For some device groups, such as RL78/F24, there is a special register called RAMSAR (RAM Start Setting Register) which defines the start of the RAM area. You can use the file cstartup.s to set up the RAMSAR register dynamically via the linker-defined symbol _RAMSAR_ADDR, defined in the linker configuration file (.icf). By default, this symbol is set to enable the whole RAM area.