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Re: disk image emulator on the GS - possible?
In article <fHc3m.511$P5.394@nwrddc02.gnilink.net>,
"Charlie" <charlieDOTd@verEYEzon.net> wrote:
> "John B. Matthews" <nospam@nospam.invalid> wrote in message
> nospam-39CE32.17130802072009@news.aioe.org">news:nospam-39CE32.17130802072009@news.aioe.org...
> > In article <1OydnUMe6ZzUY9HXnZ2dnUVZ_gidnZ2d@giganews.com>,
> > "Michael J. Mahon" <mjmahon@aol.com> wrote:
> >
> >> Charlie wrote:
> >>
> >> > Also the 2mg format allows for comments anywhere in the image.
> >> > Although I've never seen an image that used this feature, it
> >> > wouldn't surprise me to find one.
> >>
> >> That sounds like a *terrible* idea.
> >>
> >> A disk image is a regular structure that can be indexed into.
> >> Interpolating "comments" demands sequential reading for a
> >> negligible value.
> >>
> >> This "feature" should be removed from the .2mg "standard".
> >>
> >> If the comments are part of the header, they would make it a
> >> variable-length header, which is also a big downside for a
> >> small upside.
> >>
> >> The only "minimally-distruptive" way to add comments to an
> >> image would be to suffix them--and even that makes the size
> >> of an image vary, which most would interpret as an error.
> >
> > Quite right. It looks like the (optional) comment is found after the
> > disk image data itself.
> >
> > <http://apple2.org.za/gswv/a2zine/Docs/DiskImage_2MG_Info.txt>
> >
>
> Yes, that is version 1 of the 2mg format (documented by Eric Shepherd in
> 1999 I believe). It is the better of the two I've seen since it spells
> out where the comments and creator data should go. Unfortunately, there
> is an earlier version 0 (I believe documented by Thomas Fok who wrote
> Revival) of the format that does not define where the comments and
> creator data should go (I don't have a working link to that version any
> more). When writing a program to use a 2mg image you could look at the
> version number to determine whether you need to delve further into the
> structure of the file. Or you could just assume that nobody has ever
> put a comment in a stupid place. My luck, 5 minutes after I put such a
> program on the internet I'd get an email from somebody complaining about
> how my program couldn't read their 2mg files :-)
Interesting. I recall a 2mg image passing by recently; it appeared to be
v1 in having a suffix comment. Andy McFadden puzzled it out for
CiderPress, but I've misplaced the thread.
--
John B. Matthews
trashgod at gmail dot com
<http://sites.google.com/site/drjohnbmatthews>