NIELIT 2017 DEC Scientist B - Section B: 16

Question

Which of the following is added to the page table in order to track whether a page of cache has been modified since it was read from the memory?

• A. Reference bit
• B. Dirty bit
• C. Tag bit
• D. Valid bit

Answer 1

A dirty bit (or) modified bit is a bit that is associated with a block of computer memory and indicates whether or not the corresponding block of memory has been modified. The dirty bit is set when the processor writes to (modifies) this memory.
→ The bit indicates that its associated block of memory has been modified and has not been saved to storage yet. When a block of memory is to be replaced, its corresponding dirty bit is checked to see if the block needs to be written back to secondary memory before being replaced or if it can simply be removed.
→ Dirty bits are used by the CPU cache and in the page replacement algorithms of an operating system.
→ Dirty bits can also be used in Incremental computing by marking segments of data that need to be processed or have yet to be processed.

Answer 2

option B )Dirty Bit is used to track modification of a page.

                If page is modified Dirty bit=1, else Dirty bit =0