OS Structure & Operations & Multithreading Models

What is the difference between the Kernel, System Programs, and Application Programs?

• Kernel: The one program running at all times on the computer.

• System Programs: Programs that ship with the OS but are not part of the kernel (e.g., file management tools).

• Application Programs: All programs not associated with the operation of the system (e.g., web browsers).

Explain Dual-Mode Operation and the purpose of the Mode Bit.

It allows the OS to protect itself and system components from errant users. A hardware "Mode Bit" indicates the current mode: 0 for Kernel Mode (privileged instructions allowed) and 1 for User Mode

What are the three most common System Call APIs?

1. POSIX API (for POSIX-based systems like Linux/macOS)

2. Win32 API (for Windows)

3. Java API (for the JVM).

What are the three common models for mapping User Threads to Kernel Threads?

1. Many-to-One: Many user-level threads map to one kernel thread.

2. One-to-One: Each user thread maps to its own kernel thread (e.g., Linux and Windows).

3. Many-to-Many: Multiplexes many user-level threads to a smaller or equal number of kernel threads.

What are Thread Pools and what is their main benefit?

A system that creates a number of threads at startup and places them into a pool. Benefits include faster service for requests (thread already exists) and limiting the total number of threads in the system.