Save File Extensions

Research results for which save file extensions RetroDECK cores produce, and what our plugin needs to support. This informs the implementation of #196.

How RetroArch Save Extensions Work

RetroArch has two save mechanisms:

1. Standard libretro saves (libretro_saves = "true" in core info): RetroArch handles all file I/O. The extension is hardcoded to .srm (SRAM) and .rtc (real-time clock). The core exposes memory via RETRO_MEMORY_SAVE_RAM / RETRO_MEMORY_RTC, and RetroArch writes the files. Every core using this mechanism produces identical extensions.

2. Core-managed saves (libretro_saves = "false" or absent): The core handles its own file I/O, writing to RetroArch's save directory with custom extensions. RetroArch has no control over these filenames.

This means .srm/.rtc covers the vast majority of cores. Only a handful of cores use custom extensions.

RetroDECK Core Inventory

Checked against RetroDECK's installed cores (March 2026). Only cores relevant to save file sync are listed.

Cores Using Standard .srm/.rtc (libretro_saves = true)

These cores all produce .srm (and optionally .rtc). No additional extensions needed.

Platform	Cores	Notes
GB/GBC	gambatte, sameboy, gearboy, tgbdual, fixgb, DoubleCherryGB	.rtc used by Pokemon Crystal/Gold/Silver etc.
GBA	mgba, vbam, vba_next, skyemu	All GBA save types (EEPROM, SRAM, Flash) packed into single .srm
NES	fceumm, nestopia, mesen	Battery-backed saves
SNES	snes9x (all variants), bsnes (all variants), supafaust, mednafen_snes, mesen-s	Standard SRAM
N64	mupen64plus_next, parallel_n64	Packed format: EEPROM + 4 Memory Paks + SRAM + FlashRAM all in one .srm (290KB)
Genesis/MD	genesis_plus_gx, genesis_plus_gx_wide, picodrive, blastem, clownmdemu	Cartridge backup
PSX	mednafen_psx_hw, mednafen_psx, pcsx_rearmed, swanstation	Memory card 0 as raw 128KB .srm
Saturn	mednafen_saturn, kronos, yabasanshiro, yabause	Standard save
DS	desmume, desmume2015, melondsds, melonds	See DeSmuME special case below
PC Engine	mednafen_pce_fast, mednafen_pce, mednafen_supergrafx	Standard save
Lynx	handy	Standard save
NGP/NGPC	mednafen_ngp, race	Standard save
Virtual Boy	mednafen_vb	Standard save
WonderSwan	mednafen_wswan	Standard save
3DO	opera	NVRAM
Jaguar	virtualjaguar	Standard save
Pokemon Mini	pokemini	Standard save

Cores With Special Extension Behavior

DeSmuME (DS) -- .dsv

• libretro_saves = "true", so default is .srm
• However, DeSmuME has a dual-mode: its BackupDevice class can write .dsv (DeSmuME's native format) when configured in Mednafen-compatibility mode
• The core will also fall back to loading .sav files if .dsv is not found
• Action: Add .dsv as platform override for DS, so we pick up saves from users who switched modes

Genesis Plus GX (Sega CD) -- .brm

• libretro_saves = "true" for cartridge SRAM (standard .srm)
• But Sega CD BRAM (backup RAM) is managed by the core separately as .brm files
• These include: cart.brm (cartridge backup RAM) and region-specific system BRAM (scd_U.brm, scd_E.brm, scd_J.brm)
• The per-game BRAM uses the standard rom-name .brm pattern
• Action: Add .brm as platform override for Sega CD

Flycast (Dreamcast) -- .bin

• No libretro_saves field in core info -- uses its own VMU format
• Produces vmu_save_{A1-D1}.bin files and dc_nvmem.bin
• Multi-slot VMU support (games can produce multiple .bin files)
• Action: Tracked separately in #151. Not included in extension expansion.

MAME / FBNeo (Arcade) -- .nv

• libretro_saves = "false" for all MAME/FBNeo cores
• MAME manages its own NVRAM system (writes to SAVEDIR/mame/NVRAM/)
• Not using RetroArch's standard save path at all
• Action: Not relevant for save sync. MAME saves use a completely different directory structure.

gpsp (GBA) -- no libretro_saves

• libretro_saves = "false" -- manages its own saves
• In practice, gpsp writes .sav files in its own format
• However, mgba/vbam are the recommended GBA cores in RetroDECK
• Action: Add .sav defensively to GBA overrides for gpsp users

mednafen_gba (Beetle GBA) -- custom format

• No libretro_saves field -- uses Mednafen's native format
• Can write .{md5}.sav and .{md5}.eep with MD5 hash in filename
• Has a dual mode: Mednafen method (own files) vs libretro method (.srm)
• Rarely used in RetroDECK (mgba is the default)
• Action: Not worth supporting the MD5-hashed filenames. Users of this core should use libretro mode.

Comparison With Other RomM Clients

Grout (Go client)

Uses a flat global allowlist of 11 extensions with no per-platform mapping:

.srm, .sav, .dsv, .mcr, .mcd, .brm, .eep, .sra, .fla, .mpk, .nv

This is intentionally broad -- Grout targets multiple CFWs (muOS, NextUI, MinUI) where standalone emulators produce these formats. For RetroArch-only setups like RetroDECK, most of these are unnecessary.

Argosy (Android client)

Uses a per-emulator mapping:

Emulator	Extensions
RetroArch	.srm, .sav
Mupen64Plus FZ (standalone)	.sra, .eep, .fla, .mpk
DraStic (standalone DS)	.dsv, .sav
melonDS (standalone)	.sav
DuckStation (standalone)	.mcd
MAME	.nv

Again, standalone emulator formats that don't apply to RetroDECK's RetroArch-based setup.

Implementation Decision

For decky-romm-sync (RetroDECK-only):

Extension	Include?	Reason
.srm	Yes (already supported)	Standard RetroArch SRAM
.rtc	Yes (already supported)	Standard RetroArch RTC
.dsv	Yes (add as DS override)	DeSmuME native format
.brm	Yes (add as Sega CD override)	Genesis Plus GX Sega CD BRAM
.sav	Yes (add to defaults)	gpsp fallback, DeSmuME fallback, defensive
.bin	No (separate ticket #151)	Flycast Dreamcast VMU -- needs special handling
.eep, .sra, .fla, .mpk	No	N64 standalone formats, Mupen64Plus packs into .srm
.mcr, .mcd	No	PSX Mednafen/DuckStation formats, RetroArch cores use .srm
.nv	No	MAME NVRAM, completely different directory structure

Platform Override Map

_DEFAULT_EXTENSIONS = (".srm", ".rtc", ".sav")

_PLATFORM_OVERRIDES = {
    "nds": (".srm", ".rtc", ".sav", ".dsv"),
    "segacd": (".srm", ".rtc", ".sav", ".brm"),
}

Adding .sav to defaults is conservative but safe -- it ensures we pick up saves from any core that uses .sav as an alternative SRAM format, without false-matching on non-save files.