AVR-Chat

Peter, 

[ snip ]

> In my application, I only wanted the most significant 16 
> bits. there are two easy ways to shed them:
> 
> w = ((long)x * y) / 65536;  //takes 782 cycles (toooo long at 16MHz)
> 
> This seems to trigger a full divide with the compiler missing 
> the optimisation, so I gave it a clue:

Actually, the compiler is doing the correct thing in implementing a full
divide.

Consider (1 * -1) / 65536.  That's -1L / 65536 which is 0.

> w = ((long)x * y) >> 16;  // takes 79 cycles (a shade under 5us)

Consider (1 * -1) >> 16.  That's -1L >> 16 which is still -1.

(Actually, there *is* a way to implement a signed divide by a power of
two without using the clockwork method.)

Just FYI.

--
Paul Curtis, Rowley Associates Ltd  http://www.rowley.co.uk
CrossWorks for MSP430, ARM, AVR and (soon) MAXQ processors