* add exception for privileged instructions.  Should you separate them in the `match` or just put a guard into the instructions, and if the latter, can
  you use the emulator's errors to surface the execption up to the `step_internal` function but not to `System`?

* make it possible to break out of the current execution, into the debugger
* how can you add 68030 support?  Should it be all one module that checks maybe during decode if the instruction is supported?  Should it 'inherit' from the MC68010 object
* make tests for each instruction

* unimplemented: ABCD, ADDX, BKPT, CHK, EXG, ILLEGAL, MOVEfromCCR, MOVEP, RTR, RTD, SBCD, SUBX
* undecoded: ADDX, SUBX
* modify execution for >=MC68020: DIVSL, DIVUL, EXTB, LINK, MOVEM, MULSL, MULUL, RTM, TRAPcc, UNPK
* implement the full extension word for MC68020+

* >=MC68020 instructions: BFCHG, BFCLR, BFEXTS, BFEXTU, BFFFO, BFINS, BFSET, BFTST, CALLM, CAS, CAS2, CHK2, CMP2, PACK
* Coprocessor instructions: cpBcc, cpDBcc, cpGEN, cpScc, cpTRAPcc


* should you simulate clock ticks, and only step devices when they next request it
* how can you have multple CPUs
* should you simulate bus arbitration?


* check all instructions in the docs