Halloween with the Camels

It’s time to close out this year’s RetroChallenge project. Since it’s October, why not use the Forth interpreter that I’ve been slowly building up all month to do a sort of demo? I hear decorative squash is hot this time of season. The goal My mission was to augment my nascent CamelForth interpreter with a Halloween-themed demo, and I wanted part of that demo to be 16x16 sprites. The TI TMS9918A video chip at the heart of the Sega SG-1000 supports these enormous sprites1, but I’d never worked with them before. In fact, before last week, I’d never worked with any sprites on this VDP before. To really stretch things, I figured I’d need to do animation. And in order to do animation, I’d need to understand how Forth does control flow. Let’s start by investigating infinite loops. How to write an infinite loop? Forth has an infinite looping primitive known as BEGIN/AGAIN. An infinite loop can be changed to be exited with WHILE if you use REPEAT instead of AGAIN, as seen in the below piece…