These are relevant to the Game Boy because of command transfer to the SNES, as described in https://gbdev.io/pandocs/SGB_Command_Packet.html.
Significant values for rJOYP
:
- Initiate command transfer:
%00000000
- Send 1 bit:
%00010000
- Send 0 bit:
%00100000
- After bit transfer:
%00110000