Revisiting Colourflow

Choosing a RetroChallenge project is a fine line between picking something exciting and challenging, while making sure that it’s possible to complete in 30 days. Having crashed and burned on the last Winter Warmup, I’m trying to pick something a bit more achievable this time around.

Assembler Editor CartridgeBack in 1983, when I was 15, I started dabbling in 6502 assembly language after acquiring Atari’s Assembler Editor cartridge. This cartridge, along with De Re Atari, was a powerful combination. One of the first things I made was a neat little colour effect that looked like coloured venetian blinds scrolling down the screen. The code was crude and relied on a tight timing loop that just happened to create the effect.

I bundled up my assembly into a BASIC loader that put the Atari fuji on the display (blatantly stolen from one of Atari’s loaders), and submitted the program to Page 6 magazine. Surprisingly, my program was accepted, and showed up in the September/October 1984 issue (issue 11). “Colourflow” was my one and only published article in print.
Page 6 Magazine Issue 11So, what’s this got to do with RetroChallenge? Well, after reading 15-year-old me, I noticed that I had thrown down a challenge in my article.

The machine is locked in an endless loop so the only way out is to press SYSTEM RESET. I tried to incorporate code which sensed a keypress but this destroyed the precise timing loop. Maybe other readers can come up with a solution?

Sounds like a challenge to me! My RC2015/07 RetroChallenge entry is to re-visit Colourflow and address my 32-year-old challenge. Actually, there’s a second problem that I need to address. I wrote the original program on a PAL Atari 400 in the UK. The timing is slightly different on NTSC, and the effect doesn’t work at all on an NTSC machine. So, not only do I have to address my original challenge, I need to make the code work on NTSC as well as PAL.

To make it interesting, I want to recreate my original coding environment as closely as possible. This was written on an Atari 400, with the Assembler Editor cartridge. My sole form of storage was a cassette recorder. I’m not going to be so medieval as use a tape recorder, but I will try to code this using real hardware and the Assembler Editor.

Let’s see if I can complete another RetroChallenge! This year’s challenge runs from July 1 to July 31, 2015.

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