Sean H. Breheny wrote:
> I'm not familiar with "bit-slice parts". What are these?

Bit slices were 4-bit logic units that were designed to be combined into
CPUs with wider word lengths. You also had to add PROM memory for the
microcode, and some glue to make it all stick together. Slices were
popular for a while, before it was possible to design high-power CPUs on
a single chip. Nobody designs computers that way now; if any bit slice
parts are still available, it's to allow continued production and repair
of old designs.

> It would be a lot easier if you built your own CPU using an FPGA. That way,
> you would eliminate the need to buy all the various different logic
> functions and assemble them, you could make changes just by editing your
> file in the design software, AND if you have decent software, you could
> even simulate part or all of your design and see what is going on for
> debugging purposes.
>
> We did this for a simple 8 bit CPU way back in our first class on logic
> design. It was fun and not all that difficult. You might even be able to
> build something useful that you would want to use later on!

If your goal is to learn how to design FPGAs, this sounds great, and
should be a useful exercise for people who are learning current system
design practice.

On the other hand, if you want a CPU that you can dig into with a scope,
it won't do at all, because all the workings will be hidden inside the
FPGA. You're no better off than you are if you build a system around a
stock microprocessor or microcontroller.

The lesson: first decide what your goals are. Then act accordingly.

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