Have you read the descriptions of DATA, IDATA, PDATA, and XDATA in the Keil C51 Manual?

See also the 8052 Tutorial on this very site:
http://www.8052.com/tut8052.phtml

PDATA is a 256-byte page of XDATA (in C51, it defaults to the first 256-byte page of XDATA).

The compiler generates MOVX @Rn instructions to access PDATA.

See the Addressing Modes section of the 8051 tutorial:
http://www.8052.com/tutaddr.p...20Indirect

Note that DATA accesses are quickest, and use least code; XDATA accesses are slowest, and use most code.
You can make a significant difference to your C51 code size & speed by careful attention to your memory space usage...