Peter,

Could you please tell me what would happen if you use a port pin to dynamically change the status of the EA pin? Specifically what will the contents of the Program Counter be? Will it start executing from the current value of PC, except in External memory, or will the PC be reset to the beginning of the External Flash?

I read the Atmel and Phillips datasheets, but could not find any mention of this scenario.

I guess the easiest way to start executing from the external memory would be to force the value of PC to greater than 0FFFh (for 4 kb internal flash) Would a simple jump opcode acheive this? e.g. whenever I want to start executing from flash, I say:
ljmp 0fffh
or
ljmp 1000h
Will the next instruction be from fetched from the external Flash?

I too plan to design a similar kit, where the bootloader program resides in the internal Flash, and the executable in some external memory module. Since I don't own a programmer, and am tired of having to run to a friends house whenever I need one :-)), I think this would allow me the reprogrammability I seek.

I want to feed the executable code from the PC parallel port or serial port to the uC, which will then enter it into the external Flash/RAM/ NV-RAM. What would be the best way to achieve this? By dynamically setting the EA Pin low or by jumping to 1000h? I believe if I simply jump to external memory, the interrupt vectors will still reside in the internal memory, and my executable will somehow have to "register" its ISRs with the loader program. This might well evolve into having my own Operating System running in the internal flash!! :))

Could you also advise me as regards the hardware aspect of this? Which (Atmel)microcontroller would be most suitable. That is, what sort of external memory to use, RAM, NV-RAM or Flash? Which memory ICs would be best? I like the idea of using an IC having a separate internal bootloader memmory and program memory, but since I only have access to an Atmel programmer, I would need an equivalent Atmel microcontroller.

I could probably design such a kit on my own, but it would end up being clumsy and inefficient. I need your experience and expert advice to guide me to an elegant solution :)

Kundi