We all are making errors - and learning - all the time.

One more thing. It is unlikely that your "main" program will consist of "jmp $" (infinite loop doing nothing). If the ISR will interrupt any reasonable code execution, it's a good idea to store all the resources which might be used in both the "main" and ISR - namely the flags (PSW), of which you actually change at least C (by cjne) - and on the other hand, it's a bad idea to store and pass the received byte in A (ACC) (and maybe not even in B).

The point of this forum is, that we won't come up with a ready made solution, that will teach you nothing. But we are happy if we can corect your attempts - provided you are willing to learn from what we tell you.

Jan Waclawek