Software-controlled bank switching to emulate multi-bank N64 Controller Paks

[gobbledygoober]

Edit: added a few other related community projects for reference.

Summary

Suggest implementing software-controlled bank switching for N64 Controller Paks on BlueRetro.

If BlueRetro reserves 128KB of flash for memory card emulation, this would allow for a 4x sized Controller Pak. However if this partition can be increased, the SDKs and the protocol allow for up to a 62x sized Controller Pak, or just under 2MiB (4x and 16x Controller Paks were sold commercially).

In other words, this change would enable emulating a larger contiguous, single Controller Pak, instead of (4) separate smaller Controller Paks that require physical button presses.

This should hopefully be possible assuming HW1 or HW2 can emulate Joybus accessory pin A15/CE2. The interface is documented on N64brew's wiki, as well as the filesystem structure.

Using such Controller Paks with > 32 KiB of storage is supported by original retail games, the associated libultra SDK, and as of recently, the open source libdragon SDK and bryc/cyrb's mpkedit.

Motivation

BlueRetro support would have several benefits:

• Complement the current feature that exposes (4) discrete 32 KiB virtual Controller Paks, and lets users cycle banks via a controller button and configuration web UI
• Enable support for a single, larger Controller Pak. Physical Controller Paks of this kind were manufactured by just a few third-party accessory brands (Blaze and Datel) and are difficult, and expensive to find. BlueRetro support could help bring this functionality to the masses.
• Eliminate the need to press a physical button to cycle the active Controller Pak bank, thereby freeing the MQ button for other potential uses

Implementation Details and Project References

Bank switching has recently received significant community attention, so any implementation questions can likely be answered:

• The libdragon project's preview branch added support for larger, banked Controller Paks (main API, CPak FS, and CLI tools)
• bryc/cyrb added multi-bank support in mpkedit (https://bryc.github.io/mpkedit/), enabling easy creation and testing of multi-bank files
• Hunter R and Jhynjhiruu recently showcased their Dōbutsu no Mori (Animal Crossing) / Famicom emulator / Controller Pak research
  • CPAK2040-Zero: This project, created by zwenergy, utilizes a Raspberry Pi Pico to create a modern, non-volatile Controller Pak. It supports multiple virtual paks
  • As detailed at hunter-r.com, Hunter and Jhynjhiruu leveraged zwenergy's CPAK2040-zero design and software-based bank switching to implement a proof-of-concept to handle the larger save file requirements of injected NES ROMs for use with the retail game's built-in Famicom emulator
  • This replicated the functionality of an official Nintendo Ice Climber promotional Pak for Animal Crossing that saw a very limited commercial release
  • The prototype CPAK2040-zero software has not been released because its implementation involved removing some established features and hardcodings for the Ice Climber ROM.
  • Zwenergy has said on N64brew's Discord that they are working on an implementation ([Feature request] Consider supporting banked controller paks zwenergy/CPAK2040-Zero#1).
• Before the CPAK2040-Zero implementation, Jhynjhiruu also used a Raspberry Pi Pico to emulate the entire controller. Their banking code can be found here.
• The Ares emulator also has an implementation, but is currently slightly non-compliant, because it was based on older reverse engineering work (it responds to any writes above 0x8000, rather than ignoring writes above 0xC000).
  

In short, as the N64 homebrew scene continues to evolve, it is likely that more projects will take advantage of larger memory formats. This complements the existing retail game support.

Given its prevalence in tinkerer's setups, BlueRetro is uniquely positioned to make software-controlled bank switching widespread. This would continue to ensure that BlueRetro remains a versatile and powerful tool for N64 enthusiasts.

Thanks for reading.