Paging algorithm

Today I implemented three paging algorithms: FIFO, Second-Chance(SC), and LRU.

There is some different and heuristic methods when doing SC and LRU.

1. When doing SC, if we found the oldest has been referenced, then it can be either moved to the memory list tail or stay there. and both methods we will have to set the oldest page's referenced bit to false and its load time current time and search for the next oldest.

2. When doing LRU(NFU for software simulation), I think the length of aging will be the  clock tic between the next set zero. Because when all the aging bits are set, it is hard to  judge which one is older.