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Vol. 3 5-3
PROTECTION
procedure. The term current privilege level (CPL) refers to the setting of this
field.
• User/supervisor (U/S) flag — (Bit 2 of paging-structure entries.) Determines
the type of page: user or supervisor.
• Read/write (R/W) flag — (Bit 1 of paging-structure entries.) Determines the
type of access allowed to a page: read-only or read/write.
• Execute-disable (XD) flag — (Bit 63 of certain paging-structure entries.)
Determines the type of access allowed to a page: executable or not-executable.
Figure 5-1 shows the location of the various fields and flags in the data, code, and
system- segment descriptors; Figure 3-6 shows the location of the RPL (or CPL) field
in a segment selector (or the CS register); and Chapter 4 identifies the locations of
the U/S, R/W, and XD flags in the paging-structure entries.