!ADFFS is a software written by Jonathan Abbott, which allows booting floppy disk images inside RISC OS desktop. Thanks to his fantastic work you can play many classic games released for Acorn Archimedes on modern RISC OS machines, including Raspberry Pi.
The latest version available for Raspberry Pi at the moment is 2.50 beta, which can be downloaded from The Archimedes Software Preservation Project forum. Open !NetSurf, go to http://forums.jaspp.org.uk:9000/forum/viewtopic.php?f=9&t=228 and click on the post attachment (adffs250b.zip). A popup window "NetSurf Download" with a progress bar will appear. When download finishes click on the file icon (green square labeled "Data") and drag it to the SD card icon located on the iconbar. Double click the SD card icon to open the root folder of the card. You should see a green icon labelled "file" - it's the file you just downloaded. It's a zip archive containing !ADFFS, but the system does not recognize it, yet. Click with the middle mouse button on the file and select "File->Set type". The default value is "Data", change it to "Zip" and press Enter. The icon should turn blue with a big letter "Z" on it, which means that RISC OS now properly recognizes the file as a zip archive. Double click on it with the left mouse button to open the archive contents. You should see an icon labeled !ADFFS. Drag it to the folder you want to have it - I keep it in "Utilities" - and the installation is complete!
Now you need some games to play. I will use one of my favourite ones, Gods, as an example. First, download the full game archive from http://forums.jaspp.org.uk:9000/forum/viewtopic.php?f=25&t=107 just as you downloaded !ADFFS. Open the archive, go into Discs folder and drag the disc image files to a separate folder ($.Apps.Games.Gods for example). Again, you have to make the files recognized by the operating system by setting appropriate file type. Click middle mouse button on each file, select "File->Set type" and change "Data" to "Floppy". Observe the icons, they should change to !ADFFS-like.
When a game consists of only one floppy image, you can finish here.
With multi-disc games, you also need to change the image file names. It's because !ADFFS emulates a single drive and if you want to swap floppies their order is recognized by the suffix of the file name, so "Gods 1" becomes floppy number one, "Gods 2" becomes floppy number two, etc. To change the image name to conform to !ADFFS requirements click on the file with the middle mouse button, select "File->Rename", enter new file name and press Enter.
Finally, you can play the game! First, open the SD card, enter Utilities and run !SparkFS (if you opened any zip archive before it should already be running in the iconbar). !SparkFS comes preinstalled with RISC OS on Raspberry Pi, but if for some reason you don't have it, you can download a free read-only version (it's enough for !ADFFS) from RISC OS Open website.
Now run !ADFFS itself - it's icon appears on the iconbar. Drag "Gods 1" onto ADFFS icon on the iconbar to put the floppy image into the virtual disk drive. Click with the middle mouse button on the ADFFS icon on the iconbar and choose "Boot floppy". When asked about "Boot floppy via a simulated Shift-Break?" answear "Yes", and voila - the game starts. After playing the intro you will be asked to insert disc 2, which you can do by pressing CTRL-SHIFT-F2 (disc one is CTRL-SHIFT-F1, disc three is CTRL-SHIFT-F3, etc). To quit the game press CTRL-SHIFT-F12.
Tip: Want to play once more? Just click on ADFFS, select "Boot floppy"... why the game won't start? Yes, you put disc 2 in the drive during the game! Drag "Gods 1" over ADFFS and try booting again. Enjoy!