Virtual memory allows the execution of processes that are not entirely in memory by using:

Question

BlackTom AI · Accepted Answer

Demand paging loads only the needed pages of a process into physical memory on demand, so it is the mechanism that lets execution proceed when the entire process is not resident.

1) Relevant concepts:
- Virtual memory: abstraction that gives each process the illusion of a large contiguous address space backed by disk (backing store) and physical RAM.
- Paging: division of virtual address space and physical memory into equal-sized pages/frames.
- Demand paging: pages are brought into RAM only when a process references them (causing a page fault).
- Page table / TLB: map virtual pages to physical frames and track presence bits.
- Swapping vs. demand paging: swapping moves entire processes (or large regions) to/from disk; demand paging moves individual pages.

2) Step-by-step reasoning:
- A process’s virtual address space may contain many pages; RAM has far fewer frames.
- When the CPU references a virtual address, the MMU checks the page table. If the page’s present bit = 0, a page fault occurs.
- The OS services the page fault by locating the page on disk, choosing a victim frame if needed, loading the required page into that frame, updating the page table, and resuming the process.
- Because only referenced pages are loaded, a process can start and run even though most of its pages remain on disk. Over time, only commonly used pages occupy RAM.

3) Example (intermediate steps):
- Process with 10 pages, RAM has 4 frames. On first run, only page 0 is accessed → page fault → load page 0. Later accesses to pages 3, 7, 1 cause additional page faults and bring those pages in. The process executes while many pages still reside on disk.

4) Why the correct answer is correct:
- Demand paging is explicitly designed to allow execution without the whole process in memory by loading pages on demand.

5) Why other choices are incorrect:
- Contiguous allocation: a scheme for placing a process in one continuous block of physical memory; it does not support partial residency or on-demand loading.
- Compaction: a memory-defragmentation technique to create larger free blocks; it does not enable execution when parts of a process are on disk.
- Swapping: moves entire processes to/from disk; it can free/restore whole processes but does not provide fine-grained on-demand loading of pages.

Conclusion: The correct answer is a. Demand Paging, which matches the provided answer.

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