In addition to a reference bit, a modify bit can be added to form the bit pair (reference, modify) associated with a page to improve the second-chance algorithm used in page replacement, where the reference bit is 1 if the page has been referenced and the modify bit is 1 if the page has been updated in the past round. Divide the pages into four classes: (0,0), (0,1), (1,0), (1,1).