iAPX 286 Operating System Writers Guide 1983

USING HARDWARE PROTECTION FEATURESTI - TABLE INDICATORRPL - REQUESTED PRIVILEGE LEVEL121960-50Figure 2-10. Format of a SelectorINDEXThe index field of a selector specifies a descriptor in either the GDT or the task's LDT. The index fieldmay take on values from 0 through n -I, where n is the number of descriptors in the table. Theprocessor compares the index with the limit of the descriptor table to ensure that the index refers to adefined descriptor.TABLE INDICATORThis bit item tells which descriptor table is indexed by the selector. A value of zero specifies the GDT;one specifies the LDT. (The IDT cannot be referenced via a selector; only via an interrupt identifier.)REQUESTED PRIVILEGE LEVELSelector privilege is specified in the RPL field of a selector. Selector RPL may establish a less trustedprivilege level than the current privilege level for the use of that selector. RPL cannot effect an increasein privilege. A task's effective privilege level is the numeric maximum of the RPL and the currentprivilege level. For example, if a task is executing at PL = 2, an RPL = 3 reduces the task's effectiveprivilege to level 3 for access to that segment. On the other hand, if RPL = I, the task's effectiveprivilege level remains at 2.RPL is generally used by an operating system to ensure that selector parameters passed to the moreprivileged levels of the operating system do not give access to data at a level more privileged than thecalling procedure. The RPL field is a convenient placeto store the privilege level of the procedure thatoriginated the selector. Any use of the selector can be restricted to the usage allowed its originator.The ARPL instruction (ADJUST$RPL built-in function in PL/M-286) allows the operating system toset the RPL of a selector either to CPL or to the privilege level of the originator, whichever is (numerically)larger. Refer to Chapter 13 for more information on the use of RPL.2-16 121960-001