I felt my VRAM block copier wasn’t working, despite my rigorous on-paper vetting of the code. However, this is one of those situations where it’s hard to say what’s actually broken. There are a lot of moving parts here- my block copier, all the various indirection tables in the F18A, the frame buffer, and so on.
Any one of those things being incorrect would cause the whole system to fail. As usual, this debugging task boils down to trying to find a way to isolate pieces of this system to verify independently.
I played around with this for a while, and I suddenly realized what was wrong. I wish I could say it was a careful scientific process of eliminating variables, but sometimes debugging is just a flash of inspiration. I realized that my block copier was relying on the source data being aligned to a 256-byte boundary. This radically simplifies the code, so it’s a nice requirement to impose. My test font was not aligned to anything. In fact I even commented in the block copier that the source must be page-aligned (a “page” in 6502-speak is a 256-byte block) and then promptly forgot about my own constraint. It always pays to RTFM, especially when you wrote TFM.
8-6-2021 (August 6, 2021) is the Snakiest day of the year and it’s also this year’s CircuitPython Day! The day highlights all things CircuitPython and Python on Hardware. See you there!
Stop breadboarding and soldering – start making immediately! Adafruit’s Circuit Playground is jam-packed with LEDs, sensors, buttons, alligator clip pads and more. Build projects with Circuit Playground in a few minutes with the drag-and-drop MakeCode programming site, learn computer science using the CS Discoveries class on code.org, jump into CircuitPython to learn Python and hardware together, TinyGO, or even use the Arduino IDE. Circuit Playground Express is the newest and best Circuit Playground board, with support for CircuitPython, MakeCode, and Arduino. It has a powerful processor, 10 NeoPixels, mini speaker, InfraRed receive and transmit, two buttons, a switch, 14 alligator clip pads, and lots of sensors: capacitive touch, IR proximity, temperature, light, motion and sound. A whole wide world of electronics and coding is waiting for you, and it fits in the palm of your hand.