If one performs a two's complement on the operand being
subtracted and add it to the operand being subtracted
from, the carry flag is inverted for a borrow just as it
is using SUBWF.

This suggests to me, that internally,
SUBWF passes the operand through a two's complement
unit and then through an adder.

They are probably saving hardware by not conditioning
the resulting carry for a subtract operation.

Of course I may be WWAAYYY off! After all, I am only an
amateur engineer <G>!

Dan

On Thu, 30 Sep 1999 21:05:18 +0200, Quentin wrote:

>Robin Abbott wrote:
>>
>> Yes this is quite hard.
>Indeed, this was the most confusing part when I learned PIC's.
>> X!=Y      Z=1
> Z=0?
>> Hope this helps! (Mind you it would have been easier if Microchip had simply
>> put the logic in Hardware to "correct" the carry flag after a subtraction !
>>
>The way I remember the "incorrect" carry bit, is I tell myself that when
>I do subtraction, Carry is already set incase I need it, so it clears
>when I use it, but stay there if I don't.
>
>A bit of reverse logic.
>:)
>Quentin
>