The EnterLeave() function is called whenever the virtual console on which the server runs is entered or left (for OSs without virtual consoles, the function is called when the server starts and again when it exits). The purpose of this function is to enable and disable I/O permissions (for OSs where such is required), and to unlock and relock access to ``protected'' registers that the driver must manipulate. It is a fairly trivial function, and can be implemented by following the comments in the stub driver.