Xref: utzoo comp.lang.c++:11505 comp.std.c++:578
Path: utzoo!dciem!nttor!contact!egr
From: egr@contact.uucp (Gordan Palameta)
Newsgroups: comp.lang.c++,comp.std.c++
Subject: Re: Add-with-carry operator
Message-ID: <1991Feb7.013620.28671@contact.uucp>
Date: 7 Feb 91 01:36:20 GMT
References: <1991Feb3.202530.14874@julius.cs.uiuc.edu>
Organization: Contact Public Unix BBS. Toronto, Canada.
Lines: 17

In <1991Feb3.202530.14874@julius.cs.uiuc.edu> zweig@cs.uiuc.edu (Johnny Zweig) writes:

>In my work with the TCP/IP protocol suite, I have had to implement 16-bit
>1's-complement arithmetic in a protable, efficient way for protocol processing
>[rest of article omitted]

To add-with-carry two 16-bit numbers in a portable way:

       - convert them to unsigned 32-bit (high word zero-filled)
       - add them to get a 32-bit result (normal addition, ie 2's complement)
       - take the upper 16 bits and lower 16 bits and add them together,
         getting a 32-bit result
       - do previous step again (important!)

The lower 16 bits will be your answer (the upper 16 bits will be zero-filled).

Try the tcp-ip newsgroup for any further info...