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6502Observations - VisualChips

6502Observations

From VisualChips

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• Traces in the layout of the original 6501 part which was withdrawn after legal wrangling

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