Re: OT: ISO and Image files-semantics problem

[Uwe Dippel <udippel@uniten.edu.my>]

On Sat, 10 Jan 2004 09:46:47 -0500, alex wrote:

If you want to understand, why not try yourself with something faster (and
cheaper) ? Take any bootable floppy, insert it, and issue a 

dd if=/dev/fd0 of=demo.img

[if=input file; of=output file]

Probably, demo.img will be 
1474560 Bytes, whatever the floppy contained.
dd creates an image; a 1:1 'copy' of all bytes on the floppy; including
their allocation on the floppy.

This process is reversible. Enter a fresh, unformatted floppy into the
drive, issue

dd if=demo.img of=/dev/fd0

and the fresh floppy is a 1:1 copy of the earlier one and even boots;
since the allocation of the data on that floppy is identical to the
earlier one.

Try the same with zip. The zip-file will simply contain the content of
that boot-floppy as files; without maintaining the locations, and be
dependent in its size on the actual data on that floppy.

Try to write it back to another fresh floppy, and it might complain about
format; but surely it will not boot. zip has no clue where to write the
boot record or know that it even existed. But the data will be back, any
place.

Hopefully, this clears up the concept 'image'. The demo.img (or whatever
you call it; it is independent of the name !) preserves more than just the
data. It mirrors the actual medium.

So demo.img is an image of that floppy; the floppy itself isn't. (It
contains a number of independent files somewhere on that floppy.) It
rather is the real thing.

Similarly, you should now be able to answer your questions about the CD
and its image. .iso is simply a specific format of an image for CDs.

Does this help ?