On 29/02/2012 10:52 AM, Invisible wrote:

> Weird thing #1: For this to work, you have to write the image over the
> top of the entire drive, not just one partition of it. Which is strange,
> because I'm pretty sure that an ISO-9660 file system doesn't contain a
> valid IBM BIOS partition table at the start.

OK, so I did some digging. It turns out that for a Linux CD image to be 
usable for booting from a flash drive, 1) it must use isolinux as the 
boot loader, and 2) once the CD image has been created, you must run 
isohybrid over it, which edits the file a bit.

But what exactly does isohybrid /do/ to the file that makes it work on a 
USB flash drive?

Well, it turns out that isohybrid is just a Perl script. [That sound you 
just heard was me being sick in the corner.] Since Perl only supports 
textual manipulation, all of the "real" work must be being done by some 
external tool. So if I can just find out what it is...

...oh dear God. You know what? It turns out you /can/ perform binary 
manipulations in Perl. The world is not safe any more. >_<

So what does this script actually do? Well, since it's written in Perl, 
I can only take guesses based on the comments in the source code. But it 
/appears/ that it searches the ISO image to figure out where isolinux is 
stored, and then it creates a valid IBM BIOS partition table at the 
front of the CD image, and generates a boot block that runs isolinux.

Quite how it's possible to insert a partition table into a ISO-9660 file 
system without breaking it, I'm not sure. But that appears to be what 
the trick amounts to. I can imagine it being possible to modify a CD 
image to make a bootable flash image. But I'm quite surprised that a 
single image can simultaneously be both at once.

It /looks/ like it might even be possible to set up the partition table 
so that the rest of the flash drive can be mounted read/write. (But that 
presumably would stop the image working on a CD any more.)

In summary, being able to boot a CD image from a flash drive isn't 
automatic; the image has to be specially set up with this in mind. (So 
it might not work with /every/ Linux distro, for example.) In 
particular, if I wanted /my/ CD image to boot this way, I'd have to 
figure out how to make it boot with isolinux, and then configure it with 
this cursed Perl script.

(Random fact: There's also a C translation of the same program. No, I 
couldn't really read that either...)

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