Re: PDD 4: Internal data types

[Dan Sugalski]

At 06:49 PM 3/8/2001 +0000, Nicholas Clark wrote:
>On Thu, Mar 08, 2001 at 11:43:31AM -0500, Dan Sugalski wrote:
> > I was thinking maybe (length/4)*31-bit 2s complement to make portable
> > overflow detection easier, but that would be only if there wasn't a good C
> > library for this available to snag.
>
>The only portable integer overflow in ANSI C is unsigned integers.
>signed integer overflow does not always do what you might expect, even
>on a 2s complement machine.

That's what the 32nd bit of each word is for. If we do addition on a 
word-by-word basis and the high bit of the result is set, we know we've 
overflowed, and we can set it as a carry bit for subtraction.

                                        Dan

--------------------------------------"it's like this"-------------------
Dan Sugalski                          even samurai
[EMAIL PROTECTED]                         have teddy bears and even
                                      teddy bears get drunk