Consider a demand paging memory management system with 32-bit logical address, 20-bit physical address, and page size of 2048 bytes. Assuming that the memory is byte addressable, what is the maximum number of entries in the page table?
ANSWER
The maximum number of page table entries is , option (A).
Official answer: ASource: Official answer key, GATE 2025 CS1 (IIT Roorkee)
A page table has one entry for each page of the logical (virtual) address space, so the number of entries is the number of pages, .
The logical address is 32 bits and memory is byte addressable, so the logical address space is bytes.
The page size is bytes, so the offset takes 11 bits.
The page number therefore takes bits.
Number of page table entries .
The 20-bit physical address only fixes the number of frames, . It does not change the number of entries, because the table is indexed by page number, not by frame number. Demand paging only means some entries may be marked invalid, and the table still has one entry per page.
Hence the answer is option (A), .
KEY IDEA
The page table is indexed by virtual page number, so its size is . The physical address width does not affect it.
WATCH OUT FOR
Using the 20-bit physical address and computing (the number of frames). Another slip is picking because the physical address has 20 bits.
From the GATE 2025 CS1 paper. Every question of this paper · Download the full paper · All 10 Memory management questions, with notes
