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Signed Long and Constant Multiplication

 
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nikolai



Joined: 28 Apr 2010
Posts: 5
Location: Houston

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Signed Long and Constant Multiplication
PostPosted: Wed Apr 28, 2010 7:08 am     Reply with quote
I am using an older version of the compiler: v3.249

I believe I may have found a bug, which may have actually been corrected in recent releases, but I don't want to pay for the upgrade unless I am sure that it will resolve the issue. When I multiple a signed int16 (long) with a value of -1 (65535) by a constant, the result is not what I am expecting.

Code:
void main()
{
   signed long i=-1;
   signed long r=0;
   
   r = i * 1000;   // result should be -1000
                   // but I get -744, which is 256 too low
}


I have tried performing the calculation by hand and truncating the 32bit result with no success in finding the source of the error. Can someone try this on a more recent version of the compiler to confirm that this issue has been addressed already?
newguy



Joined: 24 Jun 2004
Posts: 1911

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PostPosted: Wed Apr 28, 2010 7:16 am     Reply with quote
http://www.ccsinfo.com/forum/viewtopic.php?t=29773&highlight=math

I ran across something similar a few years ago. Read through the posts for a fix that may work for you.
nikolai



Joined: 28 Apr 2010
Posts: 5
Location: Houston

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PostPosted: Wed Apr 28, 2010 7:32 am     Reply with quote
Thanks. Changing the order does give me the correct results. I wonder if this was ever fixed.
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