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
|file: /Techref/scenix/carryx.htm, 2KB, , updated: 2004/4/5 18:32, local time: 2021/10/18 00:19,
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