If we're speaking of a C51 compiler from Keil, then you have missplaced the variable. Memory space, that you have defined is internal RAM and this address is occupied by register R0 if memory bank 0 is used.

Your declaration would be:
unsigned char xdata thr _at_ 0x00 ;//UART BASE_ADDRESS +0

NOTE: thr is a read/write variable if put in xdata memory space. Assembly instructions used to access it would be: MOVX @DPTR,A and MOVX A,@DPTR

Regards Franc