Increasizing RAM size in AFM

Hi, I want to increase RAM size from 2^32 bits to 2^48 bits. I made the change in RAM_Device.lisa file by changing as    

resources
    {
        PARAMETER { description("Memory Size"), default(0x1000000000000), type(uint64_t) }  size;
        PARAMETER { description("Global monitor ignores non-exclusive stores"),
                    default(false), type(bool), publish(false) }  global_monitor_ignores_non_ex_store;

        MEMORY { virtual(true), read_function(debug_read), write_function(debug_write) } ram_contents[0x100000000000000];
    }

  Still it throws fault if I access range more than 2^32. Please suggest if I am missing something.