Subtract Binary Coded Decimal with extend

NAME
	SBCD -- Subtract binary coded decimal with extend

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

	Size = (Byte)

FUNCTION
	Subtracts the source operand to the destination operand along with
	the extend bit, and stores the result in the destination location.
        The subtraction 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.

	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 | 0 | 0 | 0 |Ry REGISTER| 1 | 0 | 0 | 0 | 0 |R/M|Rx REGISTER|
	-----------------------------------------------------------------

	Ry specifies destination register.
	Rx specifies source register.
	If R/M = 0: Rx and Ry are datas registers.
	If R/M = 1: Rx and Ry are address registers used  for the pre-
	decrementing.

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
	SUB  ADDI ADDQ
	ADDX
	ADD  SUBI SUBQ
	ABCD SUBX