rozdíl pamětí

v programu dependency walker je
Physical Memory Total: 1 073 070 080 (1024 MB)
Physical Memory Used: 544 661 504
Physical Memory Free: 528 408 576
Page File Memory Total: 1 373 229 056
Page File Memory Used: 453 275 648
Page File Memory Free: 919 953 408
Virtual Memory Total: 2 147 352 576
Virtual Memory Used: 37 482 496
Virtual Memory Free: 2 109 870 080

Jaký je rozdíl mezi "page file memory" a "virtual memory" ?