Re: Any more graceful way to saturate adds?

In article 
<7f1061af-6435-47c0-b2e3-38b64cb1f044@[EMAIL PROTECTED]
>
           spamtrap@[EMAIL PROTECTED]
 "Jim Leonard " writes:

> On Apr 13, 10:27 am, "Rod Pemberton"  <spamt...@[EMAIL PROTECTED]
> wrote:
> > If Jim needs to distinguish 255 from overflow as Terence noted,
perhaps this
> > is sufficient:
> 
> I do not need to distinguish 255 from overflow.
> 
> The quick background for this is that it is the clamping function for
> adding a signed 8-bit integer to an unsigned 8-bit integer.  The
> function is an adapted ADPCM routine; whereas most ADPCM works on
> signed 16-bit samples, I have a need for it to work on unsigned 8-bit
> samples.
> The inner loop of the routine has a previous unsigned sample, and a
> new signed sample, and needs to add them together to get the new
> sample.  The result needs to clamp at both 0 and 255.
> 
> With this in mind, what is the best solution, assuming the previous
> unsigned sample is in DL and the new signed sample is in AL?

Just to be absolutely clear about the problem (and without any 
attempt at optimising the code), can you confirm that the below 
is what you're after?

(unsigned sample in DL, signed "update" in AL)

        sub  dh, dh             ;DH = 0
        cbw                     ;sign-extend AL to AX
        add  dx, ax             ;signed addition
Now:
        if DH == 0, nothing to do: ans in DL
        if DH == FF, -ve overflow so clamp to 0: set DL = 0
        if DH == 1, +ve overflow so clamp to FF: set DL = FF

Is that correct?

Pete
-- 
   "We have not inherited the earth from our ancestors,
    we have borrowed it from our descendants."