[syslinux] isohybrid boot from logical partition

Ady Ady ady-sf at hotmail.com
Thu Dec 29 23:47:47 PST 2016

> Thanks for all the input everyone.
> I can understand the problem more clearly now.
> Didier Spaier wrote:
> > Sorry if this is a silly question: why not use a GUID partition
> > table instead of using logical partitions?
> Not a silly question, I had the same idea and had tried it out but it
> didn't work (same error msg).
> Ady Ady wrote:
> > The typical (modern) "MBR" supports 4 primary bootable partitions. The
> > meaning of "bootable" in that context refers to the possibility of
> > setting the "boot" flag on (one of) them.
> I've not found any need to set any boot flags on primary partitions to
> get the booting from primary partitions working, so I don't think the
> lack of ability to do this on a logical partition is an issue.
The boot flag is relevant when the bootstrap code says "look for the 
boot flag and then jump to that partitions VBR" (or similar). When GRUB 
(or whichever other boot manager / loader) says "jump to partition 
'x'", it is not using the boot flag as condition to select the relevant 

My main point was not really about the boot flag itself, but rather 
that the classic MBR code (being used by the isohybrid tool) has space 
for 4 (primary) partitions, so this code knows nothing about a 5th (or 
higher) partition. Therefore, perhaps using a special MBR boot code 
that has space for more than 4 partitions within the 512 bytes 
constraint might be a potential workaround, considering the 
self-imposed restrictions of using the particular 
"isohybrid-in-partitions" method.

> @Thomas, thanks for those links.  It seems clear that this issue has
> been around for a while and stems from the relative partition offset
> that GRUB passes when jumping to a logical partition.
> As per your earlier suggestions, I'll look at testing some other bootloaders.
Syslinux's chain.c32 as potential testing example?
> Taking the point that "Syslinux is partition-table-format agnostic"
> I'm guessing that any "fix" that should be made, should be on the GRUB
> side, rather than in the isohybrid side?
> Thanks,
> Duncan

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