Run MAME again and you'll get a similar warning, but now it will have an Open button. Click that, close System Preference, and close the warning that popped up. You should see a warning that libSDL2 isn't signed and a button to allow it anyway. While that error is up, open System Preferences, select Security & Privacy, and go to the General tab. The first time you run MAME after updating SDL on Big Sur (or the first time you build/run MAME on a new machine), you'll get a security error popup.
Drag the SDL2.Framework folder into the Frameworks folder, and then follow the normal Mac build instructions. Open it, then open a Finder window to 'Macintosh HD' or whatever your HDD/SSD name is (you may need to open the Finder's preferences and select Sidebar and then enable it for it to be shown) and open the Library folder. Get the DMG here: http:/ / release/ SDL2-2.0.14.dmg
It's recommended but not necessary on Intel if you're already up and running.
To build and run MAME 0.227 on Big Sur, updating your installed SDL framework to the new 2.0.14 release is required for M1 machines.
