The MAME 2003 set is closed. No new ROMs are being added to it. This means you can download a complete 0.078 reference set (roughly 15GB for ROMs + 80GB for CHDs) and know that every game works. Modern MAME sets are 400GB+ and constantly changing.
To tie it all together, here is the practical guide for the most common use case. MAME 2003 Reference Set - MAME 0.078 ROMs- CHDs...
What you need:
Steps:
Pro Tip: In the RetroArch Quick Menu (Hotkey + X), go to Options and turn on "Skip Warnings" . This removes the red "This game is not perfect" text that MAME 2003 shows for every single game. The MAME 2003 set is closed
The MAME 2003 Reference Set (based on MAME 0.078) is not the newest, most accurate, or most complete arcade emulation set. But it is the gold standard for single-board computers (like the Raspberry Pi 3/4), classic modded consoles (PlayStation Classic, OG Xbox), and any device with a low-power ARM processor. It represents a "performance sweet spot" where compatibility, speed, and ease of use beat bleeding-edge accuracy. Steps: