Re: 6502 illegal opcodes questions

[Eric Smith <eric@brouhaha.com>]

Jim Brain wrote:
> I don't think a PLA allows X (don't care) states.

I wrote:
> Certainly it does.  That's one of the primary distinguishing
> characteristics
> of a PLA as opposed to a ROM.

Jim Brain <brain@jbrain.com> writes:
> I'll defer.  All I know is the DTV designer said she originally did
> not include any illops in the design because she could not emulate the
> wored-or matrix in the FPGA in a space efficient manner.

That's true.

> I assumed the FPGA is a superset functionality of the PLA, and thus if it
> can't be easily dealt with with in there, I didn;t think it could in the
> PLA.

There's a lot of difference between an FPGA being conceptually a superset
of a PLA, and an FPGA being able to efficiently implement any particular
PLA.  I've been trying to cram the logic of the DEC J11 microprocessor
into an FPGA, and the decode PLA in that, which is relatively small in
the actual J11 chip, takes up an enormous amount of FPGA space.

The problem is that an FPGA cell is much more coarse-grained than a
PLA "cell", which is just a transistor.

Eric