If you can't come up with an SFR bit that the hardware does not care about, here are a couple of ideas off the top of my head that might satisfy your "general purpose solution" requirement.

• Reserve the topmost IRAM byte by changing IDATALEN in startup.a51 to 255. Unfortunately, you would have to access this indirectly, adding additional ISR overhead compared to your direct bit approach.


• In the ISR, get the return address MSB and decide whether the ISR would be returning to, and thus was invoked while running, boot or application code. This does not "cost" you an extra RAM global or reserved byte, but again adds overhead.