// tell the underlying compiler to use raw standard C
extern "C" {
  // create a symbolic alias to point to the RCC_BASE register
  // Reset and Clock Control block
  #define RCC_BASE 0x40021000

  // create a symbolic alias to point to the GPIOA_BASE register
  // access to general purpose input/output pins and port A
  #define GPIOA_BASE 0x50000000

  // create a symbolic alias to point to the RCC_IOPENR register
  // access to the input/output peripheral clock enable register
  // bit 0 enables the clock for port A
  #define RCC_IOPENR (*(volatile unsigned int *)(RCC_BASE + 0x34))

  // create a symbolic alias to point to the GPIOA_MODER register
  // basically pinMode
  // bits 10 and 11 refer to PA5 (the onboard LED)
  #define GPIOA_MODER (*(volatile unsigned int *)(GPIOA_BASE + 0x00))

  // create a symbolic alias to point to the GPIOA_ODR register
  // basically digitalWrite
  // bit 5 refers to PA5 (the onboard LED)
  #define GPIOA_ODR (*(volatile unsigned int *)(GPIOA_BASE + 0x14))

  // create a symbolic alias to point to the GPIOA_PUPDR register
  // enables the pull-up register for the pushbutton
  #define GPIOA_PUPDR (*(volatile unsigned int *)(GPIOA_BASE + 0x0C))

  // create a symbolic alias to point to the GPIOA_IDR register
  // used to read the state of pins
  #define GPIOA_IDR (*(volatile unsigned int *)(GPIOA_BASE + 0x10))

  // entry point for the program
  int main(void) {
    // peripheral clock enable for port A (bit 0)
    RCC_IOPENR |= (1 << 0);

    // turn the pinMode for the LED to output
    GPIOA_MODER &= ~(3 << (5 * 2));
    GPIOA_MODER |= (1 << (5 * 2));

    // turn the pinMode for the button to input
    GPIOA_MODER &= ~(3 << (0 * 2));

    // configure PA0 with an internal pull-up resistor
    GPIOA_PUPDR &= ~(3 << (0 * 2));
    GPIOA_PUPDR |= (1 << (0 * 2)); // 10 for pull-up resistor

    // in a loop, read the button's pin (0)
    while (1) {
      // mask everything except bit 0 (which reads the state of the pin)
      if ((GPIOA_IDR & (1 << 0)) == 0) {
        // if 0, then it's connected to ground
        // now turn the LED on
        GPIOA_ODR |= (1 << 5);
      } else {
        GPIOA_ODR &= ~(1 << 5); // otherwise turn the LED off
      }
    }
  }
}