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# FCEUmm - RetroBIOS

Type | libretro
Classification | community_fork
Source | https://github.com/libretro/libretro-fceumm
Upstream | https://sourceforge.net/projects/fceumm/
Version | SVN
Profiled | 2026-03-23
Cores | fceumm
Systems | nintendo-nes, nintendo-fds

Technical notes

FCEUmm (FCE Ultra mappers modified) is a NES/Famicom emulator forked from FCEU Ultra by CaH4e3. FDS games require the FDS BIOS ROM (disksys.rom). Game Genie emulation requires gamegenie.nes when the core option is enabled. An external palette file (nes.pal) overrides the built-in palette when the core option is set to Custom.

File path construction: src/general.c FCEU_MakeFName() joins the system directory with each filename directly - no subdirectories.

3 files | 1 required, 2 optional | 3 in repo, 0 missing

`disksys.rom` -required : FDS BIOS ROM

- System: nintendo-fds
- Size: 8.0 KB
- Validation: size
- Source: `src/fds.c:636-667 (FDSLoad), src/general.c:64-66 (FCEUMKF_FDSROM)`
- Platforms: batocera, emudeck, lakka, recalbox, retroarch, retrobat, retrodeck, retropie
- Loaded at 0xE000, mapped as 8 KB PRG bank 0. Load fails if missing or short read.

`gamegenie.nes` -optional : Game Genie add-on cartridge ROM

- System: nintendo-nes
- Source: `src/cart.c:344-398 (FCEU_OpenGenie), src/general.c:60-62 (FCEUMKF_GGROM)`
- Platforms: lakka, retroarch, retropie
- iNES format ROM (4 KB PRG + 256 bytes CHR). Core option disabled by default. Only applies to cartridge games.

`nes.pal` -optional : External NES color palette

- System: nintendo-nes
- Source: `src/palette.c:170-200 (FCEU_LoadGamePalette), src/general.c:68-70 (FCEUMKF_PALETTE), src/drivers/libretro/libretro.c:1789-1798`
- RGB triplets (64 standard or 512 with de-emphasis). Only loaded when palette core option is set to Custom.

Generated on 2026-03-25T14:56:43Z