There will be a new disk image format, STW, able to handle Dragonflight.
I will open an apart thread in Steem/Development for the description.
Meanwhile, since I asked technical questions here, I'll give the technical answers here as well.
This game creates backup disks, that you need to play, with a very special format.
1) Except the first one, each track is made up of five 1024 byte sectors and one 512 byte sectors, which allows to cram more data thanks to fewer inter-sector gaps.
2) Tracks all have the same recorded number (ID field): 178. This number is obtained by using the $F7 format code, that orders the WD1772 to issue two CRC (control) bytes. Notice that the CRC of the ID field itself is correct.
3) Each track has a special header in the pre-data gap zone (so an emulator really needs all the bytes of the disk, not just data and ID fields). This header is obtained by using the format codes $F5 (address mark) and $F7 (CRC).
Techniques of points (2) and (3) seem to have been used for fun by the author. We thank him because after much head-scratching they give us some good insights into the WD1772.
1) Contrary to what the doc states, the CRC Register isn't preset to ones
($FFFF) prior to data being shifted through the circuit, but to $CDB4.
This happens for each $A1 address mark (read or written), so the register
value after $A1 is the same no matter how many address marks.
When formatting the backup disk, Dragonflight writes a single $F5 (->$A1)
in its custom track headers and expects value $CDB4.
2) The format code $F7 may be used inside an ID field. The CRC bytes are added
to the CRC, so that this is correct. This implies that at the receipt of
$F7, the WD1772 saves the current value of the CRC (at least the lower
byte), before it is modified by adding the upper byte to the logic.
3) $F7 will trigger output of CRC only if the CRC is non null (Dragonflight
Notice that the CRC is null if after a reset you output it:
$F5 -> $A1, CRC = $CDB4
$F7 -> $CD, $B4, CRC = 0
$F7 -> $F7
So the 2nd $F7 is really written $F7 on the disk.