The "EXTRA" 256 bytes of RAM are accessed as XDATA using the MOVX instruction. If you have a C compiler that supports the PDATA data type, you should use that (only for those devices with 256 bytes of extra XDATA). The PDATA data type uses the MOVX @R0 and MOVX @R1 instructions (which are faster than loading DPTR for XDATA accesses).

NOTE: On the Dallas 520, the extra on-chip XDATA is not enabled after reset. You will probably want to enable it in your startup code BEFORE you clear memory and initialize variables.

Jon Ward
Keil Software