Affiliate disclosure: This guide contains affiliate links. As an Amazon Associate and Anbernic affiliate, we earn from qualifying purchases at no extra cost to you. Learn more.
You bought a retro handheld, you've heard the stock firmware is trash, and everyone says to flash custom firmware. Great — but which one? The three most popular options are muOS, KNULLI, and Onion OS, and they take fundamentally different approaches to the same problem: turning your handheld into the best retro gaming experience possible.
This guide compares all three on the dimensions that actually matter: device support, user interface, performance, features, update process, and ease of setup.
The Quick Answer
Onion OS: Best for the Miyoo Mini Plus. The most polished, beginner-friendly experience. If you own a Miyoo Mini, use this.
muOS: Best for Anbernic H700 devices if you want speed and simplicity. RetroArch-first, fast boot, minimal overhead.
KNULLI: Best for Anbernic H700 devices if you want a visual, feature-rich experience. EmulationStation frontend, box art, RetroAchievements, OTA updates.
Device Compatibility
This is the first and most important filter. Not every firmware runs on every device.
| Firmware | Supported Devices |
|---|---|
| Onion OS | Miyoo Mini, Miyoo Mini Plus only |
| muOS | Anbernic H700 family (RG35XX, RG35XX Plus, RG35XX H, RG35XX SP, RG35XX Pro, RG34XX SP, RG28XX, RG40XX H/V, RG CubeXX) |
| KNULLI | All muOS devices plus TrimUI Brick, TrimUI Smart Pro, PowKiddy models, GKD devices, and more |
If you own a Miyoo Mini Plus, the choice is made for you: Onion OS is the only option from this list. If you own an Anbernic H700 device, you're choosing between muOS and KNULLI. If you own a TrimUI or other non-Anbernic Linux handheld, KNULLI is likely your best bet.
User Interface
Onion OS
Onion OS uses a custom, purpose-built interface that's clean, fast, and intuitive. Games are organized by system in a simple vertical list. The UI supports box art display, and the Game Switcher feature lets you quickly hop between recently played titles. There's a blue light filter for nighttime play and a built-in box art scraper.
The interface feels intentional — every element is there for a reason, and nothing is cluttered. It's the firmware you'd install for someone who has never touched an emulator before.
muOS
muOS uses RetroArch as both its emulation engine and its primary interface. Recent versions support visual themes with icons and box art, which has dramatically improved the look and feel. But it's still RetroArch at its core — you navigate with the same menu system, configure the same settings, and use the same hotkeys.
For users comfortable with RetroArch, this is a feature, not a drawback. muOS is fast, direct, and stays out of your way. Boot times are minimal, and there's almost no lag between launching the firmware and playing a game.
KNULLI
KNULLI uses EmulationStation, which is a full game-browsing frontend with box art, metadata, descriptions, ratings, and genre tags. After running the built-in scraper, your game library looks like a curated collection rather than a pile of files.
It's visually the richest of the three and the closest to a "console-like" experience. The tradeoff is more complexity — there are more menus, more settings, and more to configure. EmulationStation is also heavier than a bare RetroArch interface, which means slightly longer boot times and a bit more memory overhead.
Features Comparison
| Feature | Onion OS | muOS | KNULLI |
|---|---|---|---|
| Box art display | Yes (built-in scraper) | Yes (via themes) | Yes (ScreenScraper integration) |
| RetroAchievements | Yes | Yes | Yes |
| OTA updates | No | No | Yes (Gladiator II+) |
| Game metadata/descriptions | Limited | No | Yes (full scraper) |
| Netplay (online multiplayer) | Limited | Yes | Yes |
| Wi-Fi file transfer | No (no Wi-Fi on device) | Yes | Yes (SMB/SAMBA) |
| PortMaster support | No | Yes | Yes |
| Theme support | Limited | Yes | Yes |
| Per-game settings | Yes | Yes | Yes |
| Game Switcher (quick resume) | Yes | No | No |
| Sleep on lid close | N/A (no clamshell) | Varies by device | Yes (for clamshell devices) |
Performance
All three firmwares run RetroArch under the hood for actual emulation, so game performance is broadly comparable. The differences are in overhead and boot speed:
muOS has the fastest boot time and lowest overhead. If you're squeezing every frame out of a demanding N64 or Dreamcast game, muOS's lighter footprint can make a marginal difference.
KNULLI adds EmulationStation on top, which uses more memory and adds a few seconds to boot time. For 99% of games this doesn't matter, but on the absolute edge of playability (like demanding PSP titles on H700), muOS may eke out a frame or two more.
Onion OS is optimized specifically for the Miyoo Mini Plus hardware, so it's extremely efficient on that device. Performance comparisons to muOS/KNULLI are apples-to-oranges since they run on different hardware.
Update Process
Onion OS: Download the new version, flash it to your SD card, done. No OTA. Your game card stays separate if you use a two-card setup.
muOS: Same process — download, reflash, done. No OTA. The two-card setup means your games and saves survive the reflash.
KNULLI: OTA updates via Wi-Fi (since Gladiator II). Navigate to Updates & Downloads in the settings, check for updates, install. Your games, saves, and settings are preserved. This is a huge quality-of-life advantage over the other two.
Ease of Setup
Onion OS is the easiest. Flash the image, copy your games, boot up. The interface is intuitive enough that you can hand the device to someone with zero emulation experience and they'll figure it out.
muOS is straightforward for anyone comfortable with file management. Flash the image, set up your game folders, configure a few settings. The RetroArch foundation means there's a learning curve if you've never used RetroArch, but it's well-documented.
KNULLI has the most involved setup due to EmulationStation's configuration options, scraper setup, and the two-card setup recommendation. It's not difficult — just more steps. The payoff is the richest feature set.
Which Should You Choose?
Choose Onion OS if: You have a Miyoo Mini Plus and want the simplest, most polished experience with zero configuration headaches.
Choose muOS if: You have an Anbernic H700 device and want maximum speed, minimal UI overhead, and a clean RetroArch-first experience. Best for users who already know RetroArch or want to learn it.
Choose KNULLI if: You have an Anbernic H700 (or TrimUI) device and want a visual game library with box art, RetroAchievements, OTA updates, and an EmulationStation frontend. Best for users who want the richest feature set and don't mind a slightly longer setup.
You can always try both muOS and KNULLI — flash one, use it for a week, flash the other. Your game card stays the same if you use a two-card setup. There's no wrong answer here; both are excellent, actively maintained, and well-supported by their communities.
For step-by-step installation, check our muOS setup guide, KNULLI setup guide, or Onion OS setup guide.