Call Gate

The Problem

Assume that the OS includes a code segment residing at privilege level zero (the most privileged level) containing a number of procedures to handle requests from other programs. Also assume that some of the procedures within this code segment should only be accessible by programs residing at privilege levels one and two. Any attempt to call one of these procedures in this code segment by a program residing at privilege level three (an applications program) should be rejected (i.e., should cause a GP exception). Also assume that some other procedures within the same OS code segment are designed to handle requests from applications programs (privilege level three).

The problem is that the code segment has one privilege level, ...

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