ZX81 Computer

Out on a run back in November of last year, I had this idea to create a 3-color screen mode of the ZX81. The idea was to use the checkered, or gray, graphic characters to add a third color. After reading about the C64 color modes, I called it chunky graphics as an homage to its wide pixel format. The pixel’s are chunky due to the 2x1 ratio and not being square. Let’s dig into the details.

After finishing the basic version of the cat animation, I wanted to improve the frame speed. This meant converting from BASIC to machine code. My first thought was to do the conversion with MCODER II. I dropped that idea, deciding to focus on a custom z80 Assembly version. Ergo, cat written in assembly or Fast Cat is, well, a faster version and a bit more.

The holiday's provided me a nice break to enjoy writing code. During that time, I revisited my digital rain program. For ZX81 Rain, I cleaned up the colors and added a little randomness to where the drops start. Nothing super exciting, but the overall effect is much nicer and looked good in both black/white and color on the ZX81.

Reading some old programming books from the eighties, I saw that the ZX81 had an ABS command that made me think I could improve my FizzBuzz program. It was late and my mind mixed that command with a modulus one, which the ZX81 doesn’t have. In any case, it offered me an excuse to fix the latency introduced by printing numbers.

I had this idea the other day. One of the problems I’ve had with the ZX81 is printing numbers, which is pretty slow. Due to this, many of my games avoided printing them. delaying showing score until after play has ended. My idea was to try to find a way to speed that up. Not only did I find a way, I decided to implement it in an older game of mine: E.T.

A recent post in a Facebook BASIC group of a cat animation. A couple of other members converted it to their BASIC code of choice. Mine being the ZX81, I promptly worked on a version for it. I created a mostly full screen version, which tends to show how slow the ZX81 really was. In any case, it was quite a fun little exercise.

I missed the Dec 26 deadline for submitting, but did get a star working on the ZX81. My attempt isn’t spectacular, but it does work. I recommend checking out the other entries for Vintage Christmas Star Challenge.

I’ve been on a retro computing binge lately and was recently watching one on programming on what it was like to program in the 80’s. Having lived and programmed during that time, I know full well what that was like. But the narrator was showing off an Apple ][+, a computer I didn’t own. Although I programmed on an Apple ][ in high school, my memory of it is vague at best. But what was really interesting was what he was programming a snake game. Being something I’d never tried myself, I thought now would be as good time as any.

My frist isometric test was with a static background. Although that works well, it limits the size of the playfield. Thus, I decided to expand the playfield and scroll it around the player instead. It ended up being a lot more complex than I expected.

Looking at various ideas for retro style games, something I’ve been wanting to try has been isometric graphics. I have a pretty good hand on tile based graphics now and wondered if I could do something similar with an isometric set. Hunkering down, here is how my testing is going.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.