The Classic Amiga Preservation Society (CAPS) Software Preservation Society István Fábián

Introduction  CAPS founded in 2001  Ex-games developer  Frustrated with the state of software preservation  Aims of CAPS  CAPS changing into SPS (Software Preservation Society)

Game Preservation  GAMES ARE ART  Imprisoned by media  All disks will eventually die  Help from the community

Technical Issues

Technical Issues – Disk Formats  Disk Format is not copy protection  Floppy-based systems have standard “known” formats  Floppy Disk Controller (FDC) defines format on Atari ST/PC  Amiga FDC operation done mostly in software  System has to know format to be able to read it

Technical Issues – Copy Protection  Attempt to keep piracy under control  Non-standard “effects” created on media  Allow reading/detection data that cannot be written

Technical Issues – Protection Examples  Changing width of “bitcells”  Flakey/weak bits  “Stealth” data

Technical Issues – Disk Properties  Preserving the normal disk data is not enough  There are other sorts of information that is needed  Density of the bits across a track  Format description  Track geometry  Data “behaviour”

Media Unsuitable for Preservation

Media Unsuitable for Preservation - Errors  Damage to disks caused by:  Physical damage  Drive failure  Bit rot Magnetic Force Microscope visualisation of a damaged area of magnetic disk  Need detection of these errors  Integrity algorithms  Special handling of media that lacks integrity information

Media Unsuitable for Preservation - Modification  Modified disks not repairable  Disks manufactured write enabled  Sources of modification include:  Save games  Hi-scores  Virus damage  User hacking  Etc.  A very big problem

Media Unsuitable for Preservation - Copies  Copying of a disk over a (perhaps broken) original  Cracks are easy to see by anyone  Hardware copies  Not easy to see without using our technology  Analogue in nature  Like copying a VHS tape  Generation copies will eventually fail  Modification can not be determined after it has been copied

Cracks  Most common form of disk images  Represent piracy and “warez”, not developer friendly!  Frequently do not work, perhaps fail later in game  Game features often missing  Graffiti in credits and hi-score  Modifies game code!

The Solution

The Solution – Disk Imaging  Data normally read through FDC is not the same as on disk  DSP techniques used to get what is on disk surface bit-for-bit  Disk imaging technology produces raw signal information  To be usable, the data needs to be described

The Solution – Commercial Mastering (1)  Why define disk formats?  Same in commercial mastering Trace 7500 ST Duplicator  Data meaning  Data verification  Master “gold” disk  Format description (Freeform)  Only then can we master disks

The Solution – Commercial Mastering (2)  Following commercial mastering principles  New formats researched and documented  “Layered” format descriptions  Result is IPF (Interchangeable Preservation Format) file  Contain both signal data and descriptions  Act exactly as real media

The Solution – Integrity  Integrity information described  Algorithms reproduced  Some formats do not contain integrity info  Check by encoding  Check by comparison  Some games have mastering errors, have to find good versions  If a disk does not pass integrity checks, it is useless for preservation

The Solution – Authenticity  Floppy systems are mechanical devices  Leave a “fingerprint” when writing  Commercial duplicators use high quality components and have very different fingerprints to those written on home equipment  We can see when a disk has been modified – as the fingerprints are different for one or more tracks  99.9% of games commercially mastered  Detection of copies in the same way  The shear amount of modified games is a very big problem

The Solution – Preservation Framework Layer 1: Raw Image Layer 2: Track Geometry Layer 3: Disk Geometry Layer 4: IPF File Layer 5: Meta data Layer 6: Digital Libraries Density/Timing + Data Descriptors Self-describing format Software collections Description/Categorisation All Layer 2 for Disk

Wrapping Up  We search until we find a game unmodified with no errors  IPF file describes everything the game needs to work. No program code is changed!  An IPF file is only sent to contributors and authors of the game  Currently focused on Amiga, as is the most complicated system.  Other systems supported by describing their disk format(s)  Resources available  Preserving software for the future...

The Classic Amiga Preservation Society (CAPS) Software Preservation Society