This emulator was written originally by Mike Chambers.
I have made a few modifications to it, the biggest of which is that you now pass it a pointer to a struct holding the CPU's state. This means that your program may now emulate more than one 6502 concurrently. This was necessitated by stoc, my superoptimiser, which needs to run two programs in parallel to check them for equivalence.
The emulator as I found it, can be for an ordinary 6502 or for a 2A03 (rando variant found in some games console). I've taken the liberty of adding a CMOS support; this adds a few opcodes here and there). I might get round to adding support for other chips in the future.
I've put a test harness around this. Coverage is currently at around 90%, which includes all of the documented NMOS instructions, and increasing as and when.