[Coco] high density floppy controller mods

[Warren Hoslet]

>jdaggett at gate.net wrote:
>>Do you have enough time to lay down (36) 256 byte sectors before the index 
>>mark comes back around?
>>
>>james
>>
>
>I should. the transfer rate is doubled, the rotational speed is the same.
>
>The only thing I didn't do, which I wasn't sure about, was to modify the 
>delay time of the write signal.
>
>The controller has a LS221 one-shot between the controller and the drive's 
>write data line. It's set for a pretty short time constant. I was going to 
>reduce the time constant by half during HD mode, but I tested without it 
>and it seems to work fine. (?)
>
>I'm really not clear as to what the function of that one shot is anyway. 
>None of the example circuits using the MB8877A I've seen show it.
>
>I did verify that my HD formatted disks are not readable in LD mode, which 
>is a good sign.
>
>If I knew where the dskini routine is, I'd poke it into making 36 sector 
>tracks and see if they can be read back.
>
-
The extra gaps required for the additional 18 sectors may make things pretty 
tight. Isn't that why PC disks went to 512 bytes per sector (to double the 
data bytes per-track without increasing the sector count)?

In any event, to modify Disk Basic 1.1 (2.1) to format more 
sectors-per-track, you need to poke the new sector count into the following 
addresses:

$D5B6   $D5B8   $D5C5   $D5C9   $D5D2   $D67B   $D6BC

Also note that the DSKINI routine will only check to make sure that there is 
enough unreserved memory available in the 32K Basic RAM to format the 
standard 18 sector track.

If you want to allow the DSKI$ and DSKO$ commands to be able to read and 
write to the higher sector numbers, then you should poke the new sector 
count minus one into location $D540.

DISCLAIMER: I have never actually tested this, so I can't gaurantee that 
these are the ONLY changes required. You should check out the Disk Basic 
Unravelled book to learn more about the ROM routines.