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Dirty bit

Dirty bit

(also dirty flag)

Dirty bit definition

A dirty bit represents a solitary binary digit employed by computing systems to monitor whether a distinct data unit, like a cache line or a memory page, has undergone modifications since its most recent save. If the data undergoes alterations, the dirty bit switches to 1, signifying that the data necessitates being written back to the storage medium to preserve consistency. Should the dirty bit stay at 0, it indicates that the data remains unaltered and does not call for an update within the storage device.

See also: memory allocation

Dirty bit examples

  • Cache memory: When a CPU accesses data stored in cache memory, it checks the dirty bit to determine if the data has been modified. If it has, the CPU writes the updated data back to the main memory before replacing it with new data in the cache.
  • Virtual memory: Operating systems use dirty bits in their page tables to track which pages in memory have been modified. If a page is marked as dirty, it must be written back to disk when the page is swapped out.

Dirty bit vs. clean bit

A clean bit is the opposite of a dirty bit, representing that the data has not been modified since it was last saved. While a dirty bit is set to 1, a clean bit is set to 0.

Dirty bit pros and cons


  • Reduces unnecessary write operations, improving system performance and extending the lifespan of storage devices.
  • Enhances efficiency in cache and virtual memory management.


  • Requires additional storage to track the state of each unit of data.
  • Increases complexity in the system’s management of data consistency.

Dirty bit tips

  • Always use a reliable operating system with built-in mechanisms to manage dirty bits effectively.
  • Ensure regular maintenance of your system, including updates and backups, to minimize the chances of data inconsistency.

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