If you'll consider, viewed from the top, rather than the bottom, what your 805x core has to accomplish, you'll see that there has to be that "mappable space" to which I have referred, and there has to be an ALU that accomplishes all the things the instruction set demands, of which this might be a graphic representation,

http://www.8052.com/users/richard/805x01.pdf

and there has to be a set of control logic, the purpose of which is to steer the logic paths and set the timing corresponding with each opcode. A part of that, of course, is the opcode decoder.

Separating the opcodes into "classes" defined by the number of cycles, or by the number of bytes, or a combination of these two components, and/or the source and destination data used by each opcode will probably be useful.

Further, since the ALU can be used to perform operations, such as increment, decrement, add offset, etc, it can operate on the address bus as well as on the register data. Setting up those operations will also fall into the general category of control logic as well.

The "mappable space" and ALU are pretty well defined by definition. It's the control logic that will present the real challenges.

Now, that cartoon of the ALU is a very much representative, and not to be taken too literally, sketch. It can be increased or decreased in complexity, and, implicit in its architecture, there are sizeable multiplexing functions not shown in the picture. What's significant, though, is that the logic depth, aside, perhaps, from the multiplier and divider, which are large and probably separate blocks, though their tining, source, and destination are well defined and they therefore don't require a complex steering block, so the operations are similar. Yes, the NOP has not been indicated, but that's just an XOR with 0, and has the same source and destination. The NOT function, in fact, can simply be XOR with 0xFF, and there are other "reductions" easily developed. What's important in a cartoon such as this is that there be a representation of everything that's required. If I've left out a vital element, please point it out.

RE