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
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.