GATE CS 1997 Q3.10 — Memory Management and Virtual Memory

The correct option is A.

Explanation:

A dirty bit (also known as a modified bit) is a status bit associated with each page in the page table. It is set by the hardware whenever any write operation is performed on that page in main memory.

• When a page needs to be replaced:
  • If the dirty bit is 0 (not modified), the page in memory is identical to its copy on the secondary storage (disk). Therefore, the operating system can simply overwrite the page frame without writing it back to the disk.
  • If the dirty bit is 1 (modified), the page has been updated. The operating system must write the modified page back to the disk before allocating the frame to another page.

Thus, the dirty bit helps significantly reduce disk I/O overhead by avoiding unnecessary write operations on the paging device.

Why other options are incorrect:

• Option B: LRU (Least Recently Used) information is typically tracked using a reference bit or access timestamps, not the dirty bit.
• Option C: Read-only protection is enforced using protection bits (read/write access rights), not the dirty bit.