The Raspberry PI has a number of peripheral devices that are accessible over MMIO. To demonstrate how MMIO works, our application will access the system timer:

1. In your working ~/test directory, create a subdirectory called timer.
2. Use your favorite text editor to create a file named timer.cpp in the timer subdirectory.
3. Put the required headers, constants, and declarations of types into timer.cpp: 

#include <iostream>
#include <chrono>
#include <system_error>
#include <thread>

#include <fcntl.h>
#include <sys/mman.h>


constexpr uint32_t kTimerBase = 0x3F003000;

struct SystemTimer {
  uint32_t CS;
  uint32_t counter_lo;
  uint32_t counter_hi;
};

4. Add the main function, which contains all the logic of the program:

int main() {

  int memfd = open("/dev/mem", O_RDWR | O_SYNC);
  if (memfd < 0) {
  throw std::system_error(errno, std::generic_category(),
  "Failed to open /dev/mem. Make sure you run as root.");
  }

  SystemTimer *timer = (SystemTimer*)mmap(NULL, sizeof(SystemTimer),
  PROT_READ|PROT_WRITE, MAP_SHARED,
  memfd, kTimerBase);
  if (timer == MAP_FAILED) {
  throw std::system_error(errno, std::generic_category(),
  "Memory mapping failed");
  }

  uint64_t prev = 0;
  for (int i = 0; i < 10; i++) {
   uint64_t time = ((uint64_t)timer->counter_hi << 32) + timer->counter_lo;
   std::cout << "System timer: " << time;
   if (i > 0) {
   std::cout << ", diff " << time - prev;
    }
    prev = time;
    std::cout << std::endl;
    std::this_thread::sleep_for(std::chrono::milliseconds(10));
  }
  return 0;
 }

5. Create a file called CMakeLists.txt in the timer subdirectory with the following content:

cmake_minimum_required(VERSION 3.5.1)
project(timer)
add_executable(timer timer.cpp)

set(CMAKE_SYSTEM_NAME Linux)
set(CMAKE_SYSTEM_PROCESSOR arm)

SET(CMAKE_CXX_FLAGS "--std=c++11")

set(CMAKE_CXX_COMPILER /usr/bin/arm-linux-gnueabi-g++)

6. You can now build and run the application.