Initialised by the routine at NEW, read by the routines at ONE_SPACE, CLEAR, TEST_ROOM and STK_PNTRS, and updated by the routines at POINTERS, SET_MIN, E_LINE_NO, S_DECIMAL, S_FN_SBRN, STK_F_ARG, STK_ST_0, STK_FETCH, CALCULATE and STACK_NUM.