Disk Swapping

Disk Swapping & Multi-Disc Games

Multi-disc CD-based RPGs and adventures, plus disk-side games on the Famicom Disk System, all need a way to swap discs/sides without restarting the game. Emutastic handles this in two parts: an import-time auto-bundle that produces a single library entry per game, and an in-game disk swap chord that flips between images mid-play.

In-Game Disk Swap

Default chord: L3 + Start (click the left analog stick + Start). Rebindable in Preferences → Controls → Disk Swap to any two-button chord (controller or keyboard).

When the chord fires, the bottom status bar shows one of:

• Disk N / M — successfully advanced to the next disc
• Disk: side change — FDS side flip injected (Nestopia / Mesen / FCEUmm)
• Disk swap: only one disc loaded — put all discs in the same folder and re-import — game has a single image; see auto-bundling below
• Disk swap: this core doesn't support disc switching — the core never registered the libretro disk control interface

The chord is round-robin: it cycles 1 → 2 → 3 → 1 again. RetroArch's deferred-insert timing is mirrored (a ~100-frame delay between set_image_index and the matching set_eject_state(false)) so cores like Beetle PSX get their CD audio engine the spin-down they expect. The chord and disk-control handling live in EmulatorSession.cs.

Default chord rationale

Start + Select would be the obvious default, but it conflicts with the Steam overlay toggle. L3 is rarely used in retro games and free of system-level conflicts, so the default chord stays out of the way of every shipping libretro game.

Auto-Bundling on Import

Drop a folder containing a multi-disc set and Emutastic writes an .m3u playlist next to the disc files, then imports only the playlist:

MyGame (USA) (Disc 1).cue
MyGame (USA) (Disc 2).cue
MyGame (USA) (Disc 3).cue
MyGame (USA).bin sidecars (×3)

→ produces one library entry: MyGame (USA).m3u.

Detection rules

The auto-bundler looks for (Disc N), (Disk N), (CD N) patterns in filenames (case-insensitive, regex tolerant of separators and "of M" suffixes). Files are grouped when all of these match:

• Same parent folder
• Same base title (filename minus the disc tag)
• Same extension (so a .cue set and a .chd set in the same folder stay separate)

Conservative gates:

• Bundle requires at least 2 disc files
• Disc numbers must form a contiguous sequence starting at 1 — pathological pairs like (Disc 1) + (Disc 5) are left as individual imports
• Base title must be at least 3 characters to avoid false matches like cd1.cue + cd2.cue from unrelated dumps
• Disc tag must appear within 30 characters of the end of the filename to filter out matches in title-words like Audio CD 32 Sampler

When the auto-bundler folds files into an .m3u, it also:

• Adds .bin sidecars of bundled .cue files to the skip set so they don't import as separate library entries
• Honors hand-authored .m3u files in the same folder — if you've already curated a playlist, the bundler leaves it alone and imports it as the single entry
• Removes stale single-disc library rows for any file that's now part of a bundle, so re-importing converts scattered per-disc entries into one playlist entry

What this looks like in your library

Folder contents	Library result
Game.cue (single disc)	One Game entry
Game (Disc 1).cue/.bin + (Disc 2).cue/.bin	One Game entry (the auto-generated .m3u)
Game (Disc 1).chd + (Disc 2).chd	One Game entry
Game.m3u (you wrote it) + the discs it lists	One Game entry (your .m3u)
Mixed compilation: Anthology - Game V.cue + Anthology - Game VI.cue	Two entries (different game names → not bundled)

Per-Console Behavior

Console	Mechanism	Notes
FDS	JOYPAD_L injection on port 0	Nestopia / Mesen / FCEUmm convention. The chord injects the libretro NES "Disk Side Change" button for ~6 frames. Keep nestopia_fds_auto_insert = enabled (cores never register the env disk control interface for FDS, so the chord falls back to the input convention)
PS1	SET_DISK_CONTROL_INTERFACE (env 13 / 58 EXT)	Beetle PSX registers the interface in retro_init. Use an .m3u so the core sees every disc
Saturn	env 13 / 58 EXT	Kronos accepts cue / iso / mds / ccd / chd / zip / m3u. Disk swap requires .m3u (single-disc loads register the callback but disk_total = 1)
Sega CD	env 13 (legacy only)	Genesis Plus GX registers the interface, deregisters with NULL when system_hw != MCD
Amiga	env 13 / 58 EXT	PUAE supports the interface. Plain .m3u entries land in DF0 only — games that need DF1 mounted simultaneously require (MD) filename markers (PUAE convention) which the auto-bundler doesn't generate

Cores that don't expose disk control (Beetle PCE / PCE Fast / Opera 3DO) won't surface the swap chord. Most 3DO and PC Engine CD games are single-disc anyway.

Troubleshooting

Chord doesn't fire. Check the log at ~/.local/share/Emutastic/Logs/emulator.log for [DiskDiag] poll alive heartbeats. If you see ctlConnected=False, the controller isn't reaching the EmuThread. If ctrlA=False while you're clicking L3, your controller may not expose the left thumb button via SDL3 — try a different controller or rebind the chord.

Status shows "only one disc loaded". The library entry is for a single image. Put every disc file in the same folder and re-import the folder; the bundler will write the .m3u and replace the single-disc entry.

Status shows "this core doesn't support disc switching". The current core didn't register SET_DISK_CONTROL_INTERFACE. Check the per-console table above — for FDS, this is normal (chord uses input injection instead). For Sega CD it means the loaded ROM isn't an MCD image (Genesis Plus GX deregisters disk control for non-MCD content). For other consoles, try a different core via Preferences → Cores.

Game won't boot, status says "PLEASE SET DISK CARD" (FDS). You disabled nestopia_fds_auto_insert. Re-enable it — that option is the entire boot mechanism for FDS in Nestopia; without it the core waits forever for a frontend disk-control UI that doesn't exist for FDS.