which BIOS disk device the boot loader loaded
the OS from.  If the OS was not loaded from a BIOS disk, then this field
must not be present (bit 3 must be clear).  The OS may use this field as a
hint for determining its own "root" device, but is not required to.  The
boot_device field is layed out in four one-byte subfields as follows:<P>

<pre>
	+-------+-------+-------+-------+
	| drive | part1 | part2 | part3 |
	+-------+-------+-------+-------+
</pre>

The first byte contains the BIOS drive number as und