At which point is the Start located?

See if something like this will work:

CONST FALSE = 0
DIM swch AS INTEGER, chng AS INTEGER, lg AS INTEGER
swch = FALSE
chng = swch
CLS
DO
   swch = NOT swch
   IF chng <> swch THEN lg = lg + 1
LOOP UNTIL swch = chng
PRINT lg
END


However, if you place DO above the swch = FALSE line, you will get an overflow error.