CMPXCHG - Compare and Exchange

	Usage:	CMPXCHG	dest,src  (486+)
	Modifies flags: AF CF OF PF SF ZF


	Compares the accumulator (8-32 bits) with "dest".  If equal the
	"dest" is loaded with "src", otherwise the accumulator is loaded
	with "dest".


				 Clocks			Size
	Operands	 808x  286   386   486		Bytes

	reg,reg		  -	-     -     6		  2
	mem,reg		  -     -     -     7		  2


	- add 3 clocks if the "mem,reg" comparison fails
	- see Instruction Timing