111. Tube indicator
     ~~~~~~~~~~~~~~
This short Function will return TRUE if a 6502 2nd processor is active, and
FALSE if it is not. It is a useful way of either warning the user to switch
the processor off, or of making other decisions within the program. Note that
X% is effectively set to zero simply by being made LOCAL; do not omit it,
even though it apparently does nothing.

   10 IF FNtube=TRUE THEN PRINT "2nd Processor is active"
   20 END
 1000 DEFFNtube:LOCAL A%,X%,Y%
 1010 A%=&EA:Y%=&FF:IF USR(&FFF4)AND&FF00 THEN =TRUE ELSE =FALSE