of the system, has a number of key functions that competing computers share between multiple chips and integrated circuits. These comprise the following: Synchronising the screen display; Generating a 6.5 MHz clock, from which a 3.25 MHz clock is derived for the processor; Outputting an audio signal to a cassette recorder in SAVE mode; Processing the incoming cassette audio signal in LOAD mode; Sensing keystrokes; Using memory addresses provided by the CPU to decide when ROM and RAM should be active; Controlling general system timing. The ZX81's built-in RF modulator can output a video picture to a UHF 625-line television (used in the UK, Australia, and most western European countries). France and Luxembourg required a slightly modified version of the machine to match the positive video modulation of CCIR System L sets, while the US and Canada required a different ULA chip and modulator to cope with their 525-line VHF (NTSC) television systems. Both the ZX81 and its predecessor, the ZX80, have a significant drawback in the way that they handle visual output. Neither machine has enough processing power to run at full speed and simultaneously maintain the screen display. On the ZX80, this means that the screen goes blank every time the machine carries out a computation and causes an irritating flicker whenever a shorter computation – such as processing a keystroke – takes place. 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. Another hardware quirk produced one of the most distinctive aspects of the ZX81's screen display – during loading or saving, moving zigzag stripes appear across the screen. The same pin on the ULA is used to handle the video signal and the tape output, producing the stripes as an interference pattern of sorts. The ULA cannot maintain the display during SAVE and LOAD operations, as it has to operate continuously to maintain the correct baud rate for data