Memory Models

I have a small test program which continuously sends characters (ascii table) to the serial ports (ST uPSD3233). The program runs fine when the memory model is:

Memory Model: small: variables in DATA
Code ROM Size: Large: 64K program

But I get garbage when I change from "Memory Model: small: variables in DATA" to "Large: variables in XDATA".

Where should I be looking to fix the problem?

Thanks.

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