Today, MAME supports over 40,000 unique ROM sets and 7,000+ unique arcade PCBs. It is considered the single most important preservation project in video game history. In simple terms, an "index of" is a directory listing on a web server. When a web server is configured without an index.html file, it often displays a raw, clickable list of all files and subfolders within that directory. This is known as "directory indexing."
Thus, an refers to a publicly accessible web directory that contains a list of MAME ROM files (usually archived in .zip format). For example, a URL might look like: index of mame roms
: You are better off with curated legal collections like Arcade Classics on Steam, Namco Museum , or Atari 50 . These are cheap, legal, and hassle-free. Today, MAME supports over 40,000 unique ROM sets
Additionally, the MAME project itself is moving toward emulating more modern hardware (Sega Naomi, PlayStation-based arcade systems), which requires CHD files (hard disk images) that are even harder to find legitimately. If you are a preservationist, researcher, or owner of original boards : Yes, indices are invaluable for obtaining known good dumps to compare against your hardware. When a web server is configured without an index