ZX81 Computer


Showcasing monthly programs, a retro look at my my first computer, page 2.


More Mandelbrot Set Fun in Z80 Assembly

Mandelbrot Sets Machine Code, ZX81 Zooming In Screenshot, 2022 by Steven Reid

Okay, after finishing my BASIC program version of Mandelbrot Sets, I decided I needed more speed. I wasn’t sure if I could write a version in assembly. My concern was the floating point math. After a good amount of research, and trial and error, I finally got a version working. This is my story.


A little BASIC fun with Mandelbrot Sets

Mandelbrot Sets, ZX81 Screenshot, 2022 by Steven Reid

Fun with Mandelbrot sets. I never thought I’d say that, but here I am doing just that on the ZX81. I ran across some fun discussions on them. They are often used as benchmarks, but I just wanted to create some interesting pictures. This is my version using Sinclair BASIC that creates a few interesting ones.



Wasting Time Watching Circles in This ZX81 Demo

Circles, ZX81 Screenshot, 2022 by Steven Reid

I ran across a ZX81 forum post showing a fast way to print a circle in BASIC. It was based on an article for the Atari back in 1983. I was surprised how fast the routine was and wondered how it would in assembly. Long story short, I built a screen saver for the ZX81. Sure, the ZX81 really didn’t need one, but the circles are fun to watch.





Wandering Down the ZX81 Road

Road, ZX81 Screenshot, 2022 by Steven Reid

I had this idea on Sunday that led to today’s program—more about that in a minute. One of those sit down and do it programs, I banged this out pretty quick yesterday. Not wasting time, I decided today I’d share it. Road is a very simple program that I never intended to make. But here it is, so lets talk about it more.



Adding More Movement to My Running Game

Run 2 ML, ZX81 Screenshot, 2022 by Steven Reid

Well, hard to believe we are turning the corner on March and heading into April. Spring is edging its way in and it is time to get outside and work in the yard. This made me want to kick my ZX81 program to the curb, but it didn’t feel right. So, I bundled up my current progress on Run.


Working on Bones for a Future ZX81 Game

Run ML, ZX81 Screenshot, 2022 by Steven Reid

For some time now, I’ve wanted to build a running type game. The kind where there is a single key to press and releasing it makes you take an action. There are lots of variations on this theme, and it seemed like something fun to try. Idea in hand, I began by building out a rough look for the game.



Digger Deeper Into My Water Bug II Game for the ZX81

Water Bug II, ZX81 Introduction Screenshot, Steven Reid, 1984

For years, Water Bug II has been an interesting game for me. I was so excited when completing the first version, that I wanted to build another one with ten more boards. The funny thing is, the two games look a lot alike. In fact, if you weren’t paying attention, I wouldn’t be surprised if you thought you were playing the same game. Let’s dig deeper on both the similarities and the differences.


Water Bug Was My Attempt at a ZX81 Action Game

Water Bug, Board 1 ZX81 Screenshot, 1984 by Steven Reid

For most of the day, I’ve been thinking about my Water Bug game as I never really gave it a proper write-up. Finally got around to playing it again, I was surprised to find elements in that I thought were unique to Water Bug II are actually in this version. That made me want to walk through each board, understanding better what I had done, what worked, and what didn’t. Let’s dive right in.


An Early and Bad Attempt at a Yars’ Revenge on the ZX81

Yars’ Revenge, a ZX81 Screenshot, 1983 by Steven Reid

Like many teenagers in the early eighties, I was playing video games on my Atari VCS. Yars’ Revenge was a popular purchase, and I spent many hours dodging that stupid missile and trying to take down the shields of the Qotile. It then should come at no surprise that I tried to recreate that game at 13. My attempt on the ZX81 didn’t look horrible, but it sure played bad. Let’s see how much.





Your Imagination Is Required With Comp Control for the ZX81

Comp Control,  Commands Screenshot, 1984 by Steven Reid.

As a young teenager, I often wrote programs that I would integrate into other play. In this case, I had probably watched War Games again. The idea of a computer talking to you made for some fun fantasy play. Comp Control a little pretend play where my lowly ZX81 mimicked a much more sophisticated world. I’m sure I was running around my room thinking up some crazy way to save the world.


Trying My Hand at Animation by Moving a Sailboat on My ZX81

Picture In Motion, 2021 by Steven Reid

I was hoping to have finished Gem Quest by now, but May didn’t afford much time. With Summer arriving and the pandemic seeming to be winding down, at least in Boise, my attention is elsewhere. With a short list of ideas, I decided to play off last month's theme. Picture in Motion isn’t new, but it is more interesting to watch.


Having Fun Learning How to Draw Lines on My ZX81

Picture Asm, by Steven Reid, 2021

When Simeon Dwyer shared his machine language line drawing routine, I immediately thought of redoing “The Picture.” Most of my z80 assembly has been with my Gem Quest game. I decided it would be a nice diversion to work on something else, even if a quick one. Picture Asm is the result of that diversion.


Not My Best Try at Programming an Ultima Clone

ZX-Ultima, by Steven Reid, 2012

I’ve talked about ZX-Ultima before. It was first attempt at building an Ultima like game on the ZX81 using BASIC. Written in 2012, it was a pretty ambitious project for me. I really enjoyed playing Ultima III on my C64 as a teenager. I always wondered what it would look like on my ZX81. It was bad. Yet, at the same time you could see the seeds of what would come later.


A Developer Diary in Pictures for Gem Quest, a ZX81 RPG

Gem Quest, Darkness Screenshot by Steven Reid, 3/07/2021

If you’ve been following me the last couple of months, you know I’ve been revisiting Gem Quest. Intended to be an Ultima style game for the ZX81, outside of a few demo’s I really hadn’t gotten far. That isn’t the case now. I’ve spent the last month learning z80 assembly code and hacking my way through the game. Tracking my progress, I’ve been capturing that journey in pictures and movies, which I’ve uploaded into a Flickr album.