Here Is a Silly Test Program for the ZX81

This is a little test program I wrote to test out an online emulator.

September proved to be a rather busy month, at work and home. It isn’t unusual for me, and I usually find time to throw a render or program together. As this month closed, I found myself in need of a program to share and not a lot of time. While cleaning up a few things on my laptop, I found this little test program I wrote last month that fit the need.

A testing we will go.
Funny enough, I wrote the program back in August while testing out a new JavaScript emulator. It was short and sweet and did the job intended. Sadly, the export routine is a JSON file. I didn’t spent the time to figure out how to convert it to a something other emulators can use. yet, the program was short enough that I could grab a screenshot.

Test, ZX81 Listing by Steven Reid, 2020Test, ZX81 Listing by Steven Reid, 2020

The program, as is, is a fun little demo. It prints ZX81 randomly on the screen—forever! It is basically a big loop that scrolls the text upwards. Nothing super exciting, but it worked great. Specifically, it allowed me to test a few things.

First, it tests how PRINT works. Surprisingly, this is a pretty complex routine on the ZX81 and a good test of how the overall emulator works and interprets the ZX81’s ROM. In addition, I used boolean logic, functions and other stuff. Everything worked great.

The other thing it tests is the SCROLL routine, which causes the display to bounce a bit when used on a real ZX81. I suspect it because of the way the ZX81 is designed to shrink the display when not using a 16K RAM pack. Most JavaScript emulators don’t emulate the TV display, so I wasn’t expecting that to work. It didn’t, but still made for a good test.

The last part is that I added a SAVE routine to test any tape emulation. Again, most JavaScript editors don’t emulate the tape interface. As before, a good test to see what does and doesn’t work.

Recreating for publishing.
Since I just had the screenshot, I needed to get the program into a .P file I could run on most emulators. Given the program is short, I just loaded up my emulator and typed it in. Easy as every. I wish programs on my site were this easy. I spent an hour digging through my CMS code to change few lines. Of course, nostalgia only goes so far.

Test, ZX81 Screenshot by Steven Reid, 2020Test, ZX81 Screenshot by Steven Reid, 2020

I won’t get into what could be different. As a test program, it does its job and is fun to watch. Designed to be easy to enter, I actually tried it on an emulator for iOS as well. Sadly, I can’t save there either, but it was fun to play.

And there you have it. A busy month and a simple test program for your ZX81. Give me a shout if you decide to use it anywhere. Although I’m sure you can write something even better.

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