> Having wondered recently about various processor architectures, and
> realised that I have altogether too many old EPROMs, I've begun
> wondering about actually building a processor out of EPROMs shift
> registers, RAM, counters and a few gates.
>
> The problem I'm finding is this:  I don't want to deal with the ALU, and
> was hoping for a prebuilt ALU IC.  Now, while these are available, the
> only ones I can find are 4-bit.

Aren't those cascadable?

> Ideally, I'd get a 16-bit ALU, but that might be a little tough to
> find.  I've actually found makers of those, but they all claim that
> they're obsolete, and they either aren't making them anymore, or won't
> be soon.

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.

> Ganging ALUs together would probably require more circuitry than just
> building the ALU.  While I don't want to use either a PLD or an MCU for
> this, if I can't find an ALU that fits my needs, I think that the best
> solution would really be to build one out of a small scale FPGA.

Awfully long way to go to find use for a few old EPROM's, isn't it?

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.


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