OK.

I understand your points.
But I need to download the application by MODEM.
I mean by remote download over a PSTN line (telephone line).
My costumers ask me for it and I need to continue having a low cost product. That's a maket problem.

Yes, you are right! I am trying to make a simple uC look like a real uComputer.

The "bios" can be update in factory, by serial using ISP.
Sure it will be easier if "bios" part don't change.
I don't intent to change the "bios", but it will be good to think about this possibility.

Another issue: putting more things in "Bios", I can make smaller applications, so the download time is smaller too.The modem speed is just 2400 bps.

In fact, my product is a simple POS (similar credit card machines), but with a very low cost.

Annibal