At the time of writing the latest lib6502 release is v1.3; older versions are
not considered here.

Some things which work fine with lib6502 itself are not supported when using
lib6502-jit in hybrid (the default) or compiled execution modes. All of the
following will result in undefined behaviour unless interpreted mode is used:

* Modifying memory which contains 6502 code (whether executed yet or not)
  inside a read callback. (All other types of callbacks are allowed to
  modify memory freely, including modifying code.)

* Defining a callback after calling M6502_run(); for example, doing so inside
  another callback.

* Checking the B and X flags in the processor status register
  (M6502_Registers.p) inside a callback. lib6502 tracks these flags as if they
  have a real existence at all times. lib6502-jit's compiler only sets them
  appropriately when pushing a copy of the processor status register onto the
  stack. This difference is *not* visible to code executing on the emulated CPU,
  only to callbacks. In hybrid mode, which behaviour you get will depend on
  whether your callback is invoked from the interpreter or compiled code.

The following differences exist between lib6502 and lib6502-jit in all modes,
including interpreted mode:

* lib6502 is likely to be slightly faster than lib6502-jit in interpreted mode,
  since the latter's interpreter code contains additional tests to stop
  executing at certain points after n instructions have been executed.

* Illegal instructions are treated as no-ops by default in lib6502-jit; lib6502
  aborts if an illegal instruction is executed.

* Illegal instruction callbacks are a lib6502-jit extension and are not
  available in lib6502.

* Call callbacks in lib6502 always receive a 0 as the data argument;
  lib6502-jit supplies the opcode triggering the callback as the data argument.

* A few bugs in lib6502's emulation are resolved in lib6502-jit:
  - BRK clears the D flag
  - ADC/SBC exactly match the behaviour of a real 65C02 in decimal mode
  - BIT #imm only modifies the Z flag, leaving N and V untouched
  - TSB sets the Z flag correctly
  - TRB sets the Z flag and updates memory correctly

* lib6502's run6502 -B option skips every other (ROM name) argument;
  lib6502-jit's doesn't.

lib6502-jit's stance is that anything the code executing on the emulated CPU
does is fair game and must be handled, but that the library's client code has a
responsibility to cooperate and not do tricky things like those documented
above. If you have what you think is a reasonable requirement for behaviour
which is supported by lib6502 but doesn't work on lib6502-jit please get in
touch.