Open agb firm 3DS

From GameBrew

Last Updated2022/04/25

open_agb_firm is a bare metal interface for natively running GBA games and homebrew using the 3DS's built-in GBA hardware.

Note: open_agb_firm is currently in alpha. While open_agb_firm is relatively stable and safe to use, some quirks that have not been fixed. Use at your own risk.


  • A complete and better alternative to GBA VC injects (AGB_FIRM).
  • Launch GBA files directly from the SD card.
  • Write save files directly to the SD card.
  • Automatic save type configuration using an included database.
  • User configuration, such as gamma settings.
  • And more to come.


Download contains alpha version 2021-12-24 and a new build 2022-01-05 compiled by stranno.

Copy the open_agb_firm.firm file to your 3DS's SD card at /luma/payloads/ if you're using Luma3DS or elsewhere if you're using fastboot3DS.

Copy the 3ds folder to the root of your 3DS's SD card. Merge folders if asked.

Launch open_agb_firm using Luma3DS by holding Start while booting your 3DS or assign it to a slot if you're using fastboot3DS.

After open_agb_firm launches, use the file browser to navigate to a .gba ROM to run.

User guide


Settings are stored in /3ds/open_agb_firm/config.ini.

General settings:

  • u8 backlight - Backlight brightness (default: 64, values ≤64 are recommended). Possible values are:
    • Old 3DS - Between 20 and 117.
    • New 3DS - Between 16 and 142.
  • bool directBoot - Skip GBA BIOS intro at game startup (default: false).
  • bool useGbaDb - Use gba_db.bin to get save types (default: true).


  • u8 scaler - Video scaler. 0 = none, 1 = bilinear, 2 = hardware (default: 2).
  • float gbaGamma - GBA input gamma (default: 2.2).
  • float lcdGamma - Output LCD gamma (default: 1.54).
  • float contrast - Screen gain (default: 1.0).
  • float brightness - Screen lift (default: 0.0).


  • u8 saveSlot - Only intended to be used in the per-game settings (romName.ini in /3ds/open_agb_firm/saves). Savegame slot 0-9 (default: 0).

Advanced settings:

  • bool saveOverride - Open save type override menu after selecting a game (default: false).
  • u16 defaultSave - Change save type default when save type is not in gba_db.bin and cannot be autodetected. Possible values:
    • 0, 1 - EEPROM 8k.
    • 2, 3 - EEPROM 64k.
    • 4, 6, 8 - Flash 512k RTC.
    • 5, 7, 9 - Flash 512k.
    • 10, 12 - Flash 1m RTC.
    • 11, 13 - Flash 1m.
    • 14 - SRAM 256k (default value).
    • 15 - None.

Hardware Limitations

open_agb_firm using the 3DS's built-in GBA hardware. Unfortunately, this comes with limitations compared to GBA emulators. This is a list of limitations cannot be solved in software/difficult to work around:

  • >32 MiB (>256 Mbit) games and homebrew.
  • Games with extra hardware built into the cartridge (except real-time clocks). Patches are required.
  • Proper save autodetection (can't find save type during gameplay).
  • GBA serial port (aka Link Cable).
  • >32 KiB (>256 Kbit) SRAM (homebrew games/emulators).
  • Reboots are required for switching between games.
  • No save states. Very difficult to implement because no direct hardware access.
  • Sound has lots of aliasing issues. No known workaround (hardware bug).


Most emulators output EEPROM saves differently than what open_agb_firm expects, making them incompatible.

Fortunately, they are very easy to fix, using this tool by exelotl.

The tool also works vise versa, if you want to use a save generated by open_agb_firm with an emulator.


Q. Why isn't open_agb_firm a normal 3DS app?

To access the 3DS's GBA hardware, open_agb_firm needs to run with full hardware access, which can only be provided by running as a FIRM.

Q. Is this safe to use?

Of course! While open_agb_firm does run with full hardware access, a lot of work has been put in by several people to ensure that nothing unexpected happens.

Some backend code from open_agb_firm is actually used in fastboot3ds.

Q. What games work with open_agb_firm?

In theory, all of them, except those that fall within the hardware limitations.

Q. How can I increase the brightness?

Increase the value of the backlight setting in config.ini.

See Configuration for more information.

Q. Why do the colors look off?

The default gamma settings are intended to make up for the washed out colors the 3DS LCD has.

If they look weird to you, setting the outGamma setting to 2.2 might help.

Q. Why do some of my ROM hacks/homebrew games have saving issues?

open_agb_firm resorts to save autodetection when it can't find an entry for the game it's running in gba_db.bin (which only contains data for official games).

And it's a bit wonky for games that use EEPROM or misleading SDK save strings.

Q. Why doesn't my save file from an emulator work?

There's a good chance that the save you're having issues with is an EEPROM save, which most emulators output differently. See EEPROM Fixer.

Q. My game doesn't save properly!

First, please ensure that the GBA ROM you are playing is not modified in any way, and matches its No-Intro checksums.

Second, make sure you aren't using an existing .SAV file, because some may have issues for various reasons.

Third, make sure your gba_db.bin is up-to-date. If everything seems to be in order but the game still doesn't save properly, please open an issue so it can be fixed.

In the meantime, the useGbaDb and saveOverride settings may be useful (see Configuration for more information).


A/B/L/R/Start/Select - GBA buttons

Select+Y - Dump screen output to /3ds/open_agb_firm/texture_dump.bmp (press Home if it freezes)

Hold Power button - Turn off the 3DS



Known issues

  • Sleep mode is not fully implemented.
  • Using Select+Y to dump screen output to a file can freeze the screen output sometimes.
  • Save type autodetection may still fail for certain games using EEPROM.
  • Lack of settings (including brightness control during gameplay).
  • No cheats and other enhancements.


alpha build 2022-4-25

This is a build from the master branch for the impatient.

  • Added configurable scaling so only 1 build is needed instead of one per scaling method. See the README on how to configure this.
  • Fixed the missing newline in the default config.ini.
  • firmtool support was added to the build system thanks to @Midnoclose.
  • Fixed a bug that may cause the file browser not to display all 1000 files.
  • Added day of week calculation for the GBA RTC.
  • Other non-user facing improvements/fixes.

The save type database was not created by me and i can't vouch for its completeness or accuracy (i have not heard any complaints so far). Keep that in mind when using this build and backup your saves. Also note that EEPROM savegames from some emulators or even flashcarts are incompatible because they are laid out wrong (every 8 bytes block of data is reversed). This tool can fix that.

The used scale matrix is the default "Sharp + edge enhance" one.

alpha build 2021-12-24

  • Increased file browser entry limit to 1000 per folder (the real limit depends on the average file name length of all files).
  • Autoboot support has been added. Place autoboot.txt in /3ds/open_agb_firm containing the ROM path in a single line. Example path: sdmc:/rom.gba.
  • All savegames and per-game configs are now stored under /3ds/open_agb_firm/saves.
  • Per game config support has been added and with it multiple savegame slot support (up to 10). Currently this is the only officially supported config option for games. Place romName.ini (replace romName with the ROM file name) in /3ds/open_agb_firm/saves. See the README under the Game section.

Release notes.


Thanks to:

  • yellows8.
  • plutoo.
  • smea.
  • Normmatt.
  • WinterMute.
  • ctrulib devs.
  • LumaTeam.
  • devkitPro.
  • ChaN (fatfs).
  • benhoyt (inih).
  • fastboot3DS project.
  • MAME.
  • No-Intro.
  • Wolfvak, Sono and all the other people in #GodMode9 on freenode/Discord.
  • endrift, Extrems and all the other people in #mgba on freenode.
  • everyone who contributed to

Copyright (C) 2021 derrek, profi200, d0k3.

External links