Operating Systems Cheatsheet

1. Introduction to Operating Systems

Definition

• An operating system (OS) is system software that manages computer hardware, software resources, and provides services for computer programs.

Key Functions

• Process management
• Memory management
• File system management
• Device management
• Security and access control

2. Process Management

Processes

• A process is an instance of a running program.

Process States

• Running: The process is currently being executed.
• Ready: The process is waiting to be assigned to a processor.
• Blocked: The process is waiting for a resource or event.

Process Scheduling

• Preemptive Scheduling: The operating system decides to interrupt the running process and start another.
• Non-preemptive Scheduling: The currently running process voluntarily gives up the CPU.

3. Memory Management

Memory Hierarchy

• Registers: Fastest memory directly accessible by the CPU.
• Cache: Small but faster than main memory.
• Main Memory (RAM): Holds data and instructions currently in use.
• Secondary Storage: Slower but non-volatile (e.g., hard drives, SSDs).

Virtual Memory

• Allows the execution of processes that are larger than physical memory.
• Pages are swapped in and out of main memory as needed.

Memory Allocation

• Static Allocation: Memory is allocated at compile time.
• Dynamic Allocation: Memory is allocated at runtime.

4. File System Management

File Systems

• Organize and provide access to data stored on storage devices.

File Operations

• Create: Create a new file.
• Read: Read data from a file.
• Write: Write data to a file.
• Delete: Remove a file.

File System Structures

• File Control Block (FCB): Contains metadata about a file.
• Directory Structure: Organizes files into a hierarchy.

5. Device Management

Device Drivers

• Software that allows the operating system to communicate with hardware devices.

I/O Operations

• Synchronous I/O: The process waits for the I/O operation to complete.
• Asynchronous I/O: The process continues execution while the I/O operation is in progress.

Interrupt Handling

• Hardware interrupts trigger the operating system to perform tasks.
• Device drivers handle interrupts and perform necessary actions.

6. Security and Access Control

User Authentication

• Verify the identity of users accessing the system.

Access Control

• Determine what actions users are allowed to perform on the system and its resources.

Encryption

• Protect sensitive data by converting it into a form that cannot be easily understood without authorization.

7. Types of Operating Systems

Single-Tasking vs. Multi-Tasking

• Single-Tasking: Only one task can be executed at a time.
• Multi-Tasking: Multiple tasks can be executed simultaneously.

Single-User vs. Multi-User

• Single-User: Only one user can interact with the system at a time.
• Multi-User: Multiple users can interact with the system simultaneously.

Real-Time Operating Systems (RTOS)

• Designed to handle tasks with specific timing requirements.
• Used in embedded systems, control systems, etc.

8. Examples of Operating Systems

Desktop/Laptop Operating Systems

• Windows: Developed by Microsoft.
• macOS: Developed by Apple.
• Linux: Open-source OS kernel used in various distributions.

Mobile Operating Systems

• Android: Developed by Google.
• iOS: Developed by Apple.

Server Operating Systems

• Linux: Commonly used for web servers, cloud computing, etc.
• Windows Server: Developed by Microsoft for server environments.