write-back

// setup mtrr
    int fd_mtrr;
    struct mtrr_sentry sentry;
    sentry.base = 0x06b100000;
    sentry.size = 0x40000;
    sentry.type = 6; // "write-back"
    if ( ( fd_mtrr = open ("/proc/mtrr", O_WRONLY, 0) ) == -1 ){
        if (errno == ENOENT){
            fputs ("/proc/mtrr not found: not supported or you don't have a PPro?\n", stderr);
            exit (3);
        }
        fprintf (stderr, "Error opening /proc/mtrr\t%s\n", ERRSTRING);
        exit (4);
    }
    if (ioctl (fd_mtrr, MTRRIOC_ADD_ENTRY, &sentry) == -1){
        fprintf (stderr, "Error doing ioctl(2) on /dev/mtrr\t%s\n", ERRSTRING);
        exit (5);
    }
    fprintf (stderr, "MTRR created \n");

    // mmap structure addres with mtrr
    int mm, *map;
    mm = open("/dev/mem", O_RDWR);
    map =  mmap(&env, sizeof(env), PROT_READ | PROT_WRITE, MAP_PRIVATE, mm, sentry.base);
    if(map == MAP_FAILED){
        printf("Mapping Failed\n");
        return 1;
    }
    fprintf (stderr, "mapping MTRR with env is done \n");