model_6502

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Cycle accurate behavioral model for the 6502, built using a real chip as reference.

// repository documentation

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# 6502 Model

[CI Status](https://build.emulationonline.com/teams/main/pipelines/model_6502): ![Status](https://build.emulationonline.com/api/v1/teams/main/pipelines/model_6502/jobs/unit/badge)

This is a cycle accurate model of the 65C02 chip. This model is
produced with the support of the ChipLab, which provides cycle-by-cycle
traces of all external busses while executing programs on a real chip.

The 6502 chiplab, used for collecting traces of real chip execution, is available at:
https://chiplab.emulationonline.com/6502/

## Contributing
Contributions welcome! If you would like to improve the model, a good workflow is
1. Find something that isn't working. See the roadmap or join our Discord.
2. Write a 6502 program that demonstrates desired behavior
3. Run the program on the lab, and collect the signed trace.
4. Add the trace to this repo as a test case, which should fail.
5. Implement the desired functionality.

For an example of adding an instruction, [Nop and Jump](https://github.com/EmulationOnline/6502_model/commit/4f3b7fe5e87a05f72396e278232fd875bc06fc8f) are a simple example, while [basic loads and stores](https://github.com/EmulationOnline/6502_model/commit/4f3b7fe5e87a05f72396e278232fd875bc06fc8f) needed adding some more flexible uops and was thus more involved.

## Roadmap / Currently implemented
The list below gives an idea of what is currently supported.
Unchecked boxes are planned but not yet complete.
- [ ] All official instructions are implemented (sans flags or timing complications)
- [ ] page crossing timings incorporated
- [ ] Flags are added to official instructions
- [ ] NMI interrupt is implemented
- [ ] IRQ interrupt is implemented
- [ ] Unofficial instructions are implemented, + flags

### Timing Complications
Several instructions are impacted by timing complications depending on the inputs.
- Page crossing requires an additional cycle, in order to account for the carry.
- Branching takes an additional cycle.
- Branching + page crossing thus results in 2 extra cycles.

## Discord
We coordinate development discussion on Discord.
https://discord.gg/uwx87FAYMu

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