of these bits set but doesn't understand the flag or can't fulfill
the requirements it indicates for some reason, it must notify the user and
fail to load the OS image.  Bits 16-31 indicate optional features; if any
bits in this range are set but the boot loader doesn't understand them, it
can simply ignore them and proceed as usual.  Naturally, all
as-yet-undefined bits in the flags word must be set to zero in OS
images.  This way, the flags fields serves for version control as well as
simple feature selec