 The ZX81 's ROM was doubled to 8 KB , from the ZX80 's 4 KB ROM . This enabled a fuller implementation of a version of ANSI Minimal BASIC ( termed Sinclair BASIC by the company ) . Clive Sinclair re @-@ commissioned a company called Nine Tiles , which had produced the ZX80 ROM , to develop the new ROM software for the ZX81 . The code was written by John Grant , the owner of Nine Tiles , and Steve Vickers , who had joined the company in January 1980 . Grant concentrated on the software that drove the ZX81 's hardware , while Vickers developed the new BASIC and the accompanying manual . Sinclair 's brief to the pair was fairly non @-@ specific but primarily concerned remedying a key defect of the ZX80 so that the new machine could be used for practical programming and calculations . Vickers later recalled : 
