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how to avoid multiplication overflow ?

 
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spilz



Joined: 30 Jan 2012
Posts: 219

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how to avoid multiplication overflow ?
PostPosted: Fri Sep 07, 2012 3:07 am     Reply with quote
hello everybody

I need to know if there is a way to "avoid multiplication overflow"
I explain Smile :

actually I have :
Code:
int a=2,b=200,c;
c = a*b;


by this code, I obtain c = 144 , cf 0 <= c <= 255
but I would like to avoid overflow and obtain c = 255

How can I do that?

thanks for your help

Spilz
RF_Developer



Joined: 07 Feb 2011
Posts: 839

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PostPosted: Fri Sep 07, 2012 3:31 am     Reply with quote
There is no way to "avoid" the overflow; overflow behaviour is fixed in the harware; what you are wanting to do is deal with the overflow in a different way.

So you have to do it in 16 bit arithmetic and then limit/clamp the result to 8 bits:

Code:

// Don't forget ints in CCS C are 8 bit unsigned. The same method
// works for other sizes & signedness with suitable modifications.

int a=2,b=200,c;

int16 Temporary;

Temporary =  (int16) a * b;  // Only need to cast one of the operands. Casting both makes no difference; just takes longer to type!

if (Temporary > 255)
{
    c = 255;
}
else
{
    c = Temporary;    // Note this invloves an implied truncation of the result.
}


RF Developer
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