 The development of the ZX81 got under way even before the ZX80 had been launched . Sinclair 's chief engineer , Jim Westwood , was given the task of improving the ZX80 's hardware to reduce the number of components and thus bring down the cost . He also sought to fix some of the more annoying problems with the ZX80 . Westwood and his colleagues found that the component count could be reduced greatly by combining eighteen of the ZX80 's chips into a single uncommitted logic array ( ULA ) , a type of general @-@ purpose chip that allowed manufacturers to reprogram it to meet their particular requirements rather than having to develop their own customised chip at greater expense . Ferranti produced the new chip for Sinclair , who hailed Westwood 's design as a triumph of innovation : " The ZX81 had four chips when our nearest competitor in this respect , the TRS @-@ 80 , had 44 . " Only 70 % of the logic gates on the ULA were supposed to be used , but Sinclair decided to obtain maximum benefit by using them all . This resulted in the machine becoming uncomfortably warm during usage . Computing folklore held that the ZX81 had to be refrigerated by balancing a carton of cold milk on top of the case . 
