Ucall causes the processor priority to be set to priority and the specified kernel function to be called with
arguments arg0 and arg1. Priority is one of PSL_BR0, ..., PSL_BR7. Processor priority is reset to PSL_BR0
when function returns.
Ucall is allowed only if the user is the superuser. It is obviously extremely dangerous if misused. It's
only current use is at system boot time to configure system devices by calling device drivers ...