A copy-protected floppy disk was made to fool the drive that reads it. Duplicating machines wrote tracks that were too long, bits packed at the wrong density and patches that read differently every time, precisely so that an ordinary disk controller would give up or return the wrong answer. KryoFlux does not try to decode the disk the way a computer’s controller would. It sits between a PC and an ordinary floppy drive and times each magnetic flux reversal as the disk spins, to about 42 billionths of a second, and leaves the decoding for later. That makes it useful for rescuing games whose protection defeats normal copying, and for rescuing ordinary disks that have sat unread for decades.
Fast facts
- Developer: the Software Preservation Society (SPS), whose tools still carry the name of the Classic Amiga Preservation Society (CAPS).
- Maker: KryoFlux Products & Services Limited, Twickenham.
- First announced: 6 November 2009, as the C2 DiskSystem.
- Connection: USB to the host computer; a standard PC floppy drive plugs into the board.
- Sampling: a sample clock of about 24 MHz, derived from a 48 MHz master clock.
- Output: raw “stream” files, or common sector images; it can also write ADF, G64 and IPF images back to floppy.
- Licence: the host software is free for private use; libraries, archives, museums and commercial users need a licence.
Who made it
In 2004 Retro Gamer listed the Classic Amiga Preservation Society among the exhibitors at the Classic Gaming Expo UK, describing a group that “dedicates itself to the preservation of classic software, currently for the Amiga computer, but also multi-platform shortly”. The group is now the Software Preservation Society, and its tools still carry the older name: the CAPS Analyzer, the CAPS/IPF library, and a record marked “CAPS” at the start of every IPF file. KryoFlux is the society’s hardware. Its maker describes the society as “an authority in authentic floppy disk imaging and preservation”.
How it reads a disk
A floppy disk stores data as changes in the direction of magnetisation along a track, called flux reversals. A computer’s disk controller turns their timing into bits and bytes as they pass under the head, and hands on only the decoded sectors. Anything the controller does not expect is lost.
KryoFlux keeps the timings themselves. According to Jean Louis-Guérin’s documentation of its stream format, the board runs a master clock of about 48.05 MHz and samples flux reversals with a clock of half that, about 24.03 MHz, counting the ticks between one reversal and the next. A high-density track can produce “about 500,000 flux reversals per second”, all of which have to be streamed down the USB cable as the disk turns. The stream file on the PC “is actually an exact copy of the byte Stream Protocol” the device sends, along with the timing of each index pulse, the signal that marks the start of a revolution.
To capture protection, it is not enough to read a track once. “Especially for games, multiple revolutions of data should be captured,” Louis-Guérin wrote, and the SPS analyser needs at least five revolutions of each track to produce a preservation image. Reading several revolutions shows which bits are stable and which change from read to read, as deliberately “fuzzy” protection bits do.
Because the board times the reversals instead of relying on the drive’s motor speed, the maker says reading and writing are “completely independent of real drive speed”, so a normal fixed-speed drive can read “zoned” disks such as Apple’s, which were written at different rates on different parts of the disk. Its website lists machines from the Acorn Electron, BBC Micro and ZX Spectrum to the Commodore 64, Amiga, Atari ST, IBM PC and NEC PC-8801, and even sampler disks for E-mu’s Emulator keyboards.
From flux to IPF
The raw stream is the input to preservation, not the end of it. In the SPS’s process, Louis-Guérin wrote in his documentation of the society’s Interchangeable Preservation Format (IPF), the stream “is loaded into an Analyzer (CTA CAPS Analyzer) where known disk formats are semi-automatically detected”. The result is an IPF file holding “the disk data, how it is stored (and hence how to write it back), and also a few other things needed to store copy protection (like how dense the data is at any part of each track)”.
IPF deliberately does not keep the measurements. The aim is to recover the description the disk was mastered from, as a Trace duplicating machine would have used it. “The timing information measured by the preservation tool is not stored,” Louis-Guérin explained, “but the ‘format description’ can for example indicate that specific sectors of a track use a well-known bit-cell density variation. The result is therefore perfect as the timings are not subject to measurement errors.” An IPF file can be written back to a real floppy with a KryoFlux board, or loaded into an emulator such as WinUAE. Disk protection and Copylock describe the schemes it has to capture.
Licence terms
KryoFlux’s host software is “free for private and personal use only. Commercial or other non-private and non-personal use (e.g. at a library, archive or museum) requires a licence.” Private buyers order the board from the maker’s web shop; institutions are asked to contact the company for a quote.
The IPF format is open in a narrower sense. After what Louis-Guérin called “long debate”, the source code of the IPF decoder library was released under the MAME licence, so other programs can read IPF files through it. But the format itself was “completely undocumented”: files are meant to be read through the library and “written only by SPS people”. His own description of the format is a reconstruction, and he warned that it “inevitably must contain errors”.
Where it has been used
The maker says KryoFlux has become “the de-facto standard at many universities, museums and archives around the globe”. One widely reported case came in 2014, when members of Carnegie Mellon University’s Computer Club used a KryoFlux to read around forty floppy disks from Andy Warhol’s archive. Ten of them held images believed to have been made by Warhol on a Commodore Amiga in 1985 and 1986, when Commodore had asked him to show off the machine’s graphics.
Sources
- Jean Louis-Guérin, KryoFlux Stream File Documentation, revision 1.1, 1 December 2013 (web, secondary): the clocks, the stream protocol, flux reversals per second, and multiple revolutions. https://www.kryoflux.com/download/kryoflux_stream_protocol_rev1.1.pdf
- Jean Louis-Guérin, “Interchangeable Preservation Format (IPF) Documentation”, version 1.6, April 2018, sections 1.2–1.5 and 3 (web, secondary): the SPS workflow, the CTA CAPS Analyzer, what IPF stores, writing IPF back with a KryoFlux board, and the MAME-licensed decoder library. https://www.kryoflux.com/download/ipf_documentation_v1.6.pdf
- Retro Gamer issue 5, 2004, Classic Gaming Expo UK preview: the Classic Amiga Preservation Society.
- KryoFlux website, “Features” and “About” pages (web, secondary): reading independent of drive speed, zoned-CAV disks, supported systems, stream and sector output, writing ADF, G64 and IPF, the Software Preservation Society, buying, and the maker’s address. https://www.kryoflux.com/
- KryoFlux website, “Download” page (web, secondary): the host software licence terms and documentation. https://www.kryoflux.com/?page=download
- Wikipedia, “KryoFlux” (accessed September 2026) (web, secondary): the C2 DiskSystem announcement of 6 November 2009. https://en.wikipedia.org/wiki/KryoFlux
- Neil McAllister, “Lost artworks by Andy Warhol found on 80s-era FLOPPY DISKS”, The Register, 25 April 2014 (web, secondary). https://www.theregister.com/2014/04/25/warhol_amiga_artworks_found/