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 use them all to squeeze more functions in. 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. 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: The new ROM incorporated trigonometric and floating-point functions, which its predecessor had lacked – the ZX80 could only deal with whole numbers. Grant came up with one of the ZX81's more novel features, a syntax checker that indicated errors in BASIC code as soon as it was entered (rather than, as was standard at the time, only disclosing coding errors when a program was run). Unfortunately for Vickers, he introduced a briefly notorious error – the so-called "square-root bug" that caused the square root of 0.25 to be returned erroneously as 1.3591409 – as a result of problems with integrating the ZX Printer code into the ROM. Although it was eventually fixed, the bug became the subject of controversy and Sinclair was forced to replace some of the ZX81s sold to early customers. On a more positive note, Vickers' work on the manual was received favourably, being described in 1983 as "one of the classic texts on BASIC". Max Phillips commented in a What Micro? retrospective: The task of designing the ZX81's case again fell to Rick Dickinson, who produced an updated version of the ZX80's wedge-shaped case. This time round, the design team were