D format:

A D-format disc has the general format:

<free space map>
<00 filler to sector end>
<root directory>
<rest of disc>

Free space map
--------------

This is a standard OldMap

Root Directory
--------------

This is a 77-entry, 'new' directory at disc address &400.


D format floppies
-----------------

These are normally formatted with 1024-bytes sectors, 5 to a track. This
means that there is 512 bytes of 00 filler between the map and the root
directory.

The number of heads in the disc record for a D format floppy is 2, meaning
that the sides are interleaved, not sequenced.
