Modbus registers

I have a Master meter octave with a modbus module. I'm using a mirco820.
The MB register (net signed volume) int32 [long AB CD represention, 2 registers]. Need to convert the two register into one register using the Mirco820?? Any ideas. Thanks

Copy both incoming registers to DINT's. Multiply the high word by 2^16, this will shift the bits to the left, then add to the low word.