Multi disc games will prompt for the other CD when you reach a certain point of the game, back and forth, in this example I’m using Riven because it’s probably the game that uses more discs I know, in PS1 it has 5 discs, and it’s interesting because it will prompt you to swap back to the first discs since you need to come back to the first areas to progress in the game.
Saves are usually stored separately for each game, this applies to multi discs, so if you, say, reach the second disc in Metal Gear, the save file will be named something like Metal Gear (USA) (Disc 1).srm (the save file will follow your rom file name), and it won’t be recognized in the following disc since it will try to find the save for the Metal Gear (USA) (Disc 2).srm file. Using a .m3u file will fix this because the save will follow the same name for all discs, you can name it Metal Gear (USA).m3u and the save will be named Metal Gear (USA).srm for all discs.
Here you can see an example, a folder specific for multi disc games so it will be easier to sort out the m3u files.
A M3U file is a simple plain text file (with the .m3u extension) that contains only a list of the discs inside, like this:
*Note that as my games are compressed in CHD I have only listed the compressed file, in case you have them in Bin+cue, you should only list the .cue file instead.
All you have to do is to copy the disc names and paste inside this .m3u file and launch the (in this case) Riven - The Sequel to Myst (USA).m3u in retroarch, currently supported by Playstation and Saturn.
Load the .m3u file instead of the Bins, isos or CHDs in order to get the swapping discs functionality
When you reach the disc swapping part:
Press F1 (or the button you assigned to enter Retroarch’s menu in your game-pad), it should open the Quick Menu of the core you`re currently on. In case you’re in the home menu, enter Quick Menu pressing Enter or the correspondent button in your game-pad.
Below you’ll find Disk Control, again press Enter or the correspondent button in your game-pad.
Disk Index shows the current disc you’re on, press Enter on Disk Cycle Tray Status and it will open the disc tray, check yellow message below.
Go up to Disk Index and press forward in the keyboard or in your game-pad (forward and back cycles through discs, choose the next disc, in this case, I need to select Disc 2, but depending on the area of this game, it can prompt you to go directly to Disc 3 or 1, you can actually jump to Disc 3 if you know how from the start, but you cannot do much but only visit a limited part of that area.
After selecting the next disc, press Enter again in Disk Cycle Tray Status to close it (check yellow message)
Press F1 (or the correspondent game-pad menu button) to go back to the game), it will load the next disc.
Game running with the next disc:
For frontend users, like me, using .m3u files and loading them via Retroarch has a bonus, it makes browsing though games much easier and cleaner, since it won’t need to show more than one entry for a game that has multiple discs.