ZX81 Computer


Showcasing monthly programs, a retro look at my my first computer.
A little BASIC fun with Mandelbrot Sets

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.

Posted 4 days ago by Steven 4 min read

Creating a Fun Little ZX81 Implementation of Fizzbuzz

Hey, it’s October. Well, it has been for a few weeks now. In any case, it is also time for a ZX81 program. I recently joined an 8-Bit Programming & Engineering Facebook group and its cover is a photo of a FizzBuzz program in Pascal. As a quick exercise, I thought I’d write my own in ZX81 BASIC.

Posted Oct 15 by Steven 5 min read

Wasting Time Watching Circles in This ZX81 Demo

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.

Posted Sep 01 by Steven 4 min read

A Super Fast Scrolling Demo on the ZX81

Totally divergence from working on Gem Quest. I got around to cleaning up my scrolling demo from last month. In doing so, it made me wonder would it would be like to code it in assembly. The answer is both bland and interesting. Let’s discuss why that is.

Posted Aug 16 by Steven 4 min read

Super Simple ZX81 Scrolling Example

Although I've been busy working with the ZX81 this month, it has been with my Gem Quest game. Not much to share yet there, but I am working on it. To get in something for the month, I thought I’d share an updated demo program.

Posted Jul 31 by Steven 3 min read

The Monster From the Deep Arrives on Your ZX81

This month’s program is a super simple graphic program I created in 1984 called The Monster. Probably developed as a title screen, the graphic looks like some swamp creature with seaweed hanging off it. I’m not positive what I was going for, but it does look rather imposing.

Posted Jun 30 by Steven 4 min read

Wandering Down the ZX81 Road

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.

Posted May 03 by Steven 7 min read

Racking up a New Score in This ZX81 Animation

Not a lot of time for programming this month, but I did get around to adding coins to my running game. Funny how little things take time to hack about. I’m not completely happy with it, but it works. Run 3 ML does give a whole new meaning to infinite runner.

Posted Apr 30 by Steven 3 min read

Adding More Movement to My Running Game

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.

Posted Mar 30 by Steven 5 min read

Working on Bones for a Future ZX81 Game

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.

Posted Feb 19 by Steven 5 min read

Let’s Cook up a Faster Graphic Enlarger for the ZX81

I haven’t had much time for programming the last few months. To get back into things, I decided to take an old BASIC program of mine and rewrite it. Thinking about what to convert, my mind immediately went to Enlarger. It was a rather slow animation demo that could use an update.

Posted Jan 28 by Steven 7 min read

Digger Deeper Into My Water Bug II Game for the ZX81

Now, Water Bug II is an interesting game for me. I was so excited when completing the first version, that I wanted to build another version with ten more boards. The funny thing is, the 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.

Posted Dec 25, 2021 by Steven 11 min read

Water Bug Was My Attempt at a ZX81 Action Game

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.

Posted Nov 30, 2021 by Steven 17 min read

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

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.

Posted Oct 31, 2021 by Steven 4 min read

Building a Better ZX81 Digital Rain using Z80 Assembler

My original plan was to create an z80 assembler version of Moon Lander. But with the trailer release for the next Matrix movie, I found myself revisiting another program. Built entirely in machine language, Z80 Digital Rain expands on my previous version with a splash of color. Let’s dive deeper.

Posted Sep 25, 2021 by Steven 8 min read

Amping up the Visuals and Gameplay in Moon Lander V2

Last month, I shared an old moon landing program I wrote back in the early 80s. Right after that, I decided to give it a bit of a face lift, updating the visuals without changing the game play. Moon Lander v2 is my updated game, yet still the same BASIC code under hood.

Posted Aug 21, 2021 by Steven 8 min read

Try Not to Crash the Moon Lander in This ZX81 Game

When I first started programming, one of the games we would play on the old teletype machines was a simple moon lander game. The premise is you would use your thrusters to softly land on the moon. Moon Lander was my take of that program a few years later.

Posted Jul 31, 2021 by Steven 6 min read

Your Imagination Is Required With Comp Control for the ZX81

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.

Posted Jun 24, 2021 by Steven 7 min read

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

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.

Posted May 31, 2021 by Steven 5 min read

Having Fun Learning How to Draw Lines on My ZX81

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.

Posted Apr 30, 2021 by Steven 6 min read

Not My Best Try at Programming an Ultima Clone

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.

Posted Mar 31, 2021 by Steven 5 min read

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

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.

Posted Mar 07, 2021 by Steven 8 min read

How to Build a ZX81 Ultima Game Part 2: Gem Quest

After sharing Tiles, I started to dig into my previous attempts at building an adventure game. Digging through my past research, I thought about encoding the world differently. Sadly, my experiments using MCODER failed. This spurred me to try assembler again and, to my surprise, build a good start to my game. But first, a trip down memory lane and review my first attempt at building Gem Quest.

Posted Feb 25, 2021 by Steven 6 min read

How to Build a ZX81 Ultima Game

In 2012 I wanted to see if I could make an Ultima style game on the ZX81. I had an ambitious plan centered on the tile based graphics. After a few months of testing, I was able to create a working prototype called Tiles. Although the game isn’t done, the idea behind it shows promise. You can move around the map using the standard ASDW movements. Other commands are placeholders for what could be. This article explores how Tiles came to be.

Posted Jan 31, 2021 by Steven 10 min read

How I Used My ZX81 to Draw the Iconic OP Logo

As 2020 wound down, I had a couple of different ideas for new ZX81 programs. Most of them were graphic programs of various sorts. Themes ranged from dumpster fire pictures, an advent calendar, a virus action game, to a solo RPG. Although they were all great ideas, I didn’t think I’d get any complete enough to publish. In the end, I decided to enter in one of my old graphic programs of the Ocean Pacific logo.

Posted Dec 27, 2020 by Steven 4 min read