STD - Set Direction Flag

	Usage:	STD
	Modifies flags: DF


	Sets the Direction Flag to 1 causing string instructions to
	auto-decrement SI and DI instead of auto-increment.


				 Clocks			Size
	Operands	 808x  286   386   486		Bytes

	none		  2	2     2	    2		  1


	- see Instruction Timing