To aid in debugging, the compiler uses an innovative approach to checking for undefined variables (an extremely common mistake by novice programmers). It uses a diagnostic feature of the 7040 that can deliberately set areas of memory to bad parity. When a program tries to reference variables that hadn't been set, the machine takes an interrupt (handled by the Watfor runtime routines) and the error is reported to the user as an undefined variable. This has the pleasant side effect of checking for undefined variables with essentially no CPU overhead.
