GPI/GPO
PSP development kits have an extra set of inputs and outputs: 8 on/off switches, and 8 LEDs. These are called the GPI and GPO, respectively, and can be read/written using the PSP kernel functions sceKernelGetGPI/sceKernelSetGPO.
On consumer hardware, sceKernelGetGPI always returns 0 and sceKernelSetGPO does nothing.
From 1.18, PPSSPP can return custom settings for the GPI switches, and display the GPO outputs. This functionality can be reached in developer settings and in the in-game DevMenu, if enabled. As it turns out, some games actually do things with these, as can be seen in this YouTube video by PtoPOnline.
Known games with GPI/GPO functionality
- GhostBusters: The Video Game will enable a debug overlay if you turn on some GPI switches
- Parappa The Rapper displays a metronome on the GPO LEDs.
- LocoRoco will enable extra debug logging if you turn on some GPI switches