 Many non @-@ Sinclair peripherals aimed to remedy the ZX81 's flaws and provide many new capabilities . These included RAM packs providing up to 64 KB of extra memory and promising to " fit snugly ... giving a firm connection " to the computer , typewriter @-@ style keyboards , more advanced printers and sound generators , and even a hard disk interface , which Clive Sinclair thought was " quite overgilding the lily " . A wide range of software was also published . Within only a year of the ZX81 's launch , around 200 independent companies had been established to manufacture and sell Sinclair @-@ compatible hardware . The people behind the ZX81 cottage industry were very often not computer professionals but were , as the Financial Times noted , " school teachers , civil servants , electrical engineers and technicians [ who ] have set up small operations in their own time " . 
