To transfer program control directly to another code segments without privilege level change, the target procedure entry point is specified in the far form of a call/jmp instruction. The processor performs various privilege level checking before loading new data to the CS and EIP registers. Involved privilege level fields are:
- CPL (the privilege level of current code segment which contains the source call or jmp instruction)
- DPL (the privilege level of the target code segment descriptor which contains the target procedure)
- RPL (the requestor’s segment selector privilege level in the call or jmp instruction)
System software executive that needs to be protected from user privilege codes are placed in the non-conforming code segments. Execution cannot be transferred to a less privileged code segment directly through direct call or jump; otherwise a general exception will be asserted by the processor.
Some type of exception handlers (e.g., divide-error or overflow) and system software components that don’t have to access protected facilities (e.g., math libraries) could be loaded in conforming code segments. They are executed in higher privilege levels while keeping the CPL unchanged, which prevents it from accessing more privileged data. In this way, system overhead in privilege level change is alleviated.
There is no CPL change in either conforming or non-conforming form of direct call or jump to a target code segment. Since the CPL does not change, no stack switch occurs.
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