 The ZX81 's designers adopted an improved approach , involving the use of two modes called SLOW and FAST respectively . In SLOW mode , also called " compute and display " mode , the ZX81 concentrates on driving the display . It runs the current program for only about a quarter of the time – in effect slowing the machine down fourfold , although in practice the speed difference between FAST and SLOW modes depends on what computation is being done . In FAST mode , processing occurs continuously , but the display is abandoned to its own devices – equivalent to the ZX80 's standard operating mode . 
