What's different between LD and LDG (load from generic memory vs. load from global memory)

[allanmac]

The difference between generic and non-generic addressing is:

If no state space is given, perform the load using generic addressing. In generic addressing, an address maps to global memory unless it falls within a window for const, local, or shared memory. Within these windows, an address maps to the corresponding location in const, local, or shared memory, i.e., to the address formed by subtracting the window base from the generic address to form the offset in the implied state space.

Take a look at the isspacep and cvta instructions in the PTX manual.