[Coco] 720kb Diskettes

[Theodore (Alex) Evans]

On 02/28/2016 09:04 PM, Gene Heskett wrote:

> Sarcasm isn't quite the correct reaction Alex.

It was a poor choice.  Choosing dns=1 for 3.5" floppies always has 
caused more trouble than choosing dns=3 would have especially since the 
original OS-9 Level II disks for the CoCo don't have the dmode command. 
  I suppose that if one were to use a 40 track 3.5" floppy (still 
135tpi) formatted with dns=1 using a dns=3 descriptor you would have 
similar problems, but I am inclined to think that these are fairly rare 
given that the original OS-9 Level II for the CoCo shipped with an 80 
track descriptor (yes for 5.25" floppies) that used dns=3.  While this 
is covered in the NitrOS9 docs and maybe in the original Microware docs 
all that the Coo OS-9 docs indicate is for 48tpi and 96tpi for 5.25" 
floppies.  It says squat about 135 tpi and squat about 3.5" floppies. 
So, it is 1986 or so and you have just gotten a 720k 3.5" floppy drive 
for your coco.  You want to use it under OS-9.  What do you do?  Use the 
descriptor you have or edit a new?  You probably are going to use the 
one one you have.

Yes, the current consensus seems to be that we should use dns=1 for 720k 
3.5" floppies.  The point is that the argument that no one used 3.5" 
floppies with anything other than 135 tpi on a CoCo and therefore we 
should use dns=1 because we don't have to worry about double stepping 
has never held any water.  In addition to image file which have whatever 
parameters they may have, if you have a 720k 3.5" floppy and a 720k 
5.25" floppy on the same machine, you have to mess around with dmode, 
then edit LSN0 if you want to use the backup command from one to the other.