The State Of The Art
The 1970s made computer technology accessible to individuals at a relatively low budget but most still required manual assembly. However, in the late 70s, a number of pre-assembled microcomputers came on the market and in 1980, the Sinclair ZX80 arrived. This small home computer was available in the UK for under £100 ready-made or just under £80 in kit form.
Against A Dark Background
The ZX80 was built around a Zilog Z80 CPU (central processing unit) running at 3.25MHz. To keep costs low, a number of hardware compromises were made:
- rather than a full size keyboard, the ZX80 had a membrane keyboard where the keys were effectively decals with micro-contacts behind them
- the circuit board and case were both much smaller than other computers making the keyboard restrictive to use (and also contributed to a tendency for the computer to overheat)
- the ZX80 used simple hardware combined with software to generate the TV signal. This meant that if the CPU was busy doing something else such as handling a key press, the screen went blank
- the computer only had 4K ROM (read-only memory) which placed restrictions on the BASIC interpreter (see below)
- the computer also only had 1K RAM (random access memory) so by the time screen memory had been accounted for, this left very little memory for programs (384 bytes if a full screen of text was being displayed)
To work within the limits of the 4K ROM, other compromises were needed:
- the BASIC interpreter only supported integer arithmetic which meant it wasn’t suitable for the often suggested purpose of domestic accounting
- rather than typing BASIC commands directly, the keyboard had the BASIC keywords predefined e.g. typing the G key at the start of a line would insert the GOTO command. This approach allowed keywords to be stored as a single byte in memory, avoided the need to have a full BASIC language parser built into the ROM and had the useful side-effect of providing simple context-sensitive programming support
Sinclair did release a ZX80 RAM Pack which increased the memory by an additional 1K to 3K of RAM. Care had to be taken not to press the keyboard too hard – if the computer moved slightly, the RAM Pack connection could be lost causing the computer to reset. Even with the extra memory, the BASIC interpreter left much to be desired and the computer remained more of a first step into computing rather than an end in itself. Nonetheless, around 50,000 ZX80s were sold by mail order in its first year and it helped start the trend of homes having their own computer.
However, after 12 months on the market, the ZX80 was replaced by the cheaper Sinclair ZX81…