Lmst

An AI did not write this 1,000 word commentary on the ZX81 keyboard scanning routine. I did: https://drj11.github.io/2026-02-10/zx81-keyboard.html

It includes what is now one of my favourite assembler instructions: `SBC A`

#ZX81 #Z80 #NoAI

another safe run
#zx81 #mazgos

Quite pleased with how nippy the map scrolling is in my ZX81 game-in-progress. It is the first working and largely non-optimized version. It's good that it's quick and can be made quicker, because i think it will mostly determine how responsive the game feels. By comparison the other routines (that i wrote earlier) to draw the frame and the info display are shamefully slow (and mildly quadratic, haha).
#ZX81 #RetroComputing

Finally dusted my #ZX81 off again to run last year's epic 81Zombies Basic game that I wrote. It is absolutely as painfully slow as I imagined.... BUT.....it does run on real hardware from 1981 which has made me a happy Donkey #RetroComputing

Ha! My ZX81 game-in-progress has a map! I drew it on paper and typed all the tiles in:

#ZX81

Skimming Toni Baker's «Mastering Machine Code on your ZX81» and i love the chapter titles, which feature a hand-drawn illustration of the ZX81 circuit board. I believe the PCB tracks on the ZX81 were themselves hand-drawn, which was the fashion at the time. [edit: i see from the title page, that the illustrator is Cathy Lowe, and they like a bit of slightly wobbly Letraset Helvetica]

#ZX81 #MachineCode

A drawing of the ZX81 PCB. There are two large chips, one with "Chapter One" written over it, and another with "An Introduction" written over it. 2 smaller chips are also visible. Between them run a whole mass of tracks and vias and components like resistors and maybe ceramic capacitors. The tracks curve in sometimes quite organic ways, because they were designed by hand, not by computer.

Since 1984 I've kept a French book called "robotize your zx81" with some interesting electronic hacks one could make. At the time I had my zx81 plugged onto a small black and white tv. I even had the (gasp) 16kb extension. I no longer have the zx81, but I kept the book.

#zx81 #retrocomputing

A *much* better version of the Timex/Sinclair 1000 manual, thanks to the epic scanning skills of @c64whiz

https://archive.org/details/TS1000UserManual/

#RetroComputing
#TimexSinclair
#ZX81 #Sinclair

Update: got one, thanks to @c64whiz !

Is there a better scan available of the Timex Sinclair 1000 User Manual than this one on the Internet Archive?
https://archive.org/details/timex-sinclair-1000-user-manual/

It's a little bit cruddy, and every internet link seems to lead to it

#RetroComputing #TimexSinclair #ZX81

I'm re-reading parts of the ZX81 BASIC Programming Manual (because of side-side-sidequest), which is the book that came with the ZX81. I had this when i was 10, and i'm time and time again gobsmacked by the amount of educational effort that went into this pack-in provided with the cheapest micro of all time. It's more or less how i learnt trigonometry, but i wanna talk about Chapter 15, which is about software engineering lifecycle management, or, as they put it, "Making your programs work".

It comes with a flowchart (for humans): two absolutely key parts of this chart, still overlooked in many orgs: 1 "Write program with as few bugs as possible"; 2 "fix them introducing as few new ones as possible".

_As few bugs as possible_

#ZX81 #SoftwareEngineering

A flow chart illustrating program development. There is a lead-in paragraph: "EVERY PROGRAM STARTS OFF WITH BUGS.
Many programs finish up with bugs as well. There are two corollaries to this; first, you must test all your programs straight away; & second, there's no point in losing your temper every time they don't work. The general plan can be illustrated with a flowchart:"

The flow chart is mostly linear: Write program with as few bugs as possible / Test program / Does it work perfectly? [Yes goes to Finish] / Keep your hair on / Find the bugs / Fix them, introducing as few new ones as possible [it loops back to Test Program].

I implemented Bresenham's line algorithm for my ZX81 "game" (more of a demo of routines at the moment). I use it to draw the chunky grey line crossing diagonally. It's quite a fun algorithm, and apparently the second time that i have implemented it: there is a version in BASIC in Chapter 18 of the ZX81 BASIC Programming manual that i surely typed in as a child.
#ZX81 #Bresenham

A screenshot of a ZX81 emulator showing quite a busy screen. Text at the top reads "ZX81 / GREETINGS / EARTHINGS**" then there is a checkboard background below, which has been partly covered with random placed ACAB strings. Over the top of all that, a thick grey line (actually made from checkerboard characters) is drawn from near the word GREETINGS to about 1/3 of the way down the right hand column.

The first video is online, the Scandinavian Retro peertube channel is a go!

In the first episode of Scandinavian Retro I explain what this peertube channel is about, do a quick tour around the new retro room, we look at a number of items that came in the mail and do a quick test of two of those items: the Amstrad CPC 464 and the 6128.

video.chasmcity.net/w/8jDUDrBn…

#RetroComputing #Amstrad #Amstrad464 #Amstrad6128 #AmstradCPC #Atari #Amiga #Sinclair #SinclairZX81 #ZX81 #peertube #electronics @FediVideo

Screenshot of a peertube site showing a video thumbnail with two Amstrad computers and a multi-meter.

Scandinavian Retro 2026-01: Introduction, Mailbox and Amstrad CPC

https://video.chasmcity.net/w/8jDUDrBnf8fegeHQybXS8t

Behold the awesome speed of machine code. The initial screen clear is the ZX81 system which clears the screen before running the program. The initial delay before the first text appears is mostly BASIC interpreting the 2 line program, until it jumps into the machine code (at address 16514). Then my machine code program runs; almost instantaneously!
#ZX81 #Z80