8bit add with carry test macro

[Futureatwork]

I am thinking about to write a define/macro to add two bytes, test carry and if true set result to 255.

#define add8x8(A1,A2,RESULT) ??

I tried but defines does not like if-then.
Maybe a simple function would be better, but I dont know and I like to experiment.

Can someone help me?

[Guest]

I would use this:
#define _PCH
#ifndef _PCH
#byte STATUS = 0x03
#else
#byte STATUS = 0x0FD8
#endif
#bit CARRY = STATUS.0

char Add8_8(char in1, char in2)
{
char result;

CARRY = 0;
result = in1 + in2;
if (CARRY) result = 0xFF;
}

[Guest]

And of course
return result;

[Futureatwork]

And what about the carry flag behavior, does it clear automatically after a successful addition or I have to clear it myself like showed above?

[ckielstra]

The carry flag is set according to the result of the last computation, so you don't have to clear it manually as in the example above.

Also I wouldn't use the define _PCH which you would have to set yourself, but use the define __PCH__ which is automatically defined by the compiler.

And because this is such a nice short function I would make it inline so you can save two goto instructions.

New example code:

[Futureatwork]

Does MULLW affect the same carry flag bit?

I will use this for 8bit*8bit<=255 testing.

[Guest]

8X8 multiply is another story. If you use the built-in HW multiplyer then you will always get the result as a 16-bit register pair. You should use the lower half and can test the upper byte for zero

P.S.: The clear carry before the addition as suggested by myself is for "safety first" purposes. You didn't tell what PIC you are using and some of the 18F6X20 series makes really strange things...