What is the main function of a dirty bit in a memory system?

The main function of a dirty bit in a memory system is to show if a page has been written to since it was loaded into memory. When a page is loaded into memory, it is marked as clean by the dirty bit. If any modifications are made to that page, the dirty bit is set to indicate that the contents of the page in memory do not match the corresponding page in secondary storage (e.g., a hard drive).

This is crucial for managing memory efficiently, especially when it comes time to swap pages between physical RAM and disk storage. If a page is clean, it can be easily replaced without writing it back to disk, which saves processing time and I/O bandwidth. In contrast, if a page is dirty, the system knows it must write it back to the disk before replacing it, ensuring that changes are not lost. The dirty bit thus plays a key role in optimizing memory management and maintaining data integrity within a computer system.