The total size of address space in a virtual memory system is limited by: A. the length of MAR B. the available secondary storage C. the available main memory D. all of the above E. none of the above
GATE 1991 · Computer Organization and Architecture · Virtual Memory · medium
Answer: The total size of virtual address space is limited by the length of the MAR (option A).
- Analyze the role of MAR: The CPU generates a virtual address and places it in the MAR for translation. The MAR's bit-width is the hardware constraint on the maximum address value. No virtual address larger than 2^(MAR bits) - 1 can be generated. So MAR length directly limits address space.
- Check secondary storage and main memory: Secondary storage (disk) holds swapped-out pages but does not define how large an address can be. A program could have a large virtual address space with only a small amount of disk backing (some pages just can't be loaded). Main memory (RAM) is where pages reside when active; even if RAM is small, the virtual space is not limited by it — only performance is affected (more page faults). So neither B nor C limits virtual address space size.
- Select the correct answer: Only the MAR length (option A) limits the total size of the virtual address space. Options B (secondary storage) and C (main memory) are not fundamental constraints on address space size. Option D (all of the above) is incorrect. The correct answer is A.