= Solution
A <page table> entry normally supplies validity and access controls. A present/valid bit distinguishes a usable resident translation from an absent or unmapped page; an absent entry forces a <page fault>. A user/supervisor control restricts access to privileged execution or permits user access. Read/write control permits writing or enforces read-only access, protecting code, constants or shared mappings; an attempted forbidden write faults.
Where the architecture supports it, execute permission or a non-executable bit separates instruction fetch from data reads, so writable data need not be executable. Some processors offer separate read, write and execute controls; in the classic 32-bit non-PAE x86 layout matching the stated two-level geometry, there is no independent page execute-disable bit, and a read-only page can still be read and executed. Permissions of every level involved in the page walk must permit the access; the leaf entry cannot override an upper-level restriction.
Accessed/reference and dirty/modified bits are also expected, but they are management information rather than permission bits. They record use and writes for replacement and writeback decisions. Cache-control bits similarly govern memory behavior rather than access authorization. \b[Validity, privilege and write permission are fundamental; execute permission is architecture-dependent.]
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