[syslinux] Booting

Murali (முரளி கணபதி) murali at google.com
Fri Jan 25 03:44:51 PST 2008

On Jan 25, 2008 6:57 AM, H. Peter Anvin <hpa at zytor.com> wrote:
> Jacob Alifrangis wrote:
> > The boot loader on the Windows CD detects whether there is a partition
> > located on device 0x80 (First BIOS Harddrive), if it does not see one,
> > it boots from CD first, if it does it just asks you to "Press Any Key"
> >
> > The logic you are looking for doesn't exist on PCs,
> >
> > There isn't a firmware variable that is accessible from modern Operating
> > Systems, especially not one that survives a reboot.
> >
> > What you are looking to do, can be accomplished from a Com32 Module
> > though, but you'll still need to set the bios Boot Order manually to
> > always boot from USB (Which is exceedingly dangerous and insecure)
> >
> > Your best bet to accomplish this, is writing a state-file to the
> > harddrive on the boot partition(Must be 0x80h - 0x83h) or syslinux would
> > never see it, then you must write a com32 module for NTFS and FAT /
> > Fat32 so you can read the state-file, and finally act on it via an "If"
> > statement, something that syslinux doesn't support either yet.
> >
> Actually, 3.60 introduced persistent storage for extlinux, which could
> be used for this purpose.  It is currently missing installer support for
> syslinux, but the core module support is there.

For ISOLinux and PXELinux, one might be able to get away with using
the *later sectors* in Track 0 (after leaving space for the MBR) on
the first hard disk. Of course, this assumes that the "first hard
disk" remains the same across boots and that various disk formatting
utilities dont touch that area.

My "autoboot.c32" does that and I found that the assumptions seemed to be true!

> But yes, you need to set the bios boot order to boot from whatever
> medium actually has syslinux on it.
>        -hpa
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