Retro Computer Memories: Sinclair ZX81

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ZX81 is home computer produced by Sinclair Research.

It was launched in 1981 and was designed as ultra low cost entry level home computer.

While the Apple ][ cost about $1300 at that time, ZX81 was just $100 and could be bought also in form of kit so you can solder it yourself.

Here is picture of ZX81 kit content:

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ZX81 was designed to be small, simple and cheap. It uses TV tuner so you connect it to your TV and in 8KB of ROM it holds BASIC interpreter which have 1KB of RAM memory!

Its distinctive design brought its designer, Rick Dickinson, a Design Council award.

The unexpanded ZX81’s tiny memory presented a major challenge to programmers.

Simply displaying a full screen takes up to 768 bytes, the system variables take up another 125 bytes and the program, input buffer and stacks need more memory on top of that.

Nonetheless, ingenious programmers were able to achieve a surprising amount with just 1 kB. One notable example was 1K ZX Chess by David Home, which actually managed to squeeze most of the rules of chess into only 672 bytes! http://users.ox.ac.uk/~uzdm0006/scans/1kchess/

The ZX81 conserved its memory to a certain extent by representing entire BASIC commands as one-byte tokens, stored as individual “characters” in the upper reaches of the machine’s unique (non-ASCII) character set.

Let’s see what is inside:

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nice red soldermask laquer 🙂

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surprise: the top side have no protection mask – remember the PCB have to be cheap and soldermask cost money, as the components are soldered only on the bottom side there is just bottom solder mask.

As you can see from the picture above the ZX81 have just few ICs and all glue logic is in ULA Ferranti chip in the left of Z80. The two small ICs in the right side are the RAM memory and the big chip with the window is 8KB of UV-light Erasable ROM where the BASIC interpreter is.

I like ZX81 as it’s master piece for it’s time – showing how one home computer can be produced on budged and to be profitable.