Initialised by the routine at MAIN_EXEC, read by the routines at ERROR_1, GET_CHAR, MAIN_ADD and FOR, and updated by the routines at CH_ADD_1, SKIP_OVER, POINTERS, EACH_STMT, E_LINE_NO, NEXT_LINE, INPUT, IN_ASSIGN, S_DECIMAL, S_FN_SBRN, STK_VAR and val.