iAPX 286 Operating System Writers Guide 1983

USING HARDWARE PROTECTION FEATURES(from the base address toward the segment limit value that is also contained in the descriptor). A valueof one indicates that the segment expands downward (from offset FFFFH toward the limit).WRITABLEThis field applies only to data segment descriptors. A value of one permits the CPU to write into thesegment; a value of zero protects the segment from attempts to write into it. Translation tables are butone example of data that deserve the extra protection afforded by storage in a read-only segment.CONFORMINGThis field applies to executable-segment descriptors only. Ordinarily (when the conforming bit is zero)a called procedure executes at the privilege level defined by the DPL in the descriptor of the segmentin which the procedure resides. When the conforming bit of the called segment is set, however, thecalled procedure executes at the calling procedure's privilege level. This feature cannot be used todecrease (numerically) a segment's privilege level below that defined by it's DPL. Figure 2-5 showsgraphically how a conforming segment works. This feature is useful when you want to make procedures(mathematical subroutines or run-time support procedures, for example) available to a number of otherprocedures running at different privilege levels, but when you do not want to provide increased privilegewhile the subroutine is executing.Figure 2-5. Calling a Conforming Segment121960-72-6 121960-001