iAPX 286 Operating System Writers Guide 1983

USING HARDWARE PROTECTION FEATURESo 7oRESERVED FOR iAPX 386MUST BE ZERO+6PDPL1 0ED W ABASE23·16+4BASE,s_o+2LIMIToP - PRESENT BITDPL - DESCRIPTOR PRIVILEGE LEVELED - EXPANSION DIRECTIONW -WRITABLEA - ACCESSED121960-46Figure 2-2. Data Segment Descriptorreset this bit as program segments are brought into or eliminated from memory. Reference to a segmentwhose present bit is reset causes a fault, providing an opportunity for the operating system to load thesegment from virtual store. (Chapter 9 takes up implementation of virtual rhemory systems.) In systemsthat do not implement virtual memory, this bit is always set for allocated segments.DESCRIPTOR PRIVILEGE LEVELThe value of this item defines the privilege level of the segment addressed by this descriptor. Youcontrol the values in the descriptor privilege level (DPL) by the parameters you give to the builderwhen creating a static system or the resident portion of a dynamic system, or by the procedures youroperating system uses when loading segments dynamically.INTEL RESERVEDThis portion of the descriptor is reserved by Intel and should always be initialized with zeros. Otheruse of this field in a valid descriptor will prevent compatability with the iAPX 386 and other additionsto Intel's family of processors.2-3 121960-001