Creating 5.25" floppies

Jules Richardson jules.richardson99 at
Mon Aug 17 21:25:11 CDT 2015

On 08/17/2015 08:54 PM, Terry Stewart wrote:
> On Tue, Aug 18, 2015 at 1:45 PM, Fred Cisin <cisin at> wrote:
>> On Mon, 17 Aug 2015, drlegendre . wrote:
>>> Terry & all,
>>> To be clear - SD on a "360K" floppy is 180K? Yes?
>> NO.
>> Osborne SD was also single sided.
>> 10 256 byte sectors per track.  100K.
>> "360K" was 9 512 byte sectors, with TWO sides used.
> Hmm.  I don't have the machine in front of me to check (I'm at work), but I
> pretty sure my Osborne 1A drives are single sided not double sided.

Yup, single-sided, 40 tracks.

>  But I
> have the double density modification so yes, around 180-200k per disk I
> imagine.

Yup again. I've got "The Osborne Portable Computer" book in front of me now 
and it claims that the DD configuration uses 5 sectors of 1024 bytes per 
track, so 200KB over 40 tracks - but that CP/M reserves the first 20KB of 
that, leaving 180KB for the user.

I just had this nagging feeling that some of that reserved area is 
formatted at SD rather than DD (i.e. FM rather than MFM), at least for a 
CP/M boot disk, and so an FM-capable controller was needed in PC-land to 
write them, but if you've written them with one that can only do MFM then 
it suggests otherwise (and likely any ol' PC that drlegendre has will work OK).

>   Using the PC and Imagedisk I could easy write DD single-sided
> disks in a double sided (360k PC) drive though.

Yeah, like Fred says, in the PC/DOS world a "360K" drive would have 40 
tracks at 48tpi, just like the Osborne drives (albeit with two heads rather 
than one), and would format using MFM, just like the Osborne with the DD 
upgrade - but DOS would format it at 9 sectors of 512 bytes per track, 
giving 180KB per surface and 360K per disk.

Software other than DOS - e.g. Imagedisk - isn't limited to that geometry 
and so will happily read/write DD (or SD if the controller supports it) for 
other media.



More information about the cctech mailing list