What is custom firmware for retro handhelds?
Custom firmware explained: what it is, why you'd want it, and which one to use. Covers OnionOS, GarlicOS, KNULLI, MinUI, and RetroArch.
If you’ve been reading about retro handhelds, you’ve probably seen “OnionOS” or “custom firmware” mentioned everywhere. Here’s what that actually means and whether you should care.
What is custom firmware?
Custom firmware (CFW) is replacement software for your handheld. You install it on the SD card and it replaces the manufacturer’s default operating system (stock firmware) with something better.
Stock vs. custom: what changes
| Stock firmware | Custom firmware | |
|---|---|---|
| Made by | Device manufacturer | Community developers |
| Interface | Basic, often rough | Polished, themeable |
| Features | Minimal | RetroAchievements, themes, Wi-Fi sync |
| Emulators | Varies (often outdated) | Latest versions, tuned per-device |
| Updates | Rare or never | Frequent |
| Stability | Usually stable | Very stable (for mature CFWs) |
Why bother?
1. The stock experience is usually bad
Most budget handhelds ship with laggy menus, ugly interfaces, and no artwork. CFW fixes all of that.
2. Better emulation
CFW developers optimize emulator settings for each specific device. Stock firmware tends to use generic configs that waste performance.
3. Features you’ll actually use
RetroAchievements, save state sync, artwork scraping, network play — none of this exists in stock firmware.
4. Bugs get fixed
Stock firmware bugs might never get patched. CFW communities typically fix issues within days. OnionOS has shipped dozens of updates since launch.
The major options
OnionOS — the one everyone talks about
- Compatible devices: Miyoo Mini Plus, Miyoo Flip
- Type: Standalone (replaces everything)
- Best for: Beginners and enthusiasts who want the best overall experience
OnionOS is why the Miyoo Mini Plus is so popular. It turns a $59 device into something that feels like it cost three times as much: nice UI, RetroAchievements, Wi-Fi sync, a package manager, and instant sleep/resume.
Track OnionOS updates → | Installation guide →
GarlicOS — lightweight and fast
- Compatible devices: RG35XX H, RG28XX, and other Allwinner H700 devices
- Type: Standalone
- Best for: Users who want speed and simplicity
GarlicOS prioritizes fast boot times and snappy performance. It’s less feature-rich than OnionOS but incredibly responsive. If you just want to play games without bells and whistles, GarlicOS delivers.
KNULLI — the multi-device option
- Compatible devices: RG35XX H, RG28XX, TRIMUI Smart Pro, and more
- Type: Distribution (based on Batocera Linux)
- Best for: Users with multiple devices who want one consistent experience
KNULLI supports the widest range of devices. It’s based on Batocera Linux, which means a mature, well-tested foundation with extensive emulator support.
MinUI — for people who hate clutter
- Compatible devices: RG35XX H, RG28XX, Miyoo Mini Plus, Miyoo Flip
- Type: Standalone
- Best for: People who hate clutter and want the simplest possible experience
MinUI strips everything away. No artwork, no themes — just a black-and-white menu and your games. It also gets the best battery life of any CFW, probably because it’s doing so little.
RetroArch — the universal backend
- Compatible devices: All Android devices (Retroid Pocket 5, AYN Odin 2, RG405M, etc.)
- Type: Overlay/Frontend
- Best for: Android-based handhelds
RetroArch isn’t a standalone firmware — it’s an emulation frontend that runs on top of Android (or other OSes). It’s the backbone of most CFW solutions and provides 100+ emulator cores, shader support, RetroAchievements, and netplay.
Which one should you use?
Decision flowchart:
Do you have a Miyoo device?
- Yes → OnionOS (no contest)
Do you have an Anbernic H700 device (RG35XX H, RG28XX)?
- Want features → KNULLI
- Want speed → GarlicOS
- Want simplicity → MinUI
Do you have an Android device (Retroid, AYN)?
- Install RetroArch + Daijishō frontend
Is custom firmware safe?
Yes. All major custom firmware options:
- Install on the MicroSD card, not the device’s internal storage
- Are fully reversible — swap back to a stock SD card and you’re back to normal
- Are open source — code is publicly auditable on GitHub
- Will not brick your device — even a bad install just means reflashing the SD card
The only risk is downloading from unofficial sources. Always get CFW from the official GitHub repository of each project.
FAQ
Do I need custom firmware?
Not strictly — stock firmware will play games. But CFW dramatically improves the experience. We recommend it for everyone.
Will custom firmware void my warranty?
Technically, maybe — but since it’s SD card-based, the manufacturer can never tell you used it. Simply revert to stock before any warranty claims.
Can I try multiple custom firmwares?
Absolutely. Flash different CFWs on different SD cards and swap between them. Many enthusiasts keep multiple cards for different firmware experiences.
Do I need to be technical to install CFW?
Not at all. Most modern CFW installs involve: format SD card → copy files → insert card → done. Our OnionOS installation guide walks through every step.
Will I lose my save games if I switch?
Save games are stored on the SD card. When switching between CFWs that use RetroArch, saves are typically compatible. For extra safety, always back up your Saves/ folder.
Stay updated
CFW moves fast. Use our firmware tracker to monitor all major projects in one place — versions, changelogs, and download links.
Related:
- How to install OnionOS — step-by-step guide
- Best retro handhelds under $100 — device picks
- OnionOS: the complete guide — deep dive