Put 1.7MB Onto 1.44MB Floppies

1680K Floppies, Name Servers and DHCP



By Trevor Marshall



November 08, 1999

Those of you who have been following the evolution of my floppy-based Linux gateway in the August and October columns have probably noticed I have been squeezing just about as much onto a 1440K floppy disk as was humanly possible. The gateway disk is now full, yet I still want to add features, such as a caching name server and a small Web server. Yet the elegance of booting from a floppy is indisputable -- your OS can't be hacked, the floppy is write protected, and a push of the reset button puts a clean version of Linux up again.*

Microsoftt's DMF 1680K Floppies

Microsoft uses this format only for distribution of its own software, and it is essentially undocumented (a search of Microsoft's developer database elicits only the information that "DMF ... is a special read-only format for 3.5-inch floppy disks that permits storage of 1.7 MB of data").

By failing to supply Windows users with any disk-copying or formatting utilities for DMF, Microsoft was presumably trying to prevent copying of its own software-distribution diskettes.

Yet DMF is not a "read-only" format. The 1680K disks can be manipulated normally by Windows Explorer. They even work in my Mac OS machine.

Linux has supported DMF, in addition to a number of other high-density floppy formats, for several years. Up until now I have avoided using them for the gateway because proper support software under Windows was not available.

But, recently, Donovan Chun left a message on my Linux BBS pointing to a Windows Shareware product called GRDuw from GR Software, which is capable of formatting DMF diskettes. It even manipulates the larger image files in much the same way as RAWRITE does with 1.44-Mbyte disks. GRDuw also supports an associated diskette format called MDF that packs another 42K onto two additional tracks.

(Notes:

* Make sure your BIOS ROM is write protected so it can't be overwritten.)

Two additional tracks? What is going on here? How can I get 82 tracks onto an 80-track diskette drive?

Well, the 1.44-Mbyte format was devised in the late 1980s. At that time, the speeds of the servo motors used in diskette drives were not tightly controlled and the mechanical variations between drives from differing manufacturers were significant. The 1440K standard was a conservative response to disk-drive variability. Eighteen 512-byte sectors of data were defined for each of the 80 tracks on a 3.5-inch diskette. This allowed for a large gap between the sectors (the inter-sector gap) to accommodate speed variations between the various drive motors.

By the early 1990s, most manufacturers were producing drives of higher quality. More powerful motors and accurate speed controllers made the data rate more consistent, and this reduced the need for a wide inter-sector gap. So the DMF standard uses narrower gaps to squeeze 21 sectors of data on each track instead of 18, for a total capacity of 1680K.

The 1720K MDF Format

Floppy-diskette media also improved, and manufacturers were typically burnishing (or polishing) more of the rotating media than was required to hold the 80 tracks of data. In order to get 82 tracks of data onto the diskette, the 1720K MDF format makes your head move closer to the center of the floppy media. The danger with the MDF format, of course, is if you try to put 82 tracks on a cheap, low-tolerance floppy you will be forcing your heads onto unburnished areas, and rapid deterioration will result. This is not a risk with 1680K diskettes, and I recommend you only use the 1720K MDF format if you absolutely need that extra 40K of data on your disks.