This and the ZX80's lack of a mechanism to provide a hard, fixed resynchronisation of the CPU and the display beam explains one of its most obvious pecularities: the display is stable only unless and until the user types, since it can just repeat an exactly-counted fixed length loop. If the user types it branches into code that depends on which key they pressed, causing display generation temporarily to break up. On most TV's this was evident as the display jumping and bouncing, since it interrupts the sync feed.
The ZX81 adds both an [[NMI ]] generator and a WAIT generator that can allow the processor to get back into synchronisation after doing other work. The ZX81 therefore can offer a steady display but its processor is still available for non-display processing only during the vertical blank. Hence its stable display mode is known as 'SLOW' mode, and its ZX80-esque mode is 'FAST'.
The ZX81 also marks a substantial development in the machine's underlying BASIC, making it very similar to that which would ship a year later in the [[ZX Spectrum]].