Re: MISC-d Digest V96 #17

[john.r.strohm@xxxxxxx]

>Depends. Carry select is sometimes good if you pipeline it
> - it depends on your technology and speed requirements.
>If done carefully a hybrid carry-select adder can be blazingly fast.
>
>Full blown carry lookahead is often overkill. (And if you're going that far
>you could look at some other architectures such as Brent-Kung and other
>such oddities). It can be very expensive.

I can't agree with this statement.

No one does full-blown carry lookahead, because a full-up carry lookahead
generator for an n-bit adder requires n+1 gates, and the biggest gate
requires n inputs.

HOWEVER, cascading 4-bit adders in a 4-wide tree of 4-bit carry lookahead
generators gives you a carry lookahead cascade adder that is only a little
slower than a full-up n-bit carry lookahead adder.  The corresponding carry
select adder requires considerably more gates, burns more power, and is not
as fast.

Consider: an n-nit carry select adder requires TWICE AS MANY adder stages
as the corresponding ripple carry adder, and it requires the 2-1
multiplexers on every bit to do the carry select step.