iAPX 286 Operating System Writers Guide 1983

USING HARDWARE PROTECTION FEATURES• Processor-detected exception condition• Interrupt return instruction (IRET)Control transfers within the same privilege level may be either short (within same segment) or long (toanother segment). A short transfer simply specifies the offset of the instruction to which control istransferred in the same segment. A long transfer also uses a selector to identify the segment to whichcontrol is transferred.For control transfer to a different privilege level or different task, the iAPX 286 introduces gatedescriptors.Gate DescriptorsA gate descriptor is a type of descriptor used only for transferring control flow to instructions in anothersegment. Gates provide an indirect reference that is useful for binding and protection purposes. Byrequiring interlevel and intertask control transfers to reference gate descriptors, the iAPX 286 providestwo additional protection features:1. You can hide a procedure by not providing a gate for its entry point.·2. You can control access to a procedure via the privilege assigned to the gate. This allows hidingcritiCal procedures from untrusted software.Figure 2-6 illustrates the format of a gate descriptor.DESTINATION SELECTORFor call, interrupt, and trap gates, this field contains a selector for the segment descriptor of the destinationexecutable segment. For task gates, the selector in this field points to a descriptor for a taskstate segment, and the RPL field is not used.DESTINATION OFFSETFor call, interrupt, and trap gates, this field contains the offset of the entry point within the destinationexecutable segment (not used with task gates).WORD COUNTFor each privilege level within a task, there is a separate stack~ For calls through a call gate, theprocessor automatically copies parameters from the stack for the calling procedure's privilege level tothe stack for the destination's privilege level. In this field, you specify the number of words to copy.GATE TYPEFor gate descriptors, the type field distinguishes amQngthe four kinds of gates:o Call gate1 Task gate2 Interrupt gate3 Trap gate2-8 121960-001