Clamped addition of 2 signed words
 

Had a quick looky on t'interwebs... couldn't find an answer easily to this.

Say I have 2 signed words in d0 & d1 (can be using full range -32768 to 32767), and I want to add them together and clamp to full word range (-32768 to 32767)

Is there a snazzy bit-wise trick way of doing this, without having to:

It doesn't matter about the top word of the register containing crap on exit, as only the bottom word is used later

Any thoughts on this ? I know that byte clamping can be easily done with the Sxx instruction and logic operations

How about
Note: untested ;)
Edit: also, I'm now waiting on one of the true wizards to come along with a branchless and much faster version. Where's ross when you need him? :p

@roondar I'll take that one if it works :D

yeah, am expecting Ross to come along with a branchless xor version ;)

I couldn't help myself and tested it just now. It does indeed seem to work correctly in all cases (assuming I haven't missed one).

ahaha, this made my day :)

In fact I was thinking about how to solve this problem, but today doesn't seem to be the right day (I'm not in the mood :().

[off-topic]
It happens when you are gentle and helpful to everyone and you get in return some shit shoveled on you.
[/off-topic]


Luckily you cheered me up a bit :D

Well, there's only what, 4,294,967,296 different possible test cases? Easy peasy, should be able to run through them all. :D

I don't usually do this right after an add, but here it goes :

Yup, I can type really quite quickly :p

Nice!

I really need to start looking into Sxx more often. I fear I've rather underutilized these instructions so far and seeing the above example that is clearly a shame!

Yes! Awesome. I need to learn those CC's more thoroughly!

I was explaining to someone, just yesterday, the advantages of the Scc instructions, but I'm don't even remember who it was..

I need some rest :)

Wow!! that's really nice!

Adopted from my 6502 mixing routines. Should work, but untested.