Re: Arithmetics with carry

Francisco Rodriguez wrote:
> As your numbers are 32-bit wide, and overflow condition is a flag
> about the result of the _whole_ v := v + i operation, it can be determined
> by the MSB bits only (that is, from columns 31 and 30 of the addition).

Thank you! I think I used the wrong word "underflow". What I mean is:
if "i" is negative, it might be that abs(lo(i)) is greater than lo(v).
What happens then? An example:

v = 1234 0100 hex
i = 0000 0101 hex

The operation for the lo bits will result in FFFF and the carry will
be set. Then when I add the high bits, it will be 1235 when it should
actually be 1233. How do I save that the first operation was not an
overflow but a borrow? (I don't know how to call this. That's what I
mistakenly called underflow).

Glen Herrmannsfeld wrote:
> After the high halfword add, you compare the carry out to the carry out of
> the sign bit to the carry in of the sign bit.  If they are different then it
> is overflow or underflow.  The value of such bit tells you which one.

So does that mean I have to modify my architecture and set TWO flags?
A carry flag and a negative flag (if sign of last operation was
negative), and then the Add-With-Carry instruction would look at both?

Thanks a lot!