Well Stephane...it looks to me, by looking at the 'C664 data sheet that these parts have 1792 bytes of extra RAM that can be accessed as "on-board" XDATA memory. The instruction types used to access this memory is the MOVX type opcodes.

In order to be able to use the memory howeer it is first necessary to enable it. Out of reset the 'C664 has this memory disabled. In order to enable the 1792 bytes of this ERAM (as they call it) you need to clear the EXTRAM bit in the AUXR (Auxiliary Register SFR). This would be clearing bit #2 in the SFT at address 0x8E.

Hope this helps resolve your problem
Michael Karas