> Dear all,
>
> I read from datasheet regarding OV flag.
>
> "This bit is used for signed arithmetic. It indicates an overflow of the
> 7-bit magnitude, which causes the sign bit to change."
>
> Is that mean whenever the MSB toggled (not include the result from bsf &
> bcf), this flag will be set?
>
> Thanks & regards.
>
> WH Tan
>

I am assuming you are referring to an 18F series PIC.

Actually, the description is true, but rather poorly worded. What it could
say is:

This OV bit is used for signed arithmetic. When adding or subtracting two
eight-bit signed values it is possible for the correct result to not be
representable as an eight bit signed value. This bit is set in those cases.

For example:

+100        decimal
+100        decimal
------
+200         OV will be set, because +200 is not a valid eight-bit signed
value.

The same this will happen with

-100        decimal
-100        decimal
-----
-200

Note that it is never possible to get an overflow if the operands are of
opposite signs (for addition), or the same sign (for subtraction).

Bob Ammerman
RAm Systems

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