This guide will presume you have already installed the radare2 suite of binary analysis tools on your computer.
First lets run radare2:
To Disassemble PS1 Binary files (MIPS little endian) you need to map a file at offset 0x80010000 like so:
o ./SLES_032.85 0x80010000 rwx
Replace SLES_032.85 with the location of your own PSX executable.
Now we want to analyze all the functions in the executable like so: