Comment on Wolfenstein 3D for Gameboy Color on custom cartridge (2016)Comments−Dwedit4moGame Boy Color has a secret video mode where you can directly drive the LCD from the cartridge slot.CPU completely halts, so you can't use the joypad or sound anymore.https://gbdev.gg8.se/wiki/articles/Gameboy_and_Gameboy_Color...−bbbbbr4moThat CPU halting only applies for PGB mode 1.In PGB mode 2 the CPU is still able to run (within a limited address range) and can use register FF75 "PGBIO" for limited input and output on some cartridge pins (and/or the link port for IO).
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Game Boy Color has a secret video mode where you can directly drive the LCD from the cartridge slot.
CPU completely halts, so you can't use the joypad or sound anymore.
https://gbdev.gg8.se/wiki/articles/Gameboy_and_Gameboy_Color...
That CPU halting only applies for PGB mode 1.
In PGB mode 2 the CPU is still able to run (within a limited address range) and can use register FF75 "PGBIO" for limited input and output on some cartridge pins (and/or the link port for IO).