Download the modbus spec from www.modbus.org . They give two examples: a bit shift which is small on code, slow on execution and the table driven solution that is larger in code but much faster in execution. I would suggest the table driven approach as it is significantly faster - especially on a 12 clock 8051. I did some tests the other day - 65mS vs 15mS for calculating the crc on a 40 register packet.