An Operating Systems Vade Mecum

92 Space Management Chapter 3the page frame that holds this pagea Boolean Present field (if false, this page is swapped out)We are ignoring fields used for page-replacement algorithms, such as a Dirty and a Usedfield for each page-table entry.Each such entry fits within one word, and a page table has at most 256 entries, sofour complete page tables can be stored within a single page frame. We will assume thatonly one page table is stored in a page frame, so a page-frame number is enough todescribe the location of a page table. Here is a formal description of the data structuresand the address translation algorithm:1 const2 BitsPerVirtualAddress = 36;3 BitsPerPhysicalAddress = 24;4 BitsPerSegmentAddress = 18;5 BitsPerVirtualPageAddress = 8;6 BitsPerOffsetAddress = 10;7 PageSize = 1024 = 2 10 ; { words per page }8 PageLimit = 256 = 2 8 ; { pages per segment }9 NumberOfPageFrames = 2 14 ; { page frames per physical store }10 SegmentLimit = 2 18 ; { segments per virtual space }11 type12 PageFrameNumber = 0 .. NumberOfPageFrames − 1;13 AccessType = (Read, Write, Execute, Stack);14 SegmentTableEntry =15 record { fits in one word }16 PageTablePointer : PageFrameNumber;17 Size : 1 .. 256; { number of pages }18 Permissions : set of AccessType;19 Present : Boolean;20 end;21 PageTableEntry =22 record { fits in one word }23 Location : PageFrameNumber;24 Present : Boolean;25 end;26 var27 DescriptorBaseRegister : SegmentTableEntry; { in hardware }