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Disk Imaging, popular formats

Started by Petari, 09-09-2011, 17:40:10

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  Disk imaging

As is known, for Atari ST most popular disk image formats
are ST, MSA and STX .
They were created when emulators for PC arrived.
However, disk imaging started before it. The purpose
of imaging was storage, preservation, making copies
and possibility to instead sending physical disk send image
electronically or allow DL from BBS.

Today, most usage is with emulators.

ST format is just content of data sectors in order, without
any further info, data. No special info about disk's logical
format, geometry (tracks, sides, sectors/track). But those
datas are present usually in bootsector (very first sector on disk).
Practically identical to it is DSK format.
Good sides are small size and easy to use.
But if disk has no standard informations in bootsector it may be
problem to guess disk's geometry.

For such cases MSA format is much better. There is file header with
geometry informations. Plus option for simple packing (RLL) to reduce
image sizes, what is most effective if disk is not full with data.

ST and MSA formats are good for not-copy protected disks, or for
some very basic protections - Sundog for instance (only 1 file visible).
And are good for many (most) of Atari ST games with 'manual protec-
tion' .

Image formats for protected floppies:
Most used is STX (Pasti). Less known is STT (Steem).

STX means likely ST extra - beside sector datas we have extra infornmations
in image, which cover practically all used protections systems on ST.
So, STX supports tracks with non-standard sectors, fake track addresses,
slower or faster sectors (there is timing info). Then fuzzy bytes (flakey),
non-standard bytes in gaps, completely track load based track data etc.

Best thing with STX is that it can be made with regular Atari ST. Imaging of
2 sided floppy takes usually about 5 minutes. The result is then file long about
1.8 MB. That size is result of holding all sector datas + track images. Later is
mostly imaged for case - that SW maybe will perform track load for protection

STT is format less known and used. It can image some simpler protections.

STX is unfortunately read only format, and not well supported by it's author
in first place. So, no specs given, SW is made sparsame, not updated for years.
Usable only with 2 emulators : Steem and Saint.  Floppy writing support exists
only in postings of Ijor (Jorge Zwick), but nobody saw it, and we really don't
know what will be writeable, with what and when.
Maybe floppies and floppy drives will decease before some progress in that ?

Fortunately there is HxC floppy emulator, with currently improving STX
support. It is because some people worked on understanding STX format,
so now we have some specs. , good enough to do support.
I hope that after couple months STX support for HxC will be pretty high, and
that over 90% of images will work well.

The problems with  images for Atari ST on WEB: there are some sites
with many of images to DL. Or not so many. But what is common is lack
of knowledge and care. Many images are oversized without any good reason.
There is a lot of STX images of non copy protected floppies. It is not just waste of
space, but lack of writeability.  Sites as planetemu holds often same title with
3-5 diverse images, which differ only in minor details (oversized or not, some
virus check in bootsector and similar - stoopid waste of space). 
But maybe they get money by MB traffic and not by beeing rational ?
And nobody cares to fix it. Except myself, I guess :-)

PP, Sept 9, 2011 .