what is an image 2005-07-16 - By Martin Stricker
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13200178 Hendra Tampang Allo wrote:
>
> Hi friends, i am sorry i am a newbie. I still don't understand what
> image means.For example, what does boot image mean?
In this case, it means an image of a filesystem. It is a file which
contains the whole structure and files of a complete filesystem. On
Unix-like operating systems (like Linux) you can look into it (and even
change the content!) by mounting it onto the loopback device (notz to be
confused with the loopback interface which is the computer's own network
interface and always has the IP address 127.0.0.1). Look at "man mount"
and search for the option -o loop.
Filesystem images are mostly used for bootable media, because to be
bootable the filesystem on the bootable media (floppy, harddisk, CD
etc.) needs to have a bootloader in its master boot record (MBR). The
MBR is a special place and cannot be written with normal methods (it is
not accessible by normal programs). That means: If you make a copy of
all files from a bootable floppy to a blank floppy, that copied floppy
is *not* bootable - the MBR has not been copied (and even if the boot
would fail, because the location of the files on the floppy are not
identical - booting is quite complicated and difficult). So the easiest
way to send bootable meadia is to create an image of the filesystem,
because that is an *exact* copy of everything including the MBR, and
everything stays at exactly the same place.
Most commonly you will see floppy images and ISO images, the latter are
either for CDs or DVDs (depending on the size). As a side note, every
bootable CD and DVD contains a bootable floppy image, because for
booting the CD/DVD drive acts like a floppy drive (that was easier to
implement than to invent a new way of booting). So if you want to create
a bootable CD, you need a bootable floppy image and tell your CD burning
software about it (on Linux it is mkisofs -b option, and mkisofs creates
an ISO image).
To write a boot image onto a floppy, you can use dd on Linux or rawrite
on Windows. To write an ISO image onto a CD or DVD you need to tell your
burning software that you want to burn an image. You cannot write an
image as a file to a medium, because it's not a file but a filesystem.
I hope that helps and is not too complicated!
Best regards,
Martin Stricker
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