Hi,

Peter L. Peres wrote:


>> better
>
>decf    mid, F
>decf    mid, F
>decfsz  low, F
>  incf  mid, F
>incfsz  mid, F
>  decf  high, F
>
>6 T cycles, 6 words run time (constant), and no changes in W ;). I
think
>it's optimal.
>
>Peter

I like the pair of "decf" followed by an "incfsz", however this takes
the same time/space as a "movf X,f / skpz" combination.

This has several problem where it returns incorrect values. One problem
is the decfsz low,F will skip when low transfer from 1 to 0 not from 0
to FF. If access is well controlled this can be "fixed" by loading
values with 1 greater than target and reading the values with a "decf
X,w" instead of a "movf X,w". However this makes the value only useful
as a down counter.

Another problem is handling the borrow out of the mid byte. To be
sucessful  you must decrement high only if (low WAS 0 or BECOMES FF) AND
(mid WAS 0 or BECOMES FF). Any implementation which only tests mid to
determine if high should be decremented will fail when mid is FF and low
did not underflow.

There are some good ideas in this approach, I fiddled with it a bit but
did not come up with a low cost and correct version.

Chip Weller