We've found some interesting things on the 6502, from the layout level, up through circuit level to the programmer visible level.
Notes here on bugs and undocumented behaviour.
- BRK, the B bit, and other interrupts
- 6502_Taken_Branch_Delays_Interrupts Branches and interrupts whereby a taken branch delays interrupt handling
- Unsupported or undocumented opcodes such as SAX
- The ROR bug which is found only in rare early devices
Circuit and Logic
Notes here on timing fixes and non-digital circuit techniques, and departures from NMOS design style orthodoxy.
- Signs of a fix to datapath control timing
Notes here on the traces of bug fixes, and remnants of the original 6501 layout.
- Traces in the layout of the original 6501 part which was withdrawn after legal wrangling