I used the file: L51_BANK.A51, and configurated it as following:
;************************ Configuration Section ******************************* ?B_NBANKS EQU 8 ; Define maximum Number of Banks * ; ; following values are allowed: 2, 4, 8, 16, 32, 64 * ; ; for BL51 the maximum value for ?B_BANKS is 32 * ; ; for LX51 the maximum value for ?B_BANKS is 64 * ; * ?B_MODE EQU 1 ; 0 for Bank-Switching via 8051 Port * ; ; 1 for Bank-Switching via XDATA Port * ; ; 4 for user-provided bank switch code * ; * ?B_RTX EQU 0 ; 0 for applications without real-time OS * ; ; 1 for applications using the RTX-51 real-time OS * ; * ?B_VAR_BANKING EQU 1 ; Variable Banking via L51_BANK (far memory support)* ; ; 0 Variable Banking does not use L51_BANK.A51 * ; ; 1 Variable Banking uses this L51_BANK.A51 module *