DAS - Decimal Adjust for Subtraction

	Usage:	DAS
	Modifies flags: AF CF PF SF ZF (OF undefined)

	Corrects result (in AL) of a previous BCD subtraction operation.
	Contents of AL are changed to a pair of packed decimal digits.

				 Clocks			Size
	Operands	 808x  286   386   486		Bytes

	none		  4	3     4	    2		  1

	- see Instruction Timing