#if defined (__arm__) && !defined(__thumb__)
      // __arm__ may be defined in thumb mode.
      register uint32_t scno asm("r7") = __ARM_NR_cacheflush;
      asm volatile(
          "svc 0x0"
          : "=r" (beg)
          : "0" (beg), "r" (end), "r" (flg), "r" (scno));
    #else
 // so how do i call the syscall?