Super Game Boy / ICD2: Emulation of $6003 ignores multi-player bits, MLT_REQ is handled in high-level emulation.

Bananattack commented 4 years ago

Hi! I'm making a homebrew test program for the Super Game Boy that can upload custom SNES code and then use the Super Game Boy / ICD2 ports to communicate back and forth between the SNES and GB.

There's a small longstanding issue with bsnes with how multiplayer mode is activated, that makes it so that it only works if the Game Boy side first sends a MLT_REQ SGB packet through the joypad port. I've reported the issue for https://github.com/byuu/bsnes/issues/290

This particular issue also exists in bsnes-plus.

Normally the packets sent through the SGB protocol are just data in the ICD2 that can be read out of port $7000..$700F, once port $6008 indicates a full 128-bit SGB packet is ready, and the interpretation of these packets is handled entirely in the Super Game Boy BIOS software. The Super Game Boy BIOS will write to port $6003 in order to toggle the 1P/2P/4P modes.

However, unlike all other packets that get handled by the Super Game Boy BIOS, in bsnes-plus, this one is specifically done as high-level emulation and is hardcoded into the emulator when it receives joypad port writes from the GB-side: https://github.com/devinacker/bsnes-plus/blob/4cae48a58127ace5fc97b632571dc70c39b9b948/supergameboy/interface/interface.cpp#L70-L83

This is probably a mistake, as these joypad packets have no special meaning to the ICD2 itself, they only have meaning to the program running on the SNES that interfaces with the ICD2.

Writes from the SNES program to the bits of ICD2 register $6003 that affect the 1P/2P/4P modes, as well as bits to select the CPU clock speed divider, are presently are ignored: https://github.com/devinacker/bsnes-plus/blob/4cae48a58127ace5fc97b632571dc70c39b9b948/supergameboy/interface/interface.cpp#L241-L251

This document by dforce3000 mentions some information about port $6003: http://www.dforce3000.de/pub/d4s_super_gameboy_notes.pdf

Bit 4 .. 5 should affect multiplayer mode selection.
Bits 0 .. 1 should affect GB CPU's clock speed.
bsnes and higan handle the GB CPU clock speed but not the multiplayer mode: https://github.com/byuu/bsnes/blob/1afd440c863c898afd2fcc6ad20d744436e89798/bsnes/sfc/coprocessor/icd/io.cpp#L50-L68

The lack of direct $6003 write support currently makes it impossible for a custom SNES program to enable multi-player mode unless the GB sends a MLT_REQ first. However, on a real SNES running the Super Game Boy hardware, a custom 65816 program uploaded with DATA_TRN/DATA_SND + JUMP can use writes to $6003 to enable multi-joy mode in the ICD2.

[Among one of the possible reasons a custom homebrew might want to write $6003 from the SNES, is that an uploaded program can enable 2P or 4P mode to send custom data in the ICD2 joypad registers $6004 .. $6007, which can then be read out from the Game Boy side. And if the custom SNES program can enable multi-joy mode without the MLT_REQ asking for it, the GB side can use this as a verification that the new program is indeed uploaded and ready to communicate with, and potentially fall back on Super GB clone and emulators systems that don't have a true SNES hosting them.

For example, imagine a Super Game Boy enhanced game that comes with a special driver that can make full use of the SNES PPU and APU to make more powerful capabilities than the Super Game Boy BIOS allows, while still leaving communication open with the ICD2 ports so that the Game Boy can drive things. However, if it detects the host system doesn't actually emulate the full SNES, but still supports other SGB features (such as the case of a lot of GB-only emulators, or if a clone console existed with SGB support), it could fallback on a simplified mode with only borders or screen colorization, based on that information.]

While this probably won't affect any existing commercial games, being able to use this port as it works on a real SNES would open up new possibilities.

Thanks for the help!