You only need the '182 if you want the very best in performance.

You can chain an arbitrary number of '181s without '182s, but the ripple
carry delay will get out of hand.

Bob Ammerman
RAm Systems

----- Original Message -----
From: "Brendan Moran" <annirack@SHAW.CA>
To: <PICLIST@MITVMA.MIT.EDU>
Sent: Sunday, June 22, 2003 6:19 PM
Subject: Re: [EE]:Finding ALUs


> > > Now, while these are available, the
> > > only ones I can find are 4-bit.
> >
> >Aren't those cascadable?
>
> I dug out my 74 series databook, and it turns out that yes, they are.
With
> more than 2 they need a look-ahead carry generator though.  So I can gang
4
> * 74x181 with a 74x182 to get the 16-bit ALU.
>
> >A long long time ago "minicomputers" were built from many ICs and the
> >processor would take a whole board or two or three.  That's when ALU
chips
> >were used in quantity.  Nowadays an ALU is a relatively small area on a
> >processor die.  I can't think of too many applications today that would
> >want a bare ALU when adding the rest of the processor costs less than a
> >few additional packages.
>
> I agree.  It just makes more sense commercially to build the whole
> thing.  But what I'm doing here (other than entertaining myself) is trying
> out a concept I had for a processor a long time ago.  I think it will be
> interesting and informative for me.  You see I have never seen a stack
> oriented processor, and I'm wondering how hard it would be to build one
> from straight parts.  So I want to do it.
>
> I considered the option of using PLDs to handle things in various places,
> but I want to avoid them.  If I go down that road, I might as well just
put
> the whole project in one CPLD, and that's not what I'm aiming for.  It's
> not that I don't want to program them, it's that I almost see them as
> cheating for something like this; I want to at least try to design this
> without the use of programmable logic.  I don't think I'd get nearly as
> much out of it with PLDs as I would with other simpler devices.
>
> >Awfully long way to go to find use for a few old EPROM's, isn't it?
>
> Yeah it is.  I admit when I started looking at how to make the stack, I
was
> really wondering if it was worth it, but I've worked out a decent solution
> to that.
>
> >If you've really got that many EPROM's, you could make an 8 bit ALU by
> >brute force table lookup.  It'd only be 1/2Mbyte for two 8 bit in ports,
> >one 8 bit out port, and a 3 bit ALU function code.
>
> I'd considered that, but while I have a fair number of EPROMs, they're all
> between 16k and 128k.  I think I'll gang some 4-bit ALUs.
>
> Thanks everyone,
>
> --Brendan
>
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