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Unformatted text preview: 10.7. CASE STUDY: THE PENTIUM/LINUX MEMORY SYSTEM 511 Figure 10.25(a) shows the format of a PDE. When È ½ (which is always the case with Linux), the address ﬁeld contains a 20-bit physical page number that points to the beginning of the appropriate page table. Notice that this imposes a 4-KB alignment requirement on page tables. Figure 10.25(b) shows the format
31 12 11 9 8 7 6 5 4 3 2 1 0 Page table physical base addr unused G PS A CD WT U/S R/W P=1 Field P R/W U/S WT CD A PS G PT base addr Description page table is present in physical memory (1) or not (0) read-only or read-write access permission user or supervisor mode (kernel mode) access permission write-through or write-back cache policy for this page table cache disabled (1) or enabled (0) has the page been accessed? (set by MMU on reads and writes, cleared by software) page size 4K (0) or 4M (1) global page (don’t evict from TLB on task switch) 20 most signiﬁcant bits of physical page table address (a) Page Directory Entry (PDE).
31 12 11 9 8 7 6 5 4 3 2 1 0 Page physical...
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