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.

3 Comments (+add yours?)

  1. Optimum Trajectory Photography
    Jul 16, 2013 @ 03:11:32

    Aha, a 5 chip one. I have one of the 4 chip versions. Not switched it on since before 2000 so dont know if it Y2K compliant or not!

    BUT, I learnt machine code on that wee beastie, and how to write efficient code too. Maybe some at Microsoft should spend time with 1Kb of RAM to see if they can stop producing bloatware 😉

    Lovely walk down memory lane, thanks 🙂

    Reply

  2. Frank L.A.
    Jul 16, 2013 @ 12:40:47

    My first Computer; I soldered (first time ever) the kit with a 80 Watts 5mm-tip iron and it took me four or so days to get it working; after many many attempts I even had the Z80 plugged in the wrong orientation – for several hours. Without harming it. That sticker, which turned out to be the keyboard. FAST and SLOW mode (with a jump into screen memory where the End-Of-Line code 118 was a “return from interrupt”, instructing the glue chip to stop reading via a unique Z80 system state bit pattern – for sure one could learn some dirty tricks here!) .. I am sure there was no Y2K issue – since it had neither a clock nor a file system. Even with memory extension you could fill up the entire memory and produce a stack overflow (after about a minute or so with 16KB) without a single program line, just by typing the two statements LET A$=”VAL(A$)” and PRINT VAL(A$). Later on the Spectrum one could use this unique unlimitted recursion to programm a complete (universal yet inefficient) flood fill algorithm with just a handfull of lines of code. But the TRUE masterpice was the ZX printer; with two digital in and two out ports, one motor, one encoder disc, some wire and plastics you got a full screen hardcopy in less than five seconds.

    Reply

  3. arc310
    Jul 17, 2013 @ 00:26:19

    Strange to find an EPROM here. Mine had a mask ROM. And what are those flying wires ?
    I remember a trick that made it possible to store a new char set in the static 1K RAM while using the 16K RAM for the program. You had to solder a few wires so that the bitmap was read from the 1K chip instead of the ROM.
    Later, I saw HiRes (256*192 !) by software only, using an alternate NMI interrupt driver. Clever !

    Reply

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