iAPX 286 Operating System Writers Guide 1983

inter INTRODUCTION TO PROTECTED MULTITASKING ON THE iAPX 286Applied to procedures, privilege level is a measure of the degree to which you trust a procedure not tomake a mistake that might affect other procedures or data. Applied to data, privilege level is a measureof the protection you think a data structure should have from less trusted procedures.Privilege level also applies to instructions. Certain instructions (those that deal with system tables,interrupts, and I/O, for example) have such an effect on the system as a whole that only highly trustedprocedures should have the right to use them.Levels of SegmentsWith regard to privilege, you can view the segments of a task as being grouped into four levels. Levelzero is for the most privileged procedures and the most private data; level three is for the least trustedprocedures and the most public data. You (or your operating system) associate each segment of eachtask with one of these four levels of privilege. The privilege level of a segment applies to all the proceduresor all the data in that segment. Figure 1-3 illustrates the logical segregation of segments intoprivilege levels. (Later chapters explain why operating-system segments are included within the task.)LDTPL~OPL~OPL~O}PRIVILEGELEVELoPRIVILEGELEVEL1PRIVILEGELEVEL2PRIVILEGELEVEL3PL~lPL~lPL~l}PL~lPL~2PL~2PL~2PL~2PL~2PL~3PL~3}~DATABASEMANAGER~~PL~3PL~3PL~3PL~3PL~3121960-41Figure 1-3. Segment Segregation by Privilege Level within a Task1-4121960-001