Topic in Operating Systems
Memory management is how an operating system allocates main memory among processes and translates their logical addresses into physical ones. Early questions cover base and limit registers, contiguous allocation with first-fit, best-fit and worst-fit, and internal versus external fragmentation. The core of the topic is paging: splitting an address into page number and offset, page tables, multilevel and inverted page tables, and effective access time with a TLB. Virtual memory questions ask about demand paging, page faults, and page-replacement algorithms such as FIFO, Optimal and LRU, including Belady's anomaly. Segmentation, thrashing and the working-set model complete the usual syllabus.
Below are 30 practice questions from a pool of 210 Memory Management MCQs, one of 12 topics in Operating Systems. Each shows the correct answer with an explanation; when you are ready, take a timed quiz to test recall under exam conditions.
Each question below shows the correct answer with a full explanation. Use these to build conceptual understanding before attempting a timed quiz.
Q1. Paging divides:
Paging divides physical memory into fixed-size blocks called frames and logical memory into blocks of the same size called pages. A page table maps pages to frames.
Q2. The FIFO page replacement algorithm replaces:
FIFO replaces the oldest page in memory (the one that was loaded first). It is simple but can suffer from Belady's anomaly where more frames can lead to more page faults.
Q3. Belady's anomaly states that:
Belady's anomaly is the counterintuitive observation that for some algorithms (notably FIFO), increasing the number of available page frames can actually increase the number of page faults.
Q4. In demand paging, pages are loaded into memory:
Demand paging loads a page into memory only when it is referenced (accessed). This is a lazy approach that avoids loading pages that may never be used, saving memory.
Q5. Virtual memory allows:
Virtual memory allows the execution of programs that are larger than physical memory by keeping only necessary portions in RAM and storing the rest on disk, swapping pages as needed.
Q6. A page fault occurs when:
A page fault occurs when a process tries to access a page that is mapped in its virtual address space but is not currently loaded in physical memory, requiring the OS to load it from disk.
Q7. Thrashing occurs when:
Thrashing occurs when a process does not have enough frames and constantly pages in and out. The system spends most of its time handling page faults instead of useful computation.
Q8. The first-fit memory allocation algorithm:
First-fit allocates the first memory hole that is large enough to satisfy the request. It is fast because it doesn't need to search the entire list.
Q9. Physical address is:
A physical address is the actual address in the physical memory hardware. It is the address that appears on the memory address bus and is used to access data in RAM.
Q10. The Translation Lookaside Buffer (TLB) is:
The TLB is a small, fast associative cache that stores recently used page table entries. It reduces the time for address translation by avoiding page table lookups for frequently accessed pages.
Q11. What is a multi-level page table and why is it used?
Multi-level page tables use a hierarchy of page tables (e.g., outer and inner page tables). Only the outer table must be in memory; inner tables are created only for used portions of the address space, saving memory.
Q12. Logical address is:
A logical (virtual) address is generated by the CPU during program execution. It is translated to a physical address by the Memory Management Unit (MMU) before accessing actual memory.
Q13. Internal fragmentation occurs when:
Internal fragmentation occurs when the allocated memory block is larger than what was requested (e.g., due to fixed-size allocation units), wasting the difference.
Q14. Segmentation divides a program into:
Segmentation divides a program into logical units (segments) of variable sizes, such as main program, functions, data structures, stack, and library functions, reflecting the program's logical structure.
Q15. External fragmentation occurs when:
External fragmentation occurs when total free memory is sufficient but it is not contiguous. Small, scattered free blocks exist but none is large enough to satisfy a request.
Q16. The Memory Management Unit (MMU) is responsible for:
The MMU is a hardware component that translates logical (virtual) addresses generated by the CPU to physical addresses in memory at runtime.
Q17. The LRU (Least Recently Used) page replacement algorithm replaces:
LRU replaces the page that has not been referenced for the longest period of time, based on the assumption that pages used recently will be used again soon (temporal locality).
Q18. The optimal page replacement algorithm (OPT) replaces:
OPT replaces the page that will not be used for the longest period in the future. It produces the fewest page faults but is not implementable in practice since future references are unknown.
Q19. What is the working set model and how does it relate to thrashing?
The working set model estimates the set of pages a process actively uses within a recent time window. If the system cannot allocate enough frames to hold a process's working set, that process will thrash.
Q20. A page table stores:
A page table maps each logical page number to the corresponding physical frame number in memory, enabling the translation of logical addresses to physical addresses.
Q21. The best-fit memory allocation algorithm:
Best-fit searches for the smallest hole that is large enough to satisfy the request. It produces the smallest leftover fragment but requires searching the entire list and can create many small unusable fragments.
Q22. Main memory (RAM) is:
Main memory (RAM) is volatile storage directly accessible by the CPU. Programs must be loaded from secondary storage into main memory to be executed.
Q23. What is an inverted page table?
An inverted page table has one entry per physical frame (not per virtual page). Each entry contains the process ID and virtual page mapped to that frame. It saves space but makes lookups slower without hashing.
Q24. What is the slab allocator used in Linux kernel memory management?
The slab allocator creates caches of pre-allocated objects of specific sizes (e.g., PCBs, file descriptors). Objects are allocated from and returned to slabs, minimizing allocation/deallocation overhead and fragmentation.
Q25. What is the buddy system for memory allocation?
The buddy system allocates memory in blocks of 2^n. A block is split into two buddies (halves) to satisfy smaller requests. When both buddies are free, they are coalesced back into a larger block.
Q26. What is copy-on-write in the context of virtual memory?
Copy-on-write allows parent and child processes to share the same pages after fork(). When either process modifies a page, the OS creates a private copy of that page, avoiding unnecessary duplication.
Q27. What are huge pages (large pages) and when are they beneficial?
Huge pages use page sizes larger than standard (2MB or 1GB vs 4KB), reducing TLB misses and page table size. They benefit applications with large memory footprints like databases and scientific computing.
Q28. What is the page-fault frequency (PFF) approach to preventing thrashing?
PFF monitors each process's page-fault rate. If the rate exceeds an upper bound, more frames are allocated. If it falls below a lower bound, frames are removed. This directly controls thrashing.
Q29. What is the second-chance (clock) page replacement algorithm?
The second-chance algorithm enhances FIFO by checking each page's reference bit. If set, the bit is cleared and the page is given another chance. If not set, the page is replaced. It approximates LRU.
Q30. What is memory-mapped I/O?
Memory-mapped I/O maps device registers or files into the virtual address space. Programs access them using standard load/store instructions instead of special I/O instructions, simplifying programming.
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