page replacement policy (operating systems software)
  (cache management, cpu hardware)

          FIFO
PFN| 0 1 0 2 0 3 0 1 0 2 0 3    (these are VPN's)
---|--------------------------
  0| 0 0 0 0 0 3 3 3 3 2 2 2
  1|   1 1 1 1 1 0 0 0 0 0 3
  2|       2 2 2 2 1 1 1 1 1
     x x   x   x x x   x   x


          LRU
  |  0 1 0 2 0 3 0 1 0 2 0 3
-----------------------------
 0|  0 0 0 0 0 0 0 0 0 0 0 0
 1|    1 1 1 1 3 3 3 3 2 2 2
 2|        2 2 2 2 1 1 1 1 3
    x  x   x   x   x   x   x


          OPTMAL
  |  0 1 0 2 0 3 0 1 0 2 0 3
-----------------------------
 0|  0 0 0 0 0 0 0 0 0 0 0 0
 1|    1 1 1 1 1 1 1 1 2 2 2
 2|        2 2 3 3 3 3 3 3 3
     x x   x   x       x







         FIFO
  | 0 1 2 3 0 1 2 3 0 1 2 3
----------------------------
 0| 0 0 0 3 3 3 2 2 2 1 1 1
 1|   1 1 1 0 0 0 3 3 3 2 2
 2|     2 2 2 1 1 1 0 0 0 3
    x x x x x x x x x x x x


         LRU
  | 0 1 2 3 0 1 2 3 0 1 2 3
----------------------------
 0| 0 0 0 3 3 3 2 2 2 1 1 1
 1|   1 1 1 0 0 0 3 3 3 2 2
 2|     2 2 2 1 1 1 0 0 0 3
    x x x x x x x x x x x x


         OPTMAL
  | 0 1 2 3 0 1 2 3 0 1 2 3
----------------------------
 0| 0 0 0 0 0 0 0 0 0 1 1 1
 1|   1 1 1 1 1 2 2 2 2 2 2
 2|     2 3 3 3 3 3 3 3 3 3
    x x x x     x     x