1980s BASIC’s PAINT: Deep‑Dive Edition — Bugs, Tricks, and All the Ugly Details!

Video Description

Welcome to your "DVD Extras" for the previous video (https://youtu.be/f9YkL9_GfwQ)--you should definitely watch that first! Today, we assume you know the basics of the CoCo's flood-filling PAINT algorithm, and we dive much deeper, explaining why it uses a stack instead of a queue, how I did my research, the full steps to the PAINT algorithm, a walkthrough of the code and all of the ugly details I glossed over before, nifty assembly language tricks employed by the authors for sharing code and manually manipulating the stack, a BUG in PAINT I discovered while pouring through the code at such a low level (I'm unsure if anyone else knows about this 45-year-old bug!), the code behind visualizations I showed in the previous video, how I scripted the MAME emulator to show a real-time view of the stack while PAINT runs, a test case to attempt to overflow the stack while running PAINT, and finally a silly experiment I did to get multiple paints running simultaneously. WOW! DOWNLOAD the code discussed in this video: https://cocotownretro.wordpress.com/2025/12/23/paint-how-it-works-and-why-and-what-goes-wrong/ Previous video on PAINT algorithm: https://youtu.be/f9YkL9_GfwQ Source-level debugging in MAME: https://dave-br.github.io/mame-source-debugging-preview/ Scripting MAME with LUA: https://youtu.be/ySY8EN0sE6I and https://youtu.be/xBgKwqdwxDo Intro Music: https://soundcloud.com/david_broman/whistles-piano-and-nas Chapter audio clips: Go to the companion blog page, and scroll down to "The Audio Clips" section: https://cocotownretro.wordpress.com/2025/12/23/paint-how-it-works-and-why-and-what-goes-wrong/ 00:00 Intro 02:25 I: Ok, a STACK, but why? 04:56 II: Bootstrapping 08:17 III,1: The full algorithm 10:30 III,2: The Upside-Down 12:31 III,3: The Devil's in the Details 16:31 III,4: Code sharing is code caring 19:25 III,5: Stack Surgery 21:16 III,6: Stack Pruning and BUG! 30:01 III,7: One Last Small Assembly Trick 32:31 IV: A Great Time at the DMV 36:15 V: Visualization via instrumentation for obfuscation defenestration 40:41 VI,1: Thy stack overfloweth 42:44 VI,2: A multi-tasking paint 45:16 Outtakes