In virtual memory management, what is the difference between demand paging and pre paging strategies for loading pages into memory?

Introduction / Context:
Virtual memory systems map virtual pages to physical frames and use page fault handling to bring pages into memory. The operating system can choose different strategies for when to load pages. Demand paging and pre paging represent two contrasting approaches. Understanding them is important when analyzing performance and fault behavior.

Given Data / Assumptions:

• Processes are larger than physical memory, so only some pages are in memory at any time.
• A page table records which pages are resident in memory.
• Page faults occur when a process references a page that is not currently loaded.

Concept / Approach:
Demand paging is the simplest and most common strategy. Under demand paging, the operating system loads a page into memory only when the process actually references that page. If the page is not present, a page fault is raised, the operating system reads the page from secondary storage into a free or replacement frame, updates the page table, and then restarts the instruction. Pre paging attempts to improve performance by anticipating which pages will be needed soon and loading them before they are referenced. When a page fault occurs, the system may bring in not only the faulting page but also several adjacent or related pages, based on locality of reference. If predictions are good, pre paging can reduce the total number of faults, but incorrect predictions waste I or O bandwidth and memory.

Step-by-Step Solution:
Step 1: Define demand paging as loading pages lazily only when referenced.Step 2: Understand that demand paging does not read in pages speculatively; each missing page triggers a fault.Step 3: Define pre paging as proactively loading additional pages beyond the one that caused the fault, based on predicted future access patterns.Step 4: Recognize that both strategies operate in the context of virtual memory systems and use the same underlying hardware support.Step 5: Select the option that clearly contrasts loading on demand with loading pages in advance to reduce faults.

Verification / Alternative check:
Many operating systems use variations of prefetching in the file system and paging system. For example, sequential file access may trigger read ahead of blocks that have not yet been requested. Similarly, some virtual memory systems may pre page pages when a process is started or when it exhibits strong locality. Textbooks describe demand paging as the baseline approach and pre paging as an optimization. These descriptions match the explanation given in the correct option.

Why Other Options Are Wrong:
Option B: Confuses the granularity of loading. Neither demand nor pre paging requires loading entire processes at once.Option C: Suggests that demand paging is used without virtual memory, which contradicts the definition because demand paging relies on virtual memory mechanisms.Option D: Confuses memory levels, implying that pages are loaded into cache or secondary storage, which is not how paging works. Paging moves data between secondary storage and main memory.

Common Pitfalls:
One pitfall is assuming that pre paging always improves performance. If the access pattern is not predictable, pre paging can increase I or O operations without reducing faults, wasting resources. Another mistake is to think that demand paging is an outdated or naive approach. In practice, demand paging combined with good replacement policies and hardware support often performs very well.

Final Answer:
The correct answer is Demand paging loads a page into memory only when it is actually referenced and causes a page fault, while pre paging loads some pages in advance based on predicted future use to reduce the number of page faults.