2025-05-212025-05-23Lecture Notes

OS Revision

Based on OS_Revision-2025.pdf

Exam

What is CPU Scheduler?
- A CPU scheduler selects one of the processes in the ready queue to be executed by the CPU.
What is multilevel Queue?
- A multilevel queue is a scheduling strategy that partitions the ready queue into several separate queues, each with its own scheduling algorithm.
Real-Time CPU Scheduler, Priority-Based, Rate Monotonic Scheduling
- These are specialized scheduling algorithms: Priority-Based assigns processes based on priority; Rate Monotonic is for real-time systems where shorter periods get higher priority.

What is race condition?
- A race condition occurs when multiple processes access and manipulate shared data concurrently, and the outcome depends on the timing of their execution.
What is critical section problem?
- It is a problem where multiple processes access a shared resource and the outcome depends on the order of execution.
What are three requirements for a solution to solve the critical problem?
- “Mutual exclusion, progress, and bounded waiting.”
What methods can be used to solve the critical section problems?
- “Algorithms like Peterson’s solution, semaphores, mutexes, and monitors.”
What are the problems if we incorrectly use the semaphore?
- “Incorrect semaphore use can cause deadlocks, starvation, or incorrect execution sequences.”
What can semaphore help in process synchronization?
- “Semaphores help coordinate processes by signaling and waiting, ensuring mutually exclusive access to resources.”
What is Monitor?
- A monitor is a high-level synchronization construct that allows safe access to shared variables through condition variables and procedures.

What is memory management?
- Memory management handles the allocation and deallocation of memory to processes efficiently.
When does the address binding happen?
- “Address binding happens at compile time, load time, or execution time.”
What is physical memory address?
- It is the actual address in memory hardware.
What is logical memory address?
- It is the address generated by the CPU.
In which situation, the above two addresses are same? In which situation, the above two addresses are different?
- “They are the same in compile-time and load-time binding, different in execution-time binding.”
What memory allocation methods are there? Advantages? Disadvantages?
- “Contiguous allocation, paging, and segmentation. Each has trade-offs in speed, flexibility, and fragmentation.”
How to map the logical address to physical address?
- Using a memory management unit (MMU) that translates logical addresses to physical addresses using tables.
How to reduce time to find physical address?
- Using translation lookaside buffer (TLB) or cache mechanisms.
What methods are used to solve the big page table problem?
- “Hierarchical page tables, hashed page tables, or inverted page tables.”

Example: The Intel 32 and 64-bit Architectures is not included in exam

What is swapping?
- Swapping is moving processes between main memory and disk to free up memory.
What is backing store?
- Backing store is the disk space used to hold swapped-out processes.
What is page demanding?
- Demand paging loads pages into memory only when they are needed.
What is page fault?
- A page fault occurs when a page is referenced that is not in memory.
What is effective access time?
- “It is the average time the system takes to access memory, considering page faults.”
What is locality reference?
- It refers to the tendency of programs to access a set of memory locations frequently.
How to manage the free frames?
- Using free-frame lists or bitmap to track available memory blocks.
How to determine the pages to be replaced?
- “Using algorithms like FIFO, LRU, or Optimal to select a page to replace.”
How can the thrashing happen?
- Thrashing happens when excessive paging causes the system to spend more time swapping than executing.
How to allocate frames?
- Fixed allocation or priority-based dynamic allocation methods.
How to allocate kernel memory
- “Buddy system, slab allocation, and other specialized kernel memory allocation strategies.”

What is file?
- A file is a named collection of related data stored on secondary storage.
What is file system?
- A file system manages files and directories and controls how data is stored and retrieved.
What basic operations can be performed on a file?
- “Create, delete, read, write, open, close.”
What basic operations can be performed in a directory?
- “Create, delete, list, rename, traverse.”
What structures can be used to organize directories?
- “Single-level, two-level, tree-structured, acyclic-graph, and general graph directories.”
How can a file system be mounted?
- Mounting is attaching a file system to a directory structure so it can be accessed.
How can the access to a file be controlled?
- “By using permissions, user IDs, access control lists, etc.”

File locking is not included in the exam.

Why a file system is organized in layers?
- “Layering separates concerns like application interface, logical file system, and physical file system for modular design.”
What are two categories of structures used in file system implementation?
- Metadata structures and data blocks are two structure categories.
What is File Control Block or inode?
- “A file control block or inode stores metadata about a file like size, location, ownership, etc.”
What open file tables are there in an operating system? What are they used for?
- System-wide and per-process open file tables keep track of open files and their properties.
What methods are used to allocate disk blocks to files？
- “Contiguous, linked, and indexed allocation methods.”
What are main methods used to manage free space on disk?
- “Bitmaps, free lists, and grouping techniques.”

Virtual File System, Schemes in handling large files, Performance are not included in the exam

OS Revision

TosakaUCW

2025-05-21

2025-05-23