Fixed point to Floating point

Question

I have the following code, I have just copied some data from external RAM to the MCU into a buffer called "data"

double p32        = 4.294967296e+009;       /// equals to 2^32 in decimal notation
int32_t      longhigh;
uint32_t     longlow;

offset = mapdata();  //Points to the data I want, 55 bit fixed point on HW
longhigh = data[2*offset+1]; //Gets upperpart of data 
longlow =  data[2*offset]; //Gets lower part  
double floating = (longhigh*p32 + longlow); // What is this doing?  How does it work?

Can someone explain that last line of code for me? Why are we multiplying by p32? Thanks.

Answer 1

Multiplying by p32 is equivalent to a left shift by 32 bits. It also results in a type conversion for the product (from int to double), as well as for the sum. This way you can essentially keep 64-bit ints in the buffer and convert them to doubles when required.