Install Floppies (Was: Compaq Deskpro boards/hard drives from

Fred Cisin cisin at
Sun Jul 25 14:14:52 CDT 2021

>> My recollection of the DMF Microsoft period was that if you purchased a 
>> retail MS product using the DMF format and couldn't get it read on your 
>> system, a call to MS would result in a standard format copy being shipped.

On Sun, 25 Jul 2021, Grant Taylor via cctalk wrote:
> It's my understanding that The DMF disks that Microsoft (and comparable from 
> IBM with PC-DOS) used a different non-FAT file system which took up less 
> space on the disk, thus yielding more storage for data.  But that they both 
> fit on the same /standard/ ""1.44 MB disks.
> I also seem to recall that Macintosh's could get 1.7 MB on the same ""1.44 MB 
> disks.
> But I could be completely wrong.

Using a more efficient file directory system isn't a bad idea.  But, it 
won't gain enough additional, just one or two tracks.

But, no, that's not what they did.
The Microsoft DMF is a normal FAT system!  They just cheated on the 
"standards" for the intersector gaps to get 21 sectors per track, instead 
of the "normal" 18 sectors per track.

MOST?, probably not ALL, of the Macintosh 1.4M hardware is similar enough 
that, with the right software ("drivers"), it can do the same, and 
read/write the Microsoft DMF format.

The IBM XDF format is weirder.  It is based primarily on mixing different 
sector sizes on the same track.  Although read and write are not an 
issue, It is "impossible" to FORMAT disks that way with the NEC 765 
type FDC on the PC, but there are ways to cheat and fool it into doing it.
I don't know whether the Macintosh disk controllers can handle it.

Neither the DMF nor the XDF can be read or written with virtually any USB 
drive, or many other special drives, such as Floptical or LS100, because 
those drives have firmware that is locked in to the "normal" format.

Grumpy Ol' Fred     		cisin at

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