a page-table entry

If the page table is the manager's master directory, a page-table entry is a single row in it — one record describing one virtual page. Each row packs together everything the hardware needs to know to translate that page and to police access to it. Understanding what bits live in a PTE is understanding, in miniature, the whole virtual-memory contract.

Concretely, a page-table entry typically holds a physical frame number (where the page lives, if it is present) together with a set of status and control bits. The most important is the valid (or present) bit: if it is 0, the page is not in memory and any access faults. Protection bits say whether the page may be read, written, or executed (and whether a user-mode program may touch it at all). A dirty bit records whether the page has been modified since it was loaded (so the OS knows it must be written back to storage before being evicted). A reference (or accessed) bit records whether the page was used recently, which feeds replacement decisions. Some bits also control caching behaviour for that page.

Why it matters: the PTE is the atom of virtual memory — every protection check, every fault, every replacement decision reads or writes bits in a PTE. It is also where the hardware and OS meet most intimately: the hardware reads the frame number and checks the valid and protection bits automatically on each access, while the OS sets all the bits and clears reference bits periodically to track usage. A subtle but important point: the hardware updating the dirty and reference bits must be done carefully so the OS sees a consistent view.