SPL_CONFIGURABLE_BITS_BM

#define SPL_CONFIGURABLE_BITS_BM (INTERNAL_SRAM_END_8LSB - INTERNAL_SRAM_START_8LSB)

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