Re: disk image emulator on the GS - possible?

["Charlie" <charlieDOTd@verEYEzon.net>]

"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 :-)

Charlie