A.5.13 ARPL: Adjust RPL Field of Selector
ARPL r/m16,reg16 ; 63 /r [286,PRIV]
ARPL expects its two word operands to be segment selectors. It adjusts
the RPL (requested privilege level - stored in the bottom two bits of
the selector) field of the destination (first) operand to ensure that it
is no less (i.e. no more privileged than) the RPL field of the source
operand. The zero flag is set if and only if a change had to be made.