
Core	Physical execution unit
Parallelism	Threads running at the same time
Concurrency	Multiple threads making progress