Mesen - RetroBIOS¶
Technical notes
Mesen is a cycle-accurate NES/Famicom emulator by Sour (the original NES-only version, archived in 2020 in favor of Mesen2). NES cartridge games need no BIOS. Famicom Disk System games require the FDS BIOS ROM.
The core tries FdsBios.bin first, then disksys.rom as fallback. Both are loaded from the system directory root. If neither is found or the file is not exactly 8192 bytes, FDS games fail to load (BiosMissing).
The Study Box is a Famicom tape-based learning peripheral (mapper 186). Its BIOS (StudyBox.bin, 256 KB) is loaded the same way. Only needed for Study Box tape dumps (.stbx format).
MesenDB.txt (game database) is baked into the libretro binary as MesenDB.inc and loaded in retro_init(). The external file path in InitDatabase() is never reached in the libretro context.
MesenPalette.pal is an optional custom palette file, loaded only when the mesen_palette core option is set to Custom. Accepts 192 bytes (64 colors) or 1536 bytes (512 colors with emphasis combos).
HdPacks go in system/HdPacks/
File path construction: retro_load_game() sets FolderUtilities::SetHomeFolder(systemFolder). All BIOS lookups use CombinePath(GetHomeFolder(), filename) -- flat, no subdirectories.
No functional divergence between upstream and libretro for BIOS loading.
3 files | 1 required, 2 optional | 3 in repo, 0 missing
disksys.rom required in repo
FDS BIOS ROM
- System: nintendo-fds
- Size: 8.0 KB
- Aliases:
FdsBios.bin - Validation: size
- Source:
Core/FdsLoader.cpp:45-59,Core/FdsLoader.cpp:143-146 - Platforms: batocera, emudeck, lakka, recalbox, retroarch, retrobat, retrodeck, retropie, romm
- Mapped at 0xE000-0xFFFF as PRG ROM bank 0. Core checks FdsBios.bin first, falls back to disksys.rom. Size must be exactly 8192 bytes or BiosMissing is set.
StudyBox.bin optional in repo
Famicom Study Box BIOS ROM
- System: nintendo-nes
- Size: 256.0 KB
- Validation: size
- Source:
Core/StudyBoxLoader.cpp:125-133,Core/StudyBoxLoader.cpp:149-152 - 256 KB BIOS for the Study Box tape peripheral (mapper 186). Only needed for .stbx tape dumps. Size must be exactly 262144 bytes.
MesenPalette.pal optional in repo
External NES color palette
- System: nintendo-nes
- Size: min 192 B, max 1.5 KB
- Validation: size
- Source:
Libretro/libretro.cpp:301-323,Libretro/libretro.cpp:405-406 - Accepts 192 bytes (64 RGB triplets) or 1536 bytes (512 emphasis combos). Falls back to built-in default if size doesn't match.
Generated on 2026-09-14T09:03:10Z