when the system was sold outside the UK. Firmware The ROM contained the Sinclair BASIC programming language, editor, and operating system. BASIC commands were not entered by typing them out but were instead selected somewhat similarly to a programmable graphing calculator - each key had a few different functions selected by both context and modes as well as with the shift key. Expansion Other than the built-in cassette and video ports, the only provided means of expansion was a slot opening at the rear of the case, which exposed an expansion bus edge connector on the motherboard. The same slot bus was continued on the ZX81, and later the ZX Spectrum, which encouraged a small cottage industry of expansion devices, including memory packs, printers and even floppy drives. The original Sinclair ZX80 RAM Pack held either 1, 2 or 3 KB of static RAM and a later model held 16 KB of dynamic RAM (DRAM). Following the ZX81's release, a ZX81 8 KB ROM was available to upgrade the ZX80 at a cost of around 20% of a real ZX81. It came with a thin keyboard overlay and a ZX81 manual. By simply taking off the top cover of the ZX80 and prying the old ROM from its socket and carefully inserting the new ROM and adding the keyboard overlay, the ZX80 would now function almost identically to the proper ZX81 – except for SLOW mode, due to the differences in hardware between the two models. The process was easily reversed to return the ZX80 to its original configuration. One common modification by hobbyist users was to attach a full-size keyboard, optionally moving the motherboard into a larger case. This had the dual advantages of making the machine easier to type on, while increasing ventilation to the motherboard. Versions The UK version of the machine was the standard, and only changes that were absolutely necessary to sell units in other markets were made. In fact, the only real change made in most markets involved the video output frequency (the ZX80 used an external power transformer, so differences in AC line frequency and outlet were not an issue to the machine itself). One outcome of this is that the machine had some keyboard keys and characters that were distinctly British: was used instead of , instead of or , and the character set and keyboard included the Pound symbol. Reception The