Remembering The Sinclair ZX81 Computer 36 Years Later


One of the main reasons I enjoyed the Micro Men drama so much was that it featured all three computers (other than PCs) which I have owned in my life. And by owned I mean “had access to in my home as a child”.

The first such machine is the ZX81, Sinclair’s second home machine, which turned 36 today. Even though I didn’t get the original ZX80 model, owning the ZX81 meant I had a better machine than my friend Gavin, and even when you are eight years old, that’s what really matters.

If somebody produced a full-blown computer with the dimensions of a ZX81 today, they’d be lauded as having produced the ultimate netbook: it was roughly the size of a thin hardback book. However, once you compare its powers to that of a modern smartphone, its compactness loses its attractiveness.

Perhaps the best known spec of the ZX81 was that it had 1K of RAM. That’s 1,024 bytes, or a whopping 8,192 bits. That people were able to write any functioning programs given this limitation was impressive. That somebody managed to get the 1K model to play chess was a miracle.

The machine could also be expanded to 16K. However, this was not through today’s thumbnail size memory cards. Instead, users had to plug in a RAM expansion pack, remembering to follow Sir Clive Sinclair’s advice to hold it in place with Blu-Tack. This wasn’t the case in my home however: my dad, a keen electrical engineer, built his own expansion pack which was roughly the size of a shoebox.

Getting sound from the ZX81 was a triumph of creativity over technology. The computer did not have a sound chip or speakers as such. However, as a ZX81 FAQ site reminds me, there were two unofficial ways to bring the noise. The first was to attach it to a cassette recorder or amplifier, which could then be persuaded to emit something resembling a sound. The second was to take advantage of the fact that switching between the machine’s two display modes could produce a noise on the TV set you had to use for your monitor. A carefully written program could thus play what technically counted as a tune.

Another flaw in the ZX81 was that it didn’t come with a fan. It was certainly common behavior to keep the machine cool by placing a glass of iced water on it, though I’ve not been able to confirm my memory that this was official Sinclair advice.

So how on earth did the ZX81 sell a reported 1.5 million units? Well, the mere fact that you could actually program a computer in your own home (and even build one if you bought the kit rather than the assembled version) was an amazing concept in itself. That the machine only cost £69.95 (approximately £218 or $356 in today’s money) was enough to make it a must-have for any self-respecting geek.

And how you could possibly resist a machine that, 36 years later, is listed on Wikipedia with the note “There was a bug present in the original ZX81 ROM that resulted in the square root of 0.25 being calculated as 1.3591409 rather than 0.5.” Sure beats the red ring of death.

[Picture source: Flickr (CC)]

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