How to use high and low bytes?
In hexadecimal, your number is 0x8000 which is 0x80 and 0x00. To get the low byte from the input, use low=input & 0xff and to get the high byte, use high=(input>>8) & 0xff.
Get the input back from the low and high byes like so: input=low | (high<<8).
Make sure the integer types you use are big enough to store these numbers. On 16-bit systems, unsigned int/short or signed/unsigned long should be be large enough.