How to read Osborne 1 Floppies?

Jules Richardson jules.richardson99 at
Tue May 25 14:35:36 CDT 2021

On 5/24/21 1:26 PM, Fred Cisin via cctalk wrote:
> There were two Osborne floppy formats.  Both CP/M.
> IIRC, the original format was "Single Density" (FM), 10 sectors per track, 
> with 256 bytes per sector (similar to the TRS80-Model-1.  That will require 
> FM/SD capability.  Most NEC FDCs did not support that.  Dave Dunfield has a 
> test program that will tell you whether your FDC can handle it.
> Then, they switched to "Double Density" (MFM) IIRC: 5 sectors per track, 
> with 1024 bytes per sector.  Any PC FDC with access to INT13h and INT1Eh 
> will work with appropriate software.

I think there were some bootable Osborne floppies which still retained a 
single density boot track even when the rest of the media was formatted as 
double density. Regular data disks are probably consistent throughout, but 
it's just something else to consider depending on the nature of the media 
that's being accessed.

Thankfully there seem to be far more PC controllers out there that will at 
least read FM data than there are ones that can also write it.


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