Comment by spicyjpeg

I'm late to the party but, as a prolific contributor to PSn00bSDK and the PS1 homebrew scene more in general, I feel obliged to shamelessly plug my own "PlayStation 1 demystified at the absolute lowest level" repo:

https://github.com/spicyjpeg/ps1-bare-metal

It's still very work in progress - I have only covered a tiny fraction of what the console's hardware can do - but I find it fascinating to explore how little code you actually need to get started on such a simple platform, even with no external SDKs or tools (aside from a completely standard MIPS GCC toolchain).