Operating System Concepts

Faculty-Shared Solved Questions, Part 1

Q1. What are physical and logical addresses?

Physical and logical addresses relate to how a program or system interacts with the memory of the computer.

• Physical Address: It refers to actual memory addresses on the physical hardware like RAM or other storage devices. It represents the real locations in the memory hardware where data and instructions are stored
• Logical Address: It refers to the set of all addresses generated by the CPU when a program is running.

Q2. What is a semaphore?

A semaphore is simply a non negative integer variable or counter that is used to keep track of the number of available shared resources for the maximum number of threads/processes allowed to access the shared resources simultaneously.
Semaphores can be used to implement Mutual Exclusion, Co-ordination, and synchronization between multiple processes or threads.

Q3. Define Thrashing and Virtual Memory

• Thrashing: It occurs when a system is overburdened with excessive paging or swapping between the RAM and the disk, leading to poor system performance. When the OS spends the majority of it's time swapping data in and out of virtual memory instead of executing processes it leads to Thrashing.
• Virtual Memory: It is a memory management technique that gives an "idealized abstraction" of the storage resources available to a program; This creates an illusion for the user that there is almost unlimited memory available even if the system runs out of physical RAM.

Q4. What is a Wait-For Graph? Give example.

A "Wait-For Graph" is a directed graph used in OS to represent the waiting relationships between processes and resources. It is used for deadlocks detection and prevention.

• If you have two processes(P1 & P2) and two resources(R1 & R2):
  1. P1 holds R1 but waits for R2.
  2. P2 holds R2 but waits for R1
• The Wait-For Graph would look like this