PBP 16-bit ADC result math

hi, everyone!
I am using an external 16 bit ADC interfacing with PIC16F877 by SPI.
The problem is HOW TO DO a math with 16 bit result that i`m reading from ADC.
For now I have just 12 bit result math and here is the code:

B0 VAR BYTE
B1 VAR BYTE
W0 VAR WORD

ADval_MSB VAR BYTE
ADval_LSB VAR BYTE

B0 = 0
B1 = 0
W0 = 0

B0 = ADval_MSB & $F0
B1 = ADval_MSB & $0F

ADval_MSB = B0 >> 4
B1 = B1 << 4

ADVal_LSB = ADval_LSB >> 4
ADval_LSB = B1 | ADval_LSB

W0 = (ADval_MSB * 256) + ADval_LSB ' 12 bit result of conversion.

Thanx A Lot!