P, C, S, M, V: what is actually inside a Neo Geo ROM

The five kinds of ROM

The split is physical, not organisational. Each type feeds a different chip on a different bus, which is why the C-ROMs ship in pairs (the video hardware reads two at once) and why you cannot simply concatenate everything and hope. If you want the detail on how graphics are packed, that is its own subject.

Romset or .neo: which one you want

• A MAME romset is a zip holding each ROM as its own file, named the way MAME expects. This is what you want for emulators, for debugging, and for anything that needs to inspect one piece in isolation.
• A .neo is the NeoSD flashcart format: one file with a header and the ROM regions inside it. This is what you want to run a game on real hardware — copy it to the card, pick it from the menu.

Neither is more “real” than the other, and going from one to the other is repackaging rather than conversion. Any tool that can produce a Neo Geo game should be able to give you both, because they answer different questions: does it work, and does it work on the machine.

The BIOS is not part of your game

The Neo Geo has a system BIOS, and it is separate from any cartridge — the console is a base unit and the games are carts, arcade board or not. An emulator needs that BIOS before it can boot anything, which catches people out: a perfectly good romset that refuses to start is usually a missing BIOS, not a broken build.

For homebrew there is an open replacement — nullbios, part of the ngdevkit project — which is redistributable and boots a cartridge without shipping anything owned by SNK. It is what Neo Studio uses so a project can run in the browser without you supplying a BIOS of your own.