Contents copyright Herb Johnson 2012. Last update Oct 1 2012. Quoted material is copyright by me, including quotes from others as used with permission. For more info or for reuse or questions, email me via this Web link. Corrections are appreciated.
"M2FM" or "MMFM" are references to "modified modified FM", or "modified squared FM" (the two as a superscript), which was a means of encoding 1's and 0's onto 8-inch floppy diskettes in the 1970's. It's an early form of "double density" floppy disk formatting which was only used by a few companies (Intel et al). Controllers which provided this means of encoding and decoding, were made with discrete, small-scale IC's. Later, when floppy controller large-scale IC's (LSI) chips were available as floppy controllers, this method of encoding was NOT available. Those chips either implemented FM (single density) or MFM (what became "standard" double density" bit encoding. Information on this Web page, discusses ways to read MMFM or M2FM diskettes. - Herb Johnson
This Web page was created in 2010 from, and refers to, the Web pages below. They may have more information and related information.
drive.html - a collection of tech info on floppy drives, diskettes, formats, etc.
8inch_cctech.txt - a 2005 discussion about diskette formats and controller "chips".
s_intel.html - Some Intel Multibus hardware I have, and had. Also Intel original diskettes.
isis_coll.html - a specific collection of Intel "ISIS" disks I had, now sold and archived online.
s_drives_howto.html - information on older and newer hardware to read old disks including MMFM M2FM
- Herb Johnson
M2FM / MMFM and Intel, ISIS
Catweasel and M2FM / MMFM
DEC and M2FM / MMFM
Other tools, disk image archive
In September 2006, Jay Jaeger obtained many Intel ISIS 8-inch diskettes from me. He soon discovered that many of them were written in a unique format, down to the bit level. Intel's earlist floppy disk controller used a bit format called "M2FM" or modified, modified FM (MMFM). It's not single density FM, it's similar to double density MFM bit encoding. This recording scheme is apparently not readable by many floppy controllers because of that M2FM bit encoding scheme, plus it was used with sector and track standards (preambles, postambles, etc.) that are different from what became "double density". This very early format is documented in the earliest Shugart floppy diskette drive manuals, such as the Shugart SA800 and SA801 drive. It's also documented in Intel's earliest manuals. IBM, the "gold standard" for floppy disk formats, refers to M2FM and GCR as "encoding alternatives" from other manufacturers.
A Shugart document, "SA800 Diskette Storage Drive - Theory Of Operations", has a few pages on M2FM / MMFM, which IT calls "double density". The document describes the track format but only pictorially; the M2FM scheme itself is well described. "The only reliable method to seperate M2FM encoded data is through the use of a phase locked loop (VCO) type of data seperator. The VFO, once synchronized, tracks the data and generates clock and data windows, improving the bit shift tolerance over the conventional "hard" data seperators commonly used in FM recording, which use windows of fixed timing...." Prior to that reference, the document says "Shugart Associates will provide design information, as required, to SA800/801 users who desire to incorporate double capacity diskette drives in their end products."
Manuals I have, or available on the Web, which describe or mention M2FM / MMFM include:
"The IBM Diskette and Diskette Drive" by James T Engh- follow my Web link
....IBM Journal of Research and Development, Volume 25, 1981. link to IBM site
"SA800 Diskette Storage Drive - Theory Of Operations", 33 pages
SA800 Series - Application Bulletin", 33 pages
CDC Application Note: PLO and Write comp, 40 pgs.
Intel iSBC 202 manual - links above
Intellec Double Density Diskette Operating System Hardware Reference Manual.
I have some of these manuals, contact me to obtain photocopies.
At the time (2006) I thought it MAY be possible to make hardware adjustments on SOME, EARLY floppy disk controllers which support MFM / double density, to at least accept the M2FM encoding scheme. If they have an external data "detector" or phase-locked loop or PPL or VCO for double density, it MAY be adjustable or can be reworked for M2FM / MMFM.
At the bit level, the hardware issue was described by Jay at the time as follows: "Reading a track['s bit] cell image still depends on the controller being capable of correctly decoding the bit cells and clock. FM and MFM are not really density by themselves, but are instead different ways of recording self-clocking data streams." He provided some quoted text and references to on-line documents as below. He said
">In looking at what Google pointed me to [when searching for > "Intel M2FM"], I found [that].... > all Intel did was to use the same basic 3270 format and double the number > of sectors to make the OS changes easy. The gaps between real data did not > get as big from SD to Intel's DD. > > "[But] Intel made more changes than that. In addition to the use of M2FM and > and different gap sizes, the index mark, ID address mark, data mark, and > deleted data mark don't match standard FM or MFM. The gap and PLO sync > bytes are different as well. > > "It's documented in the disk controller manuals. For instance, see > pages 4-25 through 4-31 of the iSBC 202 manual at Bitsavers.org." > > "Or pages 1-4 through 1-11 of the Intellec Double Density Diskette > Operating System Hardware Reference Manual at Bitsavers.org."
In Dec 2009, Jay Jaeger referred me to the following post in "classiccmp" (a vintage computing mail list):
Subject: Catweasel support for Intel M2FM working! From: Steven Hirsch Date: 2009-10-08 23:23:41 I'm pleased to announce that the maintainer of Linux cwtool has implemented working support for reading and writing Intel M2FM "DD" diskettes as used with the Intellec development systems :-). Karsten did some analysis of raw bit images I sent him and produced a working driver within a week! As a "smoke" test (my MDS800 is not functional at the moment), I duplicated the ISIS-II system diskette and sent the copy to a person with a working system. It boots, catalogs and otherwise looks fine. I have about 20-25 original distribution diskettes for the MDS800 and will get busy imaging them ASAP. Who would be willing to host these? They are "cooked" images, so it would be possible to extract the files from them with a bit of work. However, they're obviously of the most use to folks with access to a Catweasel board (and an Intellec system). Steve
Steve found an archive for his diskette images, at "bitsavers.org", as described below.
The Catweasel Mark 4 is a PCI card produced by Individual Computers of Germany and sold by them and a few other vendors in the USA and other countries. As of 2009 a version is in current production and sold currently in various Web venues. I discuss the Catweasel and provide current information on my "drives" Web page.The software mentioned, "cwtool" and also "cw", are from Karsten Scheibler's web site. These are parts of a Linux based software package for the Catweasel Mark 4 floppy disk controller. The site has links to providers of other software tools for the Catweasel. The cw and cwtool programs are reported to copy into the image file, the sectors per track in LOGICAL order, not merely copying their PHYSICAL order. Put another way, there is no "interleave" in the image copy.
Allison Parent posted about M2FM, as part of the Catweasel discussion above. Allison posted:
They must have confused RX01 (8" SD easily read) with the custom mixed format SD and DD (and DD as M2fm) that no current or previous floppy chip can do. All the controllers (DEC, DSD and other third party) all use bit slice or 8x305 to do the format. I have all the known working varients (DEC, DSD, Plessy/Harris, sigma systems, Crislin.) as examples in working systems. The DEC and clone RX02s can do both RX01 format and the oddball 2D format.
I discussed privately with her the recent advance in reading M2FM Intel disks with a Catweasel Mark 4 PCI controller and new software. She said:
I'm sure the CW can, but DEC hardware is far easier to use than CW and I already preserved and use that system. It's far easier to boot the PDP-11 (and faster than a PC bootup) and it can handle the high level formats that are different. For example RX02 was used on PDP-8 (12bit mode) PDP-11, VAX, and PDP10 and the lost of OSs include RTS, OS8, MUMPS (PDP8,10 and pdp11), UNIX (PDP11, VAX) and more. For example PDP11 PDP-11 ran unix, RT11, RSX11, RSTS, MUMPS, TSX11. DOS, XXDP/X11, POS, and I'm certain I forgot a few. Reading the RX02 is small peice of the battle as all of those systems and software used the disk differently heck even RT-11 can't read a unix-11 disk without a utility. The CW doesn't solve this though higher level filesystem level tools can. Fortunatly most of the DEC RX02 users knew to make RX01 disks that can be read on any soft-sector 8" system for portability sake. When you consider those OSs also supported other removable media so the overlying format is usually the important issue.
I discussed with her the Intel version and said:
But I know of no other way to read Intel M2FM disks except with original hardware. If you have an old MDS "blue box", I think you'd spend $150 to buy an old Intel DD card for it (actually you need two cards), plus time to make it work. If your MDS is the pre-Multibus box, I'm not sure they came with double density floppy controllers.
It's multibus, I have one of the two boards, but in the last 12 years I've never needed to read a DD disk. But having 500K per disk would be nice as 241K is cramped. [So while] the Catweasel is really the only game in town... In my case I enjoy running and maintaining the old hardware so being able to read their disks is inherent. Since I have systems with 1771/1791/765 that covers most all but the oddball formats that are married to specific hardware. In the case of NS* I have Horizon SD/DD and an Advantage as well so I've covered all the needed cases. This problem was more real for me back in the 75-81 time frame when [my] system was only NS* SD and the rest of the world was 8"SSSD. Even than the few [diskette formats] I can't read [now] are a minority and not something I collect.
Another Catweasel program in common use for single and (MFM) double density diskette imaging is Tim Mann's program "cw2dmk" for the Catweasel, available at this Web link. Disk image files are produced with the ".DMK" extentions, and there's a "signature" with the program name at the beginning of each image file. Another program is Dave Dunfield's "imagedisk", which runs on ordinary personal computers with floppy disk controllers. It produces image files with the ".imd" extension and which have the signature "IMD 1.XX" depending on revision.
In Jan 2010 Jay Jaeger provided images of the ISIS disks he obtained from me, to Al Kossow's bitsaver.org Web site in an Intel/MDS subdirectory. Some images have been "tar"ed into collections of disk images at this link. Other disk images from my former collection may be in that "Intel" archive area. See this Web page for a reference list or catalog of the Intel diskettes I owned in 2006.
Another subdirectory at bitsavers.org, contains the Intel diskette images Steve Hirsch copied as noted above..
Thanks to Al Kossow, for his archive work and his correspondence with me about part of his Intel/ISIS archive. Thanks of course to Jay Jaeger for restoring the MDS IV system and imaging the diskettes, to Steve Hirsch and his support to Karsten Scheibler who created/adapted the M2FM tools for the Catweasel. Thanks to my friend and colleage Allison Parent for permission to put our discussion on this page.
- Herb Johnson
Copyright © 2012 Herb Johnson