Implement the USB2SNES protocol

[Screwtapello]

USB2SNES seems to be an alternate firmware for the SD2SNES (I don't know if it's available for the FX Pak or FX Pak Pro) which uses the SD2SNES' USB port as an RS232 port, which speaks a protocol that lets the host computer read and write the SNES' RAM, as well as doing things like resetting the console and managing files on the SD card.

This would be a very useful thing for automating homebrew testing and it's also used for things like item tracking in randomisers, or (presumably) RetroAchievments on real hardware, so it would be useful to support in bsnes as well.

As I understand it, the SD2SNES is presented as a regular RS232 port, which speaks a custom protocol. The protocol doesn't seem to be officially documented anywhere, but there are tools like SNI and QUsb2snes that speak it, that could probably be reverse-engineered.

Since working with serial ports can be a pain, especially on Windows, SNI and QUsb2snes wrap that protocol in their own encodings - SNI provides a gRPC-based API, while QUsb2snes provides a JSON-over-websocket API. Client applications tend to be written to use those higher-level protocols, but we don't have to implement them since SNI and QUsb2snes already exist.

To make progress on this, we're going to need to figure out:

• a detailed protocol description of the USB2SNES RS232 protocol
• how to make a virtual serial port on Windows (I assume macOS, like Linux, can use openpty(3))
• hooking this protocol into the emulation event loop (I assume memory access will be done the same way the Cheat Finder does it)

[undisbeliever]

read and write the SNES' RAM

This is not correct. usb2snes allows QUsb2Snes and SNI to read and write to only the sd2snes's RAM. The sd2snes sniffs the data buses to clone all Work-RAM, PPU and register writes to the final 16MiB of the sd2snes's RAM. This allows the usb2snes app to:

• read from ROM, Work-RAM, Save-RAM, and PPU memory.
• write to ROM and Save-RAM

The original usb2snesw has code that can write to Work-RAM, VRAM, CGRAM or OAM via an NMI hook(?) that executes 65816 code (generated by usb2snesw) to write to the SNES's memory. I have not used this feature, so I can't say how practicable it is. This feature does not exist in QUsb2Snes nor SNI.

QUsb2Snes and SNI can communicate with Snes9x-rr and BizHawk via a lua script that communicates over TCP port 65398 to read and write to the SNES memory map. It might be an easier protocol to implement instead of emulating the serial protocol.

If you are looking for a stress test of this feature, I used usb2snes to create a communication protocol between my homebrew SNES game (running on my sd2snes) and my resource compiler (written in python). It allowed me to quickly hot-load code, rooms and graphics without restarting the game.

20220714-resources-over-usb2snes-demo.mp4