When Sony created the PSP, they needed to sell digital PS1 classics (like Final Fantasy VII and Castlevania: Symphony of the Night ). Instead of brute-forcing emulation, Sony engineers wrote a highly optimized, hyper-efficient PS1 emulator called POPS. This emulator used a specific BIOS file ( psxonpsp.bin ) that was stripped of extraneous CD audio routines and boot-up sequences, focusing purely on game execution.

: To ensure you have a clean dump, the MD5 checksum should be c53ca5908936d412331790f4426c6c33 to use this BIOS? gingerbeardman/PSX - GitHub

If you have the file, here is how to ensure RetroArch recognizes it.