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MoonShine

No Carry Set When Adding Unsigned Ints

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Bug description:

When i to add two unsigned integers like:

 

unsigned int i = 0xffff + 0xffff;

 

I expect the carry flag to be set, but is cleared.

 

Steps to reproduce:

Use the above statement anywhere in your program and inspect the carry flag in the status register.

 

Expected behaviour:

The carry flag should be set because the integers are unsigned.

 

Is the problem 100% reproduceable:

Yes.

 

IDE version: 6.03

Compiler: BoostC

Compiler version: 6.03

Target device: PIC16F876A

OS: W2K SP4

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This is not a bug. C language doesn't know what is carry flag and doesn't have to behave like assembly.

 

Regards,

Pavel

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Also, the code shown is optimised out to say i = 0xFFFE :)

 

If you code it in separate statements:

 

	unsigned int x1 = 0xffff;
unsigned int x2 = 0xffff;
unsigned int x3 = x1 + x2;

then the carry flag will in fact be set correctly, but I certainly wouldn't put any dependency on it staying that way.

 

Out of interest, how would you normally code to catch this? In most C implementations you could use a check against the result because you have Long datatypes, but what would you do in BoostC?

 

Cheers,

Jason

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