Re: DSK format specification

[Andy McFadden <fadden@fadden.com>]

In comp.sys.apple2 Michael Pender <mpender@hotmail.com> wrote:
> I remember that back-in-the-day (BITD?) there were disk utilities that
> changed the interleave between DOS disk sectors to speed up file transfer.
> That is, physical sector 0 and logical sector 0 are always the same, but
> physical sector 1 and logical sector 1 could be different.  Is this a
> similar problem?

It's similar.  Sectors have a physical order (by default written straight
from 0 to 15) and a logical order (skewed around).  The mapping between
them is fixed for each filesystem, and is different for DOS, Pascal/ProDOS,
and CP/M.  (It happens that logical sector 0 is mapped to physical sector
0 in all established formats, which means that you'll see track 0 sector 0
at the start of every disk image regardless of ordering.)

The "fast skew" programs couldn't change the mapping between physical and
logical sectors, but they could write the physical sectors in a different
order by reformatting the tracks.  Quality Software's Bag of Tricks was
one such utility.  This sort of re-skewing would only be noticeable by
software that handles nibble images, because an RWTS read of sector 5
will always map to the same physical sector.

The trouble with ".DSK" images is that they were either written as a
series of 256-byte sectors (sector 0, 1, 2, 3, ...) or a series of 512-byte
ProDOS/Pascal blocks (sector 0, 2, 4, 6, ...).  If you assume that track 0
sector 1 starts 256 bytes into the disk image file, you may be disappointed.

Naming 5.25" disk images with ".DSK" is a mistake, IMHO.  They should be
".PO" or ".DO" to identify the contents unambiguously.  If I recall
correctly, early versions of KEGS assumed that 5.25" disks were in DOS
order and 3.5" disks were in ProDOS order, so failing to label a disk
image could get you into trouble.

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