I need to crack this little problem... Please can anyone help?

I have a stream of numbers which are signed 16-bit integers, ranging 
from + 32767 (7FFF) to -32767 (8001).
The MSB is the sign bit.
-1 is FFFF. etc

I wish to add these numbers to a signed 16-bit "output word". When 
negative numbers arrive, they are just added and the necessary 
subtraction seems to take place, no problems.

However, I wish to limit the output word to +32767 and -32767. My 
anti-rollover strategy is as follows:

If input and output words are both positive, add them. If the MSB has 
rolled over to 1 then just make the output word = 7FFF.
If input and output words are opposite signs, just add. The result 
should lie within bounds.
If input and output words are both negative... well this is where I go 
snow-blind.

I first thought that if the sign bit went from 1 to 0, then a rollover 
had occurred. But this didn't account for all cases.

My brain can't handle the depth of thought required until I have a bit 
more knowledge on the subject.

I wish to keep the maths as simple as possible in order to reduce CPU 
overhead.

Many thanks in advance... Have a good weekend

Matt.

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