#define SPH_CONFIGURABLE_BITS_BM (INTERNAL_SRAM_END_8MSB - INTERNAL_SRAM_START_8MSB)Subtract the MSBs of the end address from the start address to find which bits are configurable in the SPH register, since some bits will always be 0 or 1 (i.e. non-configurable) depending on the SRAM location in the memory map.