Integrate 64DD into RTC subsystem

[meeq]

N64brew Discord Thread

Highlights

• Add functions to get and set the 64DD RTC
• Change rtctest example ROM media category to C (Expandable Game Pak with optional 64DD support)
• Add support for 64DD as a clock source in RTC subsystem
• Allow switching RTC source in rtctest example (if both Joybus and DD RTCs are available)

Outstanding items

• Investigating weird issues with clock drift
• Needs more testing

Caveats

• 64DD is currently rather obnoxious to enable in emulators/flashcarts:
  • Ares only enables 64DD for N64 ROMs if they have category code C and an expansion disk is loaded
  • SC64 only enables 64DD registers if a 64DD IPL and expansion disk are provided
    • The test ROM will hang indefinitely if you try to load with just the 64DD IPL and no expansion disk
    • The test ROM will hang indefinitely on 64DD access if you detach sc64deployer
• SC64 only has one physical RTC, so things get weird when both clocks are being used
  • Still investigating the nature of this; need clarity from Korgeaux on expected behavior here

Testing

Ares

To test with Ares, you need to specify the US 64DD IPL in the Firmware settings and provide a "dummy" 64DD Disk:

ares rtctest.z64 dummy_disk.ndd

LuigiBlood has good documentation for using 64DD with Ares on 64DD.org. The critical piece is that the Game Pak ROM MUST have a Game ID Category Code of C for Ares to trigger the 64DD codepath. The F-Zero Expansion Disk makes for a good "dummy" disk.

SC64

To test with SC64, you also need to provide the US 64DD IPL and a "dummy" 64DD Disk:

sc64deployer 64dd 64DD_IPL_US.n64 dummy_disk.ndd   --rom rtctest.z64

Note that if you don't provide a dummy disk, SC64 will not fully emulate the 64DD and the test ROM will hang.