is a generic naming convention used by many emulator frontends and some ROM management tools. It is essentially a raw binary dump of the original PS1 BIOS chip. The file size is almost always 524,288 bytes (512 KB) exactly. If your file is a different size, it is either corrupted or a different BIOS version.

The PlayStation 1 is no different. It contains a BIOS ROM chip that holds Sony’s proprietary code. This code handles:

It is often considered the only legitimate way to acquire a BIOS without personal console hardware, as it is distributed by Sony in their modern system updates.

Emulators are not magic—they recreate the behavior of the PS1 hardware using software. However, Sony’s BIOS code is proprietary and copyrighted. Emulator developers cannot legally distribute it with their software. Therefore, they design the emulator to act as a "shell" that loads an external BIOS file provided by the user.