[Coco] Re: 3.5" Floppy format

[Theodore Evans (Alex)]

On Feb 3, 2004, at 7:17 PM, John E. Malmberg wrote:

> Theodore (Alex) Evans wrote:
>> Personally I feel that the "96 tpi" bit (bit 1 of byte $10 of LSN0)
>
> I am missing quite a bit of the context for your post, and an assuming 
> that this is something specific to the OS-9 operating system.

Yes, this is an OS-9 specific issue.  LSN0 is sector 1 of track 0.  (It 
may be sector 0 of track 0 on Atari OS-9/68k disks, but I can't say for 
sure).

> As you point out, since all 3.5 inch drives are 135 tpi, no double 
> stepping is ever desired, even if you are using only 35 or 40 tracks.

As I mentioned before I have seen (actually have 2) rather exotic 40 
track 3.5" floppy drives (that they are HP drives attached to a 
IEEE-488/HP-IB interface is beside the point).

> It is only 5.25 720K or 1.2M drives that should be concerned with 
> double stepping when reading 360K floppies.

It is also an issue on 8" floppies, both 48 tpi and 96 tpi 8" floppy 
drives were made.

> Is this information stored on the actual floppy?  That would make many 
> backup type operations a problem, as many of them do make complete 
> images, and that includes copying images from 360K floppies on to the 
> first 360k of a 3.5 inch drive with out changing any data.
>
> The device driver is the only place that I can see needing to know 
> this information.

Much of the information in the drive descriptor for OS-9 floppies ends 
up on LSN0.  This allows OS-9 to easily determine the actual format of 
a disk.  This is most important for 5.25" floppies where the disk may 
be single or double sided and may be 48 tpi or 96 tpi.

OS-9 refers portions of the a disk by LSN (logical sector number) which 
is translated into head, track, and sector information for actual disk 
access.  Let us say that you have a DSQD (720k) 5.25" floppy drive with 
the appropriate drive descriptor.  In this drive you can read SSDD, 
DSDD, SSQD (though you aren't likely to find them) and DSQD floppies.  
There is a file descriptor on LSN100 of a disk which is in the drive.  
If it is a SSDD disk this would be side 0, track 10, sector 11; a DSDD 
disk side 1, track 4, sector 11; a SSQD disk side 0, track 5, sector 
11; and for a DSQD disk side 1, track 2, sector 11.  This gets even 
worse considering that it is possible to format disks with fewer than 
18 sectors per track (another figure which is in the copy of the 
descriptor in LSN0), and with fewer (or even a couple more) than the 40 
or 80 tracks normally used.  Because of this descriptor, none of this 
causes any problems.