These are almost identical to local variables, except that the name of the variable must start with the underscore character. This distinction in naming identifies the variable as global to all equates (any equate can read or write the value at any time, although it must always be ensured that the variable is written to before any read takes place). System variables are simply a special case of global variable where the variable has already been declared with a value generated by parsing the GRG file.
System variables maintain their type and will complain if a different data type from that which they were defined with is written into them.
The system variable whose name is the underscore character alone represents
the system stack pointer. So doing 0>>_ for example would clear
the stack.
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