Why Have a Call Gate?

• The caller must be forced to use a designed entry point to prevent entry at an arbitrary location; for example, at a point after the parameters have been validated. This might cause the operating system to violate its own integrity or that of another application.

• The parameters, as well as the rest of the stack, must be protected from the application while in use by the operating system to prevent changes by another thread in that application.

• The return address must be protected from the application while the operating system is running to prevent other threads of the application from altering it in a way that would cause a return to the application in a privileged mode.

Note: A CALL GATE implements all of the above requirements.

Note: A CALL GATE is a system descriptor which describes an entry point in a more privileged program which is accessible to less privileged programs.