The PSX's processor actually has more documentation out there than the SNES's processor. The PSX CPU is MIPS, which at the time was a widespread architecture. The N64, PS2 and PSP also used MIPS, among game systems.
In fact, when I took the mandatory assembly language class in college, the architecture they taught was MIPS, because it was probably the easiest assembly language to learn. (If you ignored the delay slots, anyway.)
Naturally, I aced that class, because I'd been making cheat codes for PSX games for a few years by then. I ran my assignments on actual PSX rather than the MIPS emulator the class provided. =^_^=
I made the "save anywhere" cheat code for FF9. It was a monster.