Add Disk Control interface for S6D2

source/frontends/libretro/README.md

@@ -47,6 +47,71 @@ In order to have a better experience with the keyboard, one should probably enab
 Easiest way to run from the ``build`` folder:
 ``retroarch -L source/frontends/libretro/applewin_libretro.so ../bin/MASTER.DSK``
 
-It supports playlists files `.m3u` (see https://docs.libretro.com/library/vice/#m3u-and-disk-control alttough not all options are implemented).
-
 The core can be statically linked in Linux and MSYS2, pass `-DSTATIC_LINKING=ON` to `cmake`, or disable networking `-DENABLE_NETWORKING=OFF`.
+
+## M3U Playlists and Disk Control
+
+The core cupports libretro [Disk Control Interface](https://docs.libretro.com/guides/disc-swapping/), allowing disk swaps from the frontend UI (the drive is selectable via a core option).
+
+### M3U format
+
+A `.m3u` file lists disk images, one per line. Paths are relative to the M3U file's directory, similar to [VICE](https://docs.libretro.com/library/vice/#m3u-and-disk-control).
+
+```
+Ultima V - Disk A.dsk
+Ultima V - Disk B.dsk
+```
+
+### Supported directives
+
+| Directive | Description |
+| --------- | ----------- |
+| `#` | Comment (line is ignored) |
+| `#SAVEDISK:` | Creates a writable blank disk in the save directory |
+| `#LABEL:` | Sets the display label for the next file entry |
+| `#EXTINF:` | Standard M3U extended info - label follows the first comma |
+| `\|` (pipe in filename) | `path.dsk\|Label` sets a display label for that entry |
+
+Label priority: `#LABEL:` / `#EXTINF:` override pipe labels.
+
+#### `#SAVEDISK:`
+
+Adds a writable save disk to the playlist. The disk image is created (as a blank `.dsk`) in the save directory on first use and reused on subsequent launches.
+
+```
+Game - Side A.dsk
+Game - Side B.dsk
+#SAVEDISK:Character
+#SAVEDISK:
+```
+
+The label after `#SAVEDISK:` is optional. If omitted, an automatic index is used. The resulting label shown in the frontend is always prefixed with "Save Disk ".
+
+Note: the created disk is unformatted (all zeros). Most Apple II games that request a save disk will format it themselves.
+
+#### Pipe-delimited labels
+
+```
+Game - Side A (Disk 1 of 3).dsk|Side A
+Game - Side B (Disk 2 of 3).dsk|Side B
+```
+
+The text after `|` becomes the display label in the frontend's disk control UI instead of the filename stem.
+
+### MultiDrive
+
+When enabled, the second disk in the playlist is automatically inserted into DRIVE_2 at load time. This is useful for games that expect both drives populated (e.g. Ultima, Wizardry).
+
+MultiDrive is triggered by either:
+- Naming the M3U file with `(MD)` in the stem, e.g. `Ultima V (MD).m3u`
+- Enabling the **Floppy MultiDrive** core option
+
+Save disks (`#SAVEDISK:`) are never auto-inserted into DRIVE_2.
+
+### Core options
+
+| Option | Values | Description |
+| ------ | ------ | ----------- |
+| Disk Control Drive | Drive 1, 2 | Active drive for the Disk Control Interface. |
+| Playlist Start Disk | First, Previous | Whether to start from disk 1 or resume from the previously used disk |
+| Floppy MultiDrive | Disabled, Enabled | Auto-insert second disk into DRIVE_2 for all playlists |