Games on several discs

A game on several discs is one game on CabinetOS: one entry in the library, one save, and a Change disc row in its pause menu. Single-disc games show none of this.

On your RomM server

Put every disc of a game in one folder, for example Metal Gear Solid/Metal Gear Solid (Disc 1).chd and Metal Gear Solid/Metal Gear Solid (Disc 2).chd. That is RomM’s rule: loose discs side by side are separate games, each with its own save and no Change disc.
Discs are numbered from their file names. Disc 2, Disk 2, CD2, Side B, Part 2 and D2 all work, with or without brackets. With no marker at all, the part of the names that differs decides the order (game_a, game_b, game_c). An .m3u you made yourself sets the order if it names every disc.
A disc’s name in the menu is Disc N, plus any bracketed part only that disc’s file has: (Disc 2) (Soviet) shows as Disc 2 (Soviet). Without one you see plain Disc 1, Disc 2.
A disc missing from the folder shows on the game’s page as Disc 2 missing; the other discs still play.

Playing

Play starts on the disc you were last on (disc 1 the first time). The game’s page says which: Disc 2 of 4.
One save for the whole game. Every disc reads and writes the same memory card, so a save made on disc 1 is there on disc 2.
Remove deletes every disc. Your saves stay. On PlayStation and Sega CD the emulator can keep a disc file open after you leave the game, so a removed game’s space may only come back after the console restarts.

Changing discs

Pause, Change disc, pick the disc. The one in the drive is marked In the drive. Then:

Choice	What it does	When
Insert	Opens the lid, swaps the disc, closes it. The game keeps running.	The game asks for a disc: “Insert DISC 2”, “Please switch to Disk 2”.
Insert and restart	Swaps the disc and starts the console again on it, like pressing Reset.	You want a disc from its beginning: another campaign (Red Alert’s Soviet disc, Resident Evil 2’s Claire disc), back to disc 1 after the ending, or a save the game says belongs to the other disc.

Picking the disc already in the drive offers only insert and restart: it takes you back to that disc’s title screen.

Insert only when the game asks. A disc the game is not expecting can crash a PlayStation game, as it would hang a real console.

Examples:

Metal Gear Solid says Insert DISC 2. at the end of disc 1: Insert, Disc 2, then Start as the game says.
Resident Evil 2: Leon is on disc 1, Claire on disc 2. Loading Claire’s save on disc 1 says This data is Claire’s data: Insert and restart on Disc 2, then load it there.
Command & Conquer: Red Alert: Insert and restart on Disc 2 (Soviet), then New Game, for the Soviet campaign.

By system

System	Change disc
PlayStation, Dreamcast, Sega CD	Insert and Insert and restart.
Saturn	Both. Opening the lid always takes a Saturn to its CD player screen, as on the real console: press Start there to boot the new disc, then load your save. Insert and restart skips that screen.
3DO	Insert and restart only: its emulator cannot swap a disc during play.
GameCube	No menu. The game asks for the disc it needs and it goes in by itself. A GameCube game always starts from disc 1 and shows no Disc N of M.

Save states

A save state remembers its disc. Loading one puts that disc in first, so a state made on disc 2 comes back on disc 2 wherever you load it.

On 3DO a state can only load on its own disc: the console says That state is on Disc 3. Insert and restart on that disc, then Load latest state. A state picked on the game’s page starts the game on its disc by itself.

Something wrong or missing here? See this page on GitHub or open an issue.