Software-based floppy disc data separator

Chuck Guzis cclist at
Sat Jun 19 14:59:08 CDT 2010

On 19 Jun 2010 at 11:53, Fred Cisin wrote:

> ...tell you that "the
> computer is a Lear-Sigler with a Northstar Horizon external drive"...
> Just getting them to NAME the computer is a struggle.

I gave up on a customer who insisted strenuously that his system was 
a "Hazeltine 1400".  He absolutely refused to look for another name.

> Chuck is very fond of histograms.

If it doesn't pass the MFM test, that's the first place I go.  2 
peaks=probably FM, 3 peaks = probably MFM, 4 peaks = probably MMFM.  
5 peaks = maybe GCR.  

But I'm not often interested in *writing*  floppies, though I can do 
it if you cross my palm with silver.  (e.g.  Customer wants copies of 
his hard-sector 8" disks).  What I'm mostly interested in is getting 
the data off of the disk, which can extend past simply figuring out 
the filesystem and transferring the files.  Right now, I'm dealing 
with some sort of tokenized test language.  Fortunately, the customer 
still has printouts of some of his files, so I can figure a fair 
amount out.

> On the other hand, I hardly ever continued with any format that wasn't
> going to be possible to convert using relatively stock hardware.  "If
> it isn't IBM/WD, then just file the disk in the appropriate section of
> hardware incompatibles." 

On occasion, I have to build something to handle a peculiar setup.  
Catweasels aren't the end-all, nor are any others.  For example, try 
putting a Catweasel on a 2.8" DataDisk drive.

I'm surprised that no one's showed up with some floppies from a 
Memorex 651 drive yet.

> I played around with some probabalistic code to come up with what to
> try first, particularly in finding and identifying software sector
> interleaves (which sector is used after sector number 1?  Feed the
> code the start and end bytes of sectors and have it identify which
> ones are most likely to be "half a worm" (start of a "word" at the end
> of one sector, end of the word at the beginning of another sector); in
> the absence of adequate langauage text (or excessively unfamiliar
> languages) multibyte machine language instructions are adequate)

The fun ones are where there's a sector interleave and a side and 
cylinder skew.  Or where tracks are allocated on alternate sides of a 
directory that's located in the middle of the disk. 

> But for file information, nothing beats the human mind.

It's great fun if you're the sort of person who can look at a problem 
and have the patience to wait for an "ah hah!" moment rather than 
trying to beat it to death.   If you're the "gotta solve it now" type 
of person, you're probably in for a lot of headaches.


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