Read by the routine at ED_ENTER, and updated by the routines at ERROR_2, ED_DOWN, ED_ERROR, ED_COPY, MAIN_EXEC, LINE_SCAN and INPUT. The IY register is initialised to 23610 by the routine at NEW, and reinitialised by the routine at STACK_BC. This system variable is accessed by some routines via the IY register at IY+0.