The 65816 CPU

The 65816  is a 16-bit processor that is an enhanced version of the 65C02.

It has software compatibility with the 65C02 (and hence 6502) as well as new registers, instructions, and addressing modes. Most notably it has 16-bit registers and the ability to address 24 bits (16 megabytes) of memory. It was used in some computers such as the Apple IIGS and the chip is still manufactured by Western Design Center.

The chip is not hardware compatible with the 65C02. There was a 65802 version of the processor that was pin-compatible with the 65C02. Unfortunately, these are no longer manufactured and hard to obtain.

The 65816 and 6502 have very similar pinouts. It would be cool if you could build an adaptor to put a 65816 in a 6502 socket. A web search reveals that some designs for such an adaptor are around.

This one by Daryl Rictor dates to 2004. It uses three chips plus the 65816. It even has a PCB layout. However, it comes with the caveat "I have not actually built or tested this design. Use it at your own risk." I'm doubtful that such a circuit would work the first time (no circuit of any complexity does).

A more promising design is this one  by Ruud Baltissen. He reports that he used it with a VIC-20 and it worked well. It only uses one chip in addition to the 65816.

I'm seriously thinking of building this circuit and trying it on a Replica 1. There are some limitations, like not being able to address more that 16 bits of memory, but it would be fun to try out the new instructions in the 65816. For example, there is a single instruction that can copy a range of memory. The 65816 (actually a W65C816S6PG-14) chip is under $10 from Mouser  so it's not a big investment to lose if it doesn't work.