Azhar,

Excuse me if i am being "dense" here.

The effort to "toggle" EA would indicate a requirement to "shadow" the low program memory area with two devices 1) and internal PROM, 2) an external PROM.

Why can't the internal PROM "jump" to the external PROM when the conditions indicate this is appropriate? Are you intending two separate interrupt vector structs? The interrupt and RESET vectors are the only fixed location entities that seem to potentially require alternating PROMs.

If this is true you can easily accomodate alternate interrupt without having to "shadow" the internal PROM.

What am i missing?

regards,
p