sfr S0_TMOD               = 0x89; // timer mode            
  #define SM_TMOD_GATE1     0x80  // control T1 from int1as 8 bit auto-reload timer
  #define SM_TMOD_T1CM1     0x50  // T1 as 16 bit counter
  #define SM_TMOD_T1TM2     0x20  // T1 as 8 bit auto-reload timer
  #define SM_TMOD_T1TM1     0x10  // T1 as 16 bit timer
  #define SM_TMOD_GATE0     0x08  // control T0 from int0
  #define SM_TMOD_T0CM1     0x05  // T0 as 16 bit counter
  #define SM_TMOD_T0TM2     0x02  // T0 as 8 bit auto-reload timer
  #define SM_TMOD_T0TM1     0x01  // T0 as 16 bit timer

then the above MOV TMOD, #11010101B becomes
mov S0_TMOD, #SM_TMOD_GATE1+SM_TMOD_T1CM1+SM_TMOD_T0CM1