#define bMODECNFG   (* (unsigned char xdata *)0xFFFB)        
#define MODECNFG_CLKOUTEN 0x04  
#define MODECNFG_CLKSLCT 0x08   
...                                
bMODECNFG |= MODECNFG_CLKOUTEN;
bMODECNFG |= MODECNFG_CLKSLCT;
...