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A well-made pack mirrors the official MiSTer layout: Games/SNES/ , Games/Genesis/ , Games/PSX/ , _Arcade/ (with .mra files), etc. Some even include artwork, cheats, and custom wallpapers.
A typical "Mister Rom Pack" is usually distributed as a compressed archive ( Mister Rom Packs
Technically not a static download, the update_all.sh script is the de facto pack manager for MiSTer. When you run it, it downloads the official Mister Rom Packs from the internet archive automatically. A well-made pack mirrors the official MiSTer layout: