>-----Original Message-----
>From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] 
>Sent: 24 February 2005 21:34
>To: piclist@mit.edu
>Subject: [PIC]: Fast Integer Division by a constant
>
>
>Hi,
>
>I'm wondering if anyone has any clever algorithms for 
>performing 16 bit 
>integer division by a constant using the multiply instruction 
>in the 18F 
>instruction set. I've looked over the many standard division 
>routines, and 
>thought perhaps there may be ones that employ the hardware multiply to 
>really speed things up.
>
>Specifically, I need to scale a number (0 - 65535) to 0 - 2730 
>(hence a 
>divide by 24.)  Maybe I'm missing a real obvious approach, but I'd 
>appreciate any suggestions anyone might have.
>
>Thanks!
>
>- Mike

Yes there is an easy way. To divide by 24, you multiply by 1/24.
Obviously this isn't possible directly using integer math, but if you
scale the multiplier by a nice binary factor, and then apply the same
scaling to the result then things work nicely.

e.g. in your case

(x * (1/24) * 2^16)/2^16

(x * 2730)/2^16


So you simply multiply your number by 2730, giving a 32 bit result, and
then use the top 16bits to get your scaled result.

Regards

Mike

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