[sebhc] h17 and h8d disk images

Dan Lanciani ddl-cctech at danlan.com
Wed Sep 1 20:27:36 CDT 2004

"Barry Watzman" <Watzman at neo.rr.com> wrote:

|Personally, if I was writing an image program, I'd record each track's
|sectors in the order in which they physically occurred on the track AND I'd
|also record, in each sector's data contents, the track and sector number
|recorded in that sector's header.

That's easier said than done.  Unless you are lucky enough to get an entire
track to read correctly each time you have to do some fancy matching to
keep the physical information.  And it just isn't necessary in this application
since there were neither copy protected disks nor interleave schemes in
general use.

|Only by recording BOTH of these -- they physical layout AND the logical
|layout (the numbers in the header) would it be possible to physically
|reproduce a physically correct image of the diskette.

If you *do* have to worry about non-standard formats used in copy protection
schemes then merely recording that information is far from sufficient to
reproduce the disk.  Using funny sector/track numbers and positions got old
within months of its introduction.  To be able to make an accurate copy you
need to record both the data and the *clock* bits in the FM or MFM stream along
with the timing relationship to the index hole.  This is because people learned
to embed headers within data areas and play games with non-standard address
marks.  Even with the data and clock bits recorded you might not be able to
make a reliable copy if you don't know how to recompute the correct precomp

				Dan Lanciani
				ddl at danlan.*com
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