Add Binary Coded Decimal (w/extend)

NAME
	ABCD -- Add binary coded decimal

SYNOPSIS
	ABCD	Dy,Dx
	ABCD	-(Ay),-(Ax)

	Size = (Byte)

FUNCTION
	Adds the source operand to the destination operand along with
	the extend bit, and stores the result in the destination location.
	The addition is performed using binary coded decimal arithmetic.
	The operands, which are packed BCD numbers, can be addressed in
	two different ways:

	1. Data register to data register: The operands are contained in the
	   data registers specified in the instruction.

	2. Memory to memory: The operands are addressed with the predecrement
	   addressing mode using the address registers specified in the
	   instruction.

	This operation is a byte operation only.

	Normally the Z condition code bit is set via programming before the
	start of an operation. That allows successful tests for zero results
	upon completion of multiple-precision operations.

FORMAT
	-----------------------------------------------------------------
	|15 |14 |13 |12 |11 |10 | 9 | 8 | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
	|---|---|---|---|-----------|---|---|---|---|---|---|-----------|
	| 1 | 1 | 0 | 0 |    Rx     | 1 | 0 | 0 | 0 | 0 |R/M|    Ry     |
	-----------------------------------------------------------------

	R/M = 0 -> data register
	R/M = 1 -> address register
	Rx:   destination register
	Ry:   source register

RESULT
	X - Set the same as the carry bit.
	N - Undefined
	Z - Cleared if the result is non-zero. Unchanged otherwise.
	V - Undefined
	C - Set if a decimal carry was generated. Cleared otherwise.

SEE ALSO
	ADD  ADDI ADDQ
	ADDX
	SUB  SUBI SUBQ
	SBCD SUBX