Friday, March 11, 2011

Happy birthday, ZX81

Thirty years ago today the Sinclair ZX81 was introduced to the world. The world's first affordable personal computer, it wasn't in fact the first home computer I had owned. That had been a Nascom 1, a Zilog Z80 based machine with just 1Kb of RAM, which I built from a kit - all 1,400 soldered joints of it, using the same Antex soldering iron I still use today.

The ZX81 was also offered as a kit as well as a ready built version, so naturally I ordered the kit. I seem to remember it cost £49 - much less than the Nascom. I don't remember how many soldered joints there were, but there were only four main chips. It was a much easier project to build. The ZX81 also came with just 1Kb of RAM. But unlike the Nascom, it had a built-in BASIC interpreter so you could still do more with that 1Kb and you didn't have to program it in assembly code.

Innovative design was used to cut the cost of the ZX81. For example, instead of a dedicated display processor the Z80 CPU generated the display. Whenever your program executed, the screen went blank. The screen was an ordinary black and white TV. Programs were loaded and saved using a cheap cassette recorder. That was read and written by the CPU too, which generated wild patterns on the screen while it neglected its display duties. The keyboard was a plastic membrane type. It was horrible to type on.

Later I upgraded the memory to 48Kb using a third party RAM pack (the standard RAM pack sold by Sinclair was only 16Kb.) This, too was built from a kit. Like the Sinclair one, it fixed to the rear of the machine using an edge connector, with no other fixing. Every ZX81 owner is familiar with the term "wobbly RAM pack". One accidental jolt could interrupt the connection and crash the computer losing all your work. Ah, those were the days!

As a radio ham, I naturally was interested in writing ham radio software for the ZX81. I wrote several programs including a morse tutor, which used a machine code routine for sending the actual code. I think I have written morse tutors for every type of computer I have owned - it's amazing that I am still so bad at reading the code! An article describing the morse tutor and a memory keyer for the ZX81 was published in Short Wave Magazine and was one of my first published articles.

I can still remember the excitement of home computing in those early days. Today's PCs, vastly more powerful and capable though they are, just aren't as interesting. Back then, home computing was very much a hobbyist's game. We were pioneers. Now everyone and his granny has a computer, and programming has become more or less a job for professionals. I do miss those old days!

Thank you, Clive Sinclair, for bringing us these wonderful toys. Happy 30th birthday, ZX81!


Sivan said...

It was also the first home computer I used. It belonged to my younger brother, but I too logged many hours on it.

LY2SS said...

These machines or to be precise - clones of them - were wildly popular between hams in ex uSSR area (just look "ZX spectrum clones" article in wikipedia). I remember connecting 5.25" FDD controller to it! It had very innovative feature - once you push button (NMI) it immediately wrote memory dump and CPU state to the FDD. It helped to cheat a lot of games!
I still admire Spetrums...


Ricardo - CT2GQV said...

I had the ZX82, commonly called the "Spectrum"...great fun, made my first program on it. Still have it and still works! Recently someone gave me a 128K one with integrated cassette "disk".
Nice to remember.

VE3WDM said...

I won a ZX at work it was great should have held on to it. From there it was the commodore 64 and then the lightening fast 128.

PE4BAS, Bas said...

Brings back so many (programming) memories. The ZX81 was the first computer I really programmed at, it belonged to my uncle. Later he owned a Spectrum and a QL.I owned his QL for a time, that computer was ahead of it's time. But obsolete when I owned it. I remember the endless loading times of the programs on cassette tapes. The QL had something like a kind of micro ZIP drives, those were a lot faster but not used on any other brand of computer ever I believe. Great fun. But prefer the "modern' computer now. 73, Bas

sv3auw said...

I still have MY Spectrum!

Casey Bahr said...

First one I had also! I had 16KB of memory, hi hi, and the thermal printer. Used to spend many hours optimizing a program to search for primes. Wow. That set me off in a whole different direction, advanced degree in computer science, and a long career as a software engineer (due much more to my education than my first program!).

Julian, tried to leave an entry in the visitor's book, but kept saying it couldn't be added. ??

Will add you to my blog roll at


- Casey, TI2/NA7U

Paul M0PCZ said...

Very good Julian, I wouldn't know where to start with my admiration for the ZX81, well I know where it ended, and that was with my Computer Studies 'O' Level pass, the programming all done on the ZX81, rather than the BBC'B' micro which was popular in 1984.

Steve said...

Didn't the ZX80 come out before the ZX81?