Official Playstation 1 Development Kit (Hardware) (2020)
there was also custom firmware that someone had written for one of the cheat devices (i think maybe the "gameshark"?)  with that, and a $40 mail order ISA board, and some time building a MIPS cross compiling gnu toolchain, you could write programs in C and run them on the consumer/production PS1 as a hobbyist.i'm trying to remember what the OS situation was though. i remember some really thin libc that made syscalls into some rom maybe?  or maybe it was just a thin libc.  i do remember being able to access the audio device without doing anything super low level...edit: here we go! the firmware was called caetla and here's a nice 22yr old webpage that describes it:   http://hitmen.c02.at/html/psx_faq.html