Syslinux - Oct 2008 - Question about .bs and .bss style bootsectors.

Hello

Recently I've been happily experimenting with syslinux, replacing grub
and my old bootmanager. All works beautifully, but I have one question -
in case of .bss bootsectors - what exactly and under what circumstances
is patched in ?

With syslinux used as main bootmanager - bootsectors from xp64, xp32 and 
[pre-syslinux] msdos 7.1 (98se) work perfectly fine when chainloaded 
natively as .bs (and dos also as .bss).


PS.

By the way - there's another "trick" to make dos (at least
microsoft
versions) to boot from another harddisk - if you set actual physical
drive number in its bootsector and as active flag on the respective disk
(e.g. 81h instead of 80h). This way there's no need for drive swapping code.

Michal Soltys wrote:
> 
> By the way - there's another "trick" to make dos (at least
microsoft
> versions) to boot from another harddisk - if you set actual physical
> drive number in its bootsector and as active flag on the respective disk
> (e.g. 81h instead of 80h). This way there's no need for drive swapping
code.
> 
Interesting, although I much would like to avoid writing to a disk 
whenever possible.  On the other hand, perhaps one can just load the 
bootsector and patch the drive number in the boot sector in memory; that 
is very easy to do and would be a nice option.  I'll probably cook up a 
patch for that just because it's so easy.

	-hpa

Michal Soltys wrote:
> Hello
> 
> Recently I've been happily experimenting with syslinux, replacing grub
> and my old bootmanager. All works beautifully, but I have one question -
> in case of .bss bootsectors - what exactly and under what circumstances
> is patched in ?
> 
What is patched in is the FAT superblock from the Syslinux system 
volume, starting at byte 11.  However, I did just notice a dangerous 
thing: I always copy the number of bytes appropriate for FAT32, which is 
larger than for FAT12/16.  I will fix that :)

.bss is obviously only supported when booting from a FAT filesystem.

	-hpa

On Wed, Oct 22, 2008 at 11:34:36PM +0200, Michal Soltys
wrote:
> With syslinux used as main bootmanager - bootsectors from xp64, xp32 and 
> [pre-syslinux] msdos 7.1 (98se) work perfectly fine when chainloaded 
> natively as .bs (and dos also as .bss).
They should work if you obtain the .bs files by installing the
corresponding systems using their tools, and then copying the working
bootsector from the same partition; in this case the copied bootsector
already has the proper FAT superblock data inside.
> By the way - there's another "trick" to make dos (at least
microsoft
> versions) to boot from another harddisk - if you set actual physical
> drive number in its bootsector and as active flag on the respective disk
> (e.g. 81h instead of 80h). This way there's no need for drive swapping
code.
Unfortunately, at least current Linux kernels reject such partition
tables: if the active flag for any partition is neither 0 nor 0x80,
the partition table is considered broken and ignored completely.
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