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Unformatted text preview: e can be executed. Buffer overﬂow attacks exploit this omission by loading and running code directly on the user stack (Section 3.13). If there were such an execute bit, then the kernel could eliminate the threat of such attacks by restricting execute privileges to the read-only code segment. End Aside. Pentium Page Table Translation
Figure 10.26 shows how the Pentium MMU uses the two-level page table to translate a virtual address to a physical address. The 20-bit VPN is partitioned into two 10-bit chunks. VPN1 indexes a PDE in the page directory pointed at by the PDBR. The address in the PDE points to the base of some page table that is indexed by VPN2. The PPN in the PTE indexed by VPN2 is concatenated with the VPO to form the physical address.
10 10 12 VPN1 VPN2 VPO Virtual address
word offset into physical and virtual page word offset into page directory word offset into page table page directory page table PTE PDE PDBR physical address of page directory physical address of page table base (if P=1)
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