The Sinclair ZX81 was hardly the most accomplished of 1980s 8-bit microcomputers, but its ultra-low-budget hardware was certainly pressed into service for some impressive work. Perhaps the most legendary piece of commercial software in this vein was 1K Chess, which packed an entire chess engine into the user-available bytes in the unexpanded 1K ZX’s memory map. [MarquisdeGeek] has taken this vintage piece of code in 2026 and subjected it to a thorough analysis, finding all the tricks along the way.
Though hackers have since found ways to trick the ’81 into displaying bitmap graphics, using it as intended is text-only with some limited block graphics. The chess board then is text-only, and its illusion of “thinking” about moves comes courtesy of the on-screen board doubling as the play area memory. In the GitHub repository you can find decompiled and annotated versions as well as the original ZX binary, with as a bonus a screen capture of the game as it appears as BASIC with the ZX’s odd means of storing Z80 code in REM statements.
If that wasn’t enough, in his note giving us the tip he reveals that much of the work was done in a ZX emulator running in a Dragon emulator, and gives us a fun glimpse of the game running in an emulator on a Cheap Yellow Display inside 1K Chess cassette box. We like it, a lot!
If you need a greater ZX81 fix, take a look at how this machine chased the beam to make TV graphics on the cheap.
sooo…. where’s a link to the build that fits inside a cassette tape case? CLICK BAITED!!!!
Yeah, would have been nice to read more about that. Atleast its the same guy https://x.com/i/status/2038560578269073426 and he posted here https://www.reddit.com/r/ZX81/s/vSeQRwOnJl
But nothing on his website https://www.marquisdegeek.com/ (it gives of alott of windows phone 8, 10 vibes, i like).
Might it be a zx81 emulator on a pico?
Reddit post has github link
https://github.com/MarquisdeGeek/ditzy
It is mine – and I have about 10 projects not yet on the marquisdegeek.com site. But seeing as some folk were keen on this specific hack, I prioritised writing/uploading my comments for this one. Now at https://marquisdegeek.com/code_zxchess
As above, it’s just a CYD and battery jammed inside a cassette case. It could probably be ported to a pico.
Yes, this is there link to the most interesting part: https://www.reddit.com/r/ZX81/comments/1s7q9pj/i_have_the_game_of_1k_zx_chess_quite_literally_in/
There’s nothing really to say about the build: cut a hole in the back of a cassette tape, squeeze the display part of a CYD into the gap, and add a battery pack. Everything you see in the picture is all there is.
I bought my 1st ZX80 while stationed at RAF Lakenheath back on 1980 and the ZX81shortly after. Both from a basement shop in Cambridge. We spent many a day typing in code from books and magazines to save on tape so we could play games and sims after a hard night on the flight line. I salvaged parts from a broken radio/tv that ran on twelve volts to bring into the flight line tool crib to play with on breaks. I remember adding 16k of ram and a thermal printer we used to print new cassette tape covers of our mixed music in the walkman days . I also soldered and external keyboard and bought optional character rom that let us have the game characters for asteroids, pacman and others that we played in the snack bars video games. The sinclair got me started on my computer career that I have retired from at 65 this year. It all started with a little Z- 80 processor for me.
Try Atari 2600’s Video Chess. 4K ROM, 128 bytes RAM. Heck, displaying 8 sprites in a row wasn’t possible normally (Space Invaders did 6 by abusing a minor bug to triple up 2 sprites in a row). The programmer got around that by displaying 4 sprites in a row and used alternating scan lines to display one set of 4 on odd field and second set of 4 on even field.
The chess game is known to cheat on higher level by moving pieces when it’s not supposed to move. This is due to 128 bytes of RAM inside 2600.
I’ve had a chess app running with the 16k extension on my ZX81 the time beeing
It was also really impressive for a poor chess player like me!
It was probably writen in assembly language 😬
Spookily enough, one reason I didn’t get to do my own write up of this project is that I spent a portion of this week learning how to code the Atari 2600!
I remember buying 1k chess in Hull it was incredible at the time. I too added the extra ram, keyboard and graphics pack. I wrote a stats program that I used at Hull university instead of the mainframe computer.
I still have the timex Sinclair zx81. What fun that was to program. Then onto engineering and a few startups. Loved every minute of it