The Sinclair ZX81, the first mass marketed computer in the UK, was launched 40 years ago today. Compared to its successors, it was very modest in specifications (via Wikipedia):
The ZX81 has a base configuration of 1 KB of on-board memory that can officially be expanded externally to 16 KB. Its single circuit board is housed inside a wedge-shaped plastic case. There are only three other onboard chips: a 3.5 MHz Z80A 8-bit microprocessor from NEC, an uncommitted logic array (ULA) chip from Ferranti, and an 8 KB ROM providing a simple BASIC interpreter. The entire machine weighs just 350 grams (12 oz).
The ULA chip provides many key functions:
- Synchronising the screen display;
- Generating a 6.5 MHz clock, from which a 3.25 MHz clock is derived for the processor;
- Outputting an audio signal to a cassette recorder in SAVE mode;
- Processing the incoming cassette audio signal in LOAD mode;
- Sensing keystrokes;
- Using memory addresses provided by the CPU to decide when ROM and RAM should be active;
- Controlling general system timing.
Both the ZX81 and its predecessor, the ZX80, have a significant drawback in the way that they handle visual output. Neither machine has enough processing power to run at full speed and simultaneously maintain the screen display. On the ZX80, this means that the screen goes blank every time the machine carries out a computation and causes an irritating flicker whenever a shorter computation – such as processing a keystroke – takes place.
Happy Birthday!