> > 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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