S must not load any segment registers
(even just reloading the same values!) until it sets up its own GDT.
<LI>IDTR: The OS must leave interrupts disabled until it sets up its own IDT.
</UL>

However, other machine state should be left by the boot loader in "normal
working order", i.e. as initialized by the BIOS (or DOS, if that's what
the boot loader runs from).  In other words, the OS should be able to make
BIOS calls and such after being loaded, as long as it does not overwrite
the BIOS data structures b