Hi Chris,

See Atmel's documentation, AT89 Series Hardware Description: Accessing External Memory.

If EA/ is tied to ground, the uC will fetch code data *only* from external memory. If EA/ is tied high, it will use internal prom/flash from address 0 to the limits of the chip, and then start accessing external code via PSEN/, with ALE and P2|P0 providing the address. If your code doesn't exceed the internal limits, no external accesses will be required.

Another way of looking at it is to say that EA/ controls whether or not the internal code space is used.

Make sure you read the data sheet for your particular variant...some, like Atmel's 89C2051, don't have the hardware implemented to access external code.

Your ideas of partitioning your code internal and external should work fine.

Dennis