SX microcontroller CARRYX option / CF FUSEX bit

The intent of the CF_ bit is to make multiprecision adds and subtracts easier, as shown here:

   ;FF:= FF + GG with correct carry out (CF_ is 0)
	mov	W, G	;add low bytes
	clrb	STATUS.CF	;clear carry
	add	F, W
	mov	W, G+1	;add high bytes and carry
	add	F+1, W

   ;FF:= FF - GG with correct carry out (CF_ is 0)
	mov	W, G	;subtract low bytes
	setb	STATUS.CF	;set carry
	mov	W, G+1	;subtract high bytes

The following routines accomplish the same thing (although with an extra instruction), and they're compatable with the PIC 16C5x:

   ;FF:= FF + GG with correct carry out (CF_ is 1)
	mov	W, G	;add low bytes
	add	F, W
	mov	W, G+1	;get ready to add high bytes
	snb	STATUS.CF	;skip if there was no carry into high byte
	movsz	W, ++G+1	;else add in carry; if result is 0 then high
	add	F+1, W	; byte doesn't change and CF is still set

   ;FF:= FF - GG with correct carry out (CF_ is 1)
	mov	W, G	;subtract low bytes
	sub	F, W
	mov	W, G+1	;get ready to subtract high bytes
	sb	STATUS.CF	;skip if there was no borrow from high byte
	movsz	W, ++G+1	;else increase amount to subtract by 1; if it's
	sub	F+1, W	; 0 then high byte doesn't change, nor does CF

Since the carry-out is correct in all four of these routines, they can easily be extended for more bytes of precision.

We can do a double precision add in five instructions if there is a location set asideto hold a zero. Unfortunately, this can't be extended to more than two bytes of precision.

   ;FF:= FF + GG with correct carry out (CF_ is 1)
	mov	W, G	;add low bytes
	add	F, W
	mov	W, <<ZERO	;shift in carry forming either 01h or 00h
	add	W, G+1	;add carry and high bytes
	add	F+1, W

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