PPT Slide
Clock Page Replacement Algorithms
- Basic Clock algorithm:
- Hand looks at reference bit of current page frame. If ref bit == 0, this page hasn’t been referenced and it is selected to be replaced.
- If ref bit == 1, this page has been referenced. Give it a another chance by setting the ref bit to 0 and continue looking for a page frame where the ref bit == 0. Notice that this insures eventually a free page will be found when the clock hand rolls back around.
- A page has to get referenced at least once every revolution of the clock hand to keep from getting replaced.
- This is a modified FIFO algorithm (called First In, Not Used, First Out -- FINUFO or Not Recently Used -- NRU).
- One clock alg had to look an average of 13 page frames in order to find one to replace.