# Prove you changed nothing

2026-10-20. M16 reorganised the source so a second board means adding files, and I had to prove it didn't change a single byte the PB686 could see.

Source page: https://barelybooting.com/atlas286/journal/2026-10-20-prove-you-changed-nothing/

And now for something completely identical. M16 had one rule: change nothing the PB686 can see. The goal was to make adding a second board a matter of adding files, not editing the core code. M15 had a handful of places in the core that asked which board it was building for, and M16 replaced them with a small set of chipset hooks and flags, so the core never checks a board name anymore.

"Nothing changed" is easy to say, so I wanted it measured. A checker builds every variant of the BIOS, for both boards and every option, and compares each one byte for byte against M15. The only bytes it's allowed to skip are the build stamp and the checksum that goes with it. That morning it came back identical across the board.

Of course a checker that always says "identical" would have said the same thing, so its own tests feed it broken builds on purpose. It caught every one. This was all desk work, no real hardware involved, and M16 never got a tag of its own.