6502Observations

We've found some interesting things on the 6502, from the layout level, up through circuit level to the programmer visible level.

= Programmer Visible = Notes here on bugs and undocumented behaviour.

• BRK, the B bit, and other interrupts
• Timing of Interrupt Handling noting that a taken branch delays interrupt handling, also that CLI/PLP allow one further instruction to execute, unlike RTI.
• Unsupported or undocumented opcodes such as SAX and XAA
• The ROR bug which is found only in rare early devices

See also our catalogue of 6502 test programs, useful to verify simulators or emulators.

= 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

= Layout = 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• Traces in the layout of the original 6501 part which was withdrawn after legal wrangling