[syslinux] MEMDISK El Torito Emulation

Bernd Blaauw bblaauw at home.nl
Mon Nov 23 13:36:53 PST 2009


Op 23-11-2009 20:53, Shao Miller schreef:
> Bart Lagerweij wrote (June 11, 2009):
>> Sure no problem. For me this is a very old project.
>> I will change it to MIT license.
>> Will take a week or so (limited time).

which implies the sourcecode or specification of his implementation is 
available somewhere? Not seeing it on the nu2.nu site at least in the 
eltorito1.4 archive (contains only binary).

> It appears that you are suggesting that MEMDISK keep its kernel 
> command-line around (it does!) and that some Syslinux variant 
> (ISOLINUX, in this instance) in the RAM disk take a look for these 
> arguments.  This is an interesting idea, but not currently available.  
> (That I know of.)  The trickiest part would be that finding these 
> parameters (the MEMDISK command-line) would be the responsibility of 
> the booted items in the RAM disk.  Thus all .ISOs in existence would 
> need to be updated in order to find and use these parameters.
>
> At this web page[1], a user using "CD Shell" (whose web page is 
> referenced in a reply post) uses a CD Shell module to load 
> ISOLINUX.BIN and feed it a command as though typed at the "boot:" 
> prompt.  This might be interesting to you.  You might be able to 
> determine how CD Shell's author accomplishes this.

Memdisk keeps its kernel indeed, there's a few DOS programs able to 
retrieve them, based on Murali's code of IFMEMDSK. www.auersoft.eu 
contains some of these.
For all I care Isolinux just intercepts an original Memdisk commandline 
then, fine for me :)

Wasn't there some module that allowed 1 isolinux to chain into the next? 
does that allow entering some boot commandline?

> The El Torito specification goes back a little ways now, and does a 
> (fairly) simple dance of loading and jumping.  ISOLINUX.BIN is loaded 
> and jumped-into.  If it were compressed by UPX, it would have to be a 
> very special UPX which understood a lot about boot-time on a PC, would 
> need to include decompression code as well as the intelligence to 
> shuffle things around in memory so that the uncompressed ISOLINUX.BIN 
> would land where it's expecting to land.  I don't believe that UPX 
> supports this.

Never knew it worked that way. Thought it was just jumped into at the 
start and then further executed.

> It's possible that MEMDISK could be compressed if UPX supports the 
> Linux kernel format which MEMDISK is compliant with.

That could be interesting yeah.

What I wonder as well if there's some simple application that can 
exchange 1 file inside an ISO by another of the same exact name and 
size. I know WinImage and possibly ISObuster or something like that can 
do it on Windows.
Would enable very easy bootdisk part updating inside a Memdisk-ISO 
booted ISO image without needing to run MKISOFS every time when only the 
bootdisk part is updated.
(so far I'm also saving the bootdisk image as a separate 
memdisk-bootable disk image on harddisk, sometimes synchronising with 
the cdrom one).

Bernd




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