Cx51 User's Guide: xdata - Keil Embedded Development Tools for ARM, Cortex-M, Cortex-R4,
The xdata memory type may be used to declare variables only. You may not declare xdata functions. This memory is indirectly accessed using 16-bit addresses and is the external data RAM of the 8051. The amount of xdata is limited in size (to 64K or less)....