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Math operation problem

 
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sergiofil



Joined: 27 Oct 2011
Posts: 3
Location: Portugal

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Math operation problem
PostPosted: Sat Oct 13, 2012 6:45 pm     Reply with quote
Hi,

I'm programming a PIC18f4580 with PCH compiler and i was getting an unexpected performance by the pic. So, after debugging the code, I found the problem. It is the following math operation:


vars:
Code:

temp: unsigned int16;
a: BYTE , value is 81
myList.offset; unsigned int8; value is 32
myList.x; unsigned int8; value is 8
myList.y; unsigned int8; value is 12
(...)

temp=((a-myList.offset)*((myList.x/8)*myList.y))+4;

I think that the result should be:
Code:

temp = ((81-32)*((8/8)*12))+4
temp = (49*12)+4
temp = 592

But the pic result is: temp = 80 !!!!!

So, I would like to understand what is wrong, to don't repeat the mistake the next time!!



Best Regards,
Sérgio Silva
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Sat Oct 13, 2012 9:06 pm     Reply with quote
If you want the compiler to do int16 multiplication, you need to cast one
of the operands to an int16, as shown below in bold:
Quote:
temp=((int16)(a-myList.offset)*((myList.x/8)*myList.y))+4;

Then you will get 592. In other words, it doesn't do automatic type
promotion. You need to tell it what to do.

Another option is to use the _mul() function. It will return a 16-bit
result for two 8-bit operands. See the CCS manual:
http://www.ccsinfo.com/downloads/ccs_c_manual.pdf
sergiofil



Joined: 27 Oct 2011
Posts: 3
Location: Portugal

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PostPosted: Sun Oct 14, 2012 5:15 am     Reply with quote
Thanks for the explanation!

I have seen the _mul() function in manual, but i never tried to use it!


Best Regards,
Sérgio Silva
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