Memory Management What if pgm mem > main mem ?

Memory Management What if pgm mem > main mem ? Overlays – program controlled

Memory Management What if pgm mem > main mem ? Virtual Memory – OS controlled (with architecture help)

Memory Management Separate physical, logical address space Page faults Demand paging

Memory Access in VM Is access legal ? (seg fault) If page in physical memory, return mem Else Find free page Schedule disk operation Perform disk operation Update page table Restart program at offending address

Memory Access Time Assume machine characteristics 200ns clock rate Main Mem access of 5 cycles  1000ns 25 milisecond page service time

Computing Average Access Time MAT = (1 – p) * p * 25,000,000 e.g. assume page fault rate of.001 MAT =.999 * * 25,000,000 = ,000  26x penalty

Your Turn Those were 1998 numbers Assume 2x speedup in clock speed every two years, 2x speedup in disk access time every 4 years What penalty would.001 page fault rate lead to in 2010? What page fault rate would allow MAT of 2x main memory access time.

Replacement Policy (Page) FIFO Random Optimal LRU LRU approximation

Sample References

FIFO – 4 pages

Optimal – 4 pages

LRU – 4 pages

Your Turn Show FIFO, LRU, Optimal for references above but with 3 pages.