[Info-vax] OT: For MAC Lovers Only :-)

[glen herrmannsfeldt]

John E. Malmberg <wb8tyw at qsl.network> wrote:
> On 10/15/2012 5:58 AM, Johnny Billquist wrote:
>> On 2012-10-14 01:46, Steven Schweda wrote:

>>>     Hmmm.  Interesting.  I don't think that I ever tried one
>>> of those on a non-Mac system before.  (That was a "Microsoft
>>> Excel for Apple Macintosh" installation disk.  "Disk Format
>>> 800K", "Version 4.0".)

>> Apple used their own low level format for disks for a while. You can
>> totally forget to read such disks on anything else. (Of course anything
>> is possible, assuming that you can access the magnetic flux on the
>> floppy...)

> It may be possible to use the "read track" low level floppy disk 
> controller command, which basically just copies all the bits read into a 
> memory buffer and then use a program to try to separate the data.

As far as I know, no floppy controller designed to read data
blocks can read a track like that. Yes some have a read track
command, but it still decodes bytes using the byte storage
method of the given format. 

> The read track is used to verify the write track, which is what is used 
> to format a standard floppy disk.  As it needs to read everything, it 
> just reads a bit stream, paying no attention to what the data is.

Soft sectored formats have to have a special way to indicate a
sector header that is different from any data pattern. Derivatives
of the original FM format (FM, MFM, MMFM, etc.) do this by leaving
out specific clock bits. Even in read track mode the logic for
decoding that still runs, but only data bits (bytes) are written
to the buffer.

> I am not sure that the VMS I/O drivers give you access to that API.

> The catweasle products also allow access to the low level stuff, the 
> last time I looked they were all sold out, and of course no drivers are 
> available for VMS.

Yes, ones like catweasel are different. In that case, there is no
logic for decoding bits, not even a PLL to lock onto the data rate
and separate clock and data bits. They sample the output of the
drive fast enough that you can figure out where a transition
(flux change) occurs, and later decode that to data bits.

-- glen