An Operating Systems Vade Mecum

Paging 99example simple, these virtual machines are tiny: Each has a virtual main store of threepages. The figure shows the page tables maintained by V to manage the two virtualspaces, as well as a picture of main store as seen from V’s point of view. The only fieldsshown in the page tables are Page Frame and Present. Residing in P 1 is an ordinaryoperating system, which currently supports two processes, D 1 and D 2 . Together, theyoccupy all the main store available to P 1 . (In a more realistic example, we would have toleave some room in that store for P 1 ’s kernel as well.) Another operating system residesin P 2 , and it has only one client, process E 1 .Let us imagine that D 1 is currently executing. None of the five page tables shownis appropriate for the hardware to use. When D 1 refers to page 0, that should map topage 2 of P 1 , which is in page frame 6 of the main store. Similarly, D 1 ’s page 1 shouldmap to page 0 of P 1 , which is page 8 of main store. In other words, the proper page tablefor D 1 is a composite of the tables for D 1 and P 1 . In mathematical terms, this page tablecomposes the mappings specified by tables D 1 and P 1 . If P 1 were also a virtualizing kernel,more adjacent levels of the virtual-machine hierarchy would need to be composed.0 101012D 1virtual storeD 2virtual storeE 1virtual storeD1 page frame pres.0 2 Y1 0 YD0 1 21 1 0D 1 D 2 D 1P 1virtual store2 page frame pres.0n1 1 YE1012page frame120pres.YYY0 1 22 0 1E 1 E 1 E 1P 2virtual storeP 2P 1 page frame pres.page frame pres.0 8 Y0n12 6nY1242YY0 1 2 3 4 5 6 7 8 92 1 2 0P 2 P 2 P 1 P 1V physical storeFigure 3.17 Two levels of address translation