From: "Lawrence Rust" <lvr AT NOsoftSPsystemAM DOT co DOT uk>
Newsgroups: comp.os.msdos.djgpp
References: <Ysdc7.12907$da2 DOT 54030 AT news11-gui DOT server DOT ntli DOT net> <9krscn$miu$04$1 AT news DOT t-online DOT com>
Subject: Re: 128 bit integer
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Date: Thu, 9 Aug 2001 10:01:14 +0100
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"Kai Dietrich" <toepferei DOT dietrich AT t-online DOT de> wrote in message
news:9krscn$miu$04$1 AT news DOT t-online DOT com...
> Message by Lawrence Rust <lvr AT NOsoftSPsystemAM DOT co DOT uk> on: 08.08.01
17:07:35
[snip]
> is this correct:
> sum[0] = a[0] - b[0];
> carry = sum[0] > a[0] ? 1 : 0;
> sum[1] = a[1] - b[1] - carry;

Yes, that's correct.

Note that this technique relies on unsigned integer underflow and overflow
being well defined.  ANSI C guarantees this.  The method cannot be used with
signed integers since under/overflow in this case results in undefined
behaviour, although in DJGPP it's well defined.  Hence you need to implement
your own signed type with 2's complement if you wish the code to be
portable.

-- Lawrence Rust
Software Systems www.softsystem.co.uk