So, I was having an issue where I could enter a BASIC program in the emulator, but it would break immediately when I entered the RUN command. Needless to say, this was pretty frustrating and really hard to track down.

Happy to say, that I figured it out today!

It was a bug with my implementation of IN A, (n). I wasn't using the register along with the parameter to form the port address.

- var value = input.Ports.ReadByte(input.Data[1]);
+ var value = input.Ports.ReadByte((ushort) (input.State.Registers[register] << 8 | input.Data[1]));

Here's the result: