Yep. The MOVX @R0/@R1 instrucion is not consistent across the 8051 devices. Sometimes it accesses the 16-bit XDATA address calculated by P2 << 8 + R0/R1. Sometimes, there is a special register (other than P2) that is used. Sometimes, it ALWAYS accesses a particular page and P2 is not used at all. And, sometimes there are multiple ways it can work depending on the setting of a configuration bit.

Intel called memory addressed this way PDATA. It is the third best memory space on the 8051 (after directly addressable DATA and indirectly addressable DATA).

Just make sure that if you use PDATA you tell your compiler/assembler where the PDATA lives in the XDATA memory space so that relocatable XDATA variables are not placed on top of the PDATA variable space.

Jon Ward
Keil Software