Tom Ehlert wrote:
> 
> problem is, that the freedos stuff (both MBR and boot sector) don't
> know about 'DS:SI information in memory', and ignore it
> 
> what is this supposed to be (pointer into the booted partition entry)?
> any links ?
> 

Ah.

The DS:SI information is simply that the MBR is supposed to pass the 
partition information in memory to the boot sector by pointing DS:SI to 
the booted partition.  For primary partitions, this is done simply by 
pointing DS:SI to the entry in the partition table itself; however, to 
make that work with logical partitions, the partition table start value 
has to be adjusted.

The MBR code in Syslinux is one example on how this works.  It supports 
booting from logical partitions, and produces the proper DS:SI information.

http://git.kernel.org/?p=boot/syslinux/syslinux.git;a=blob;f=mbr/mbr.S;hb=HEAD

FreeDOS has the option of using either the DS:SI information, or the 
Hidden Sectors field in the FAT BPB.  Either option has its pros and 
cons, unfortunately.

        -hpa


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