extern "C" {
  #define RCC_BASE 0x40021000
  #define GPIOA_BASE 0x50000000
  #define TIM1_BASE 0x40012C00

  #define RCC_IOPENR (*(volatile unsigned int *)(RCC_BASE + 0x34))
  #define RCC_APBNR2 (*(volatile unsigned int *)(RCC_BASE + 0x40)) // for the timer pin

  #define GPIOA_MODER (*(volatile unsigned int *)(GPIOA_BASE + 0x00)) // pinmode for general purpose pins
  #define GPIOA_AFRH (*(volatile unsigned int *)(GPIOA_BASE + 0x24)) // pinmode for advanced pins

  #define TIM1_CR1 (*(volatile unsigned int *)(TIM1_BASE + 0x00)) // enables the counter when switched on
  #define TIM1_CCMR1 (*(volatile unsigned int *)(TIM1_BASE + 0x18)) // enables output + pwm mode
  // also enables shadow mode
  #define TIM1_CCER (*(volatile unsigned int *)(TIM1_BASE + 0x20)) // flips the gateway switch between the 
  // pwm modulation and the physical external pins
  #define TIM1_PSC (*(volatile unsigned int *)(TIM1_BASE + 0x28)) // determines the frequency
  #define TIM1_ARR (*(volatile unsigned int *)(TIM1_BASE + 0x2C)) // determines the counter's ceiling

  #define TIM1_CCR1 (*(volatile unsigned int *)(TIM1_BASE + 0x34)) // stores the duty cycle

  #define TIM1_BDTR1 (*(volatile unsigned int *)(TIM1_BASE + 0x44)) // master safety switch

  void ms_delay(int count) {
    while (count--) {
      for (volatile int i = 0; i < 1500; i++) {
        __asm("nop");
      }
    }
  }

  int main(void) {
    RCC_IOPENR = (1 << 0); // flips the peripheral clock enable on
    RCC_APBNR2 = (1 << 11); // do the same for the advanced clock

    GPIOA_MODER &= ~(3 << (8 * 2)); // clear the pin mode pin
    GPIOA_MODER |= (2 << (8 * 2));
    // AFRH refers to four bits
    GPIOA_AFRH &= ~(0xF << ((8 - 8) * 4)); // clear the bits
    GPIOA_AFRH |= (2 << ((8 - 8) * 4)); // hooks the timer pin PA8 to the output of timer

    TIM1_PSC = 12 - 1;
    TIM1_ARR = 100 - 1;

    TIM1_CCMR1 &= ~(7 << 4); // clear the first four bits to set the mode
    TIM1_CCMR1 |= (6 << 4) | (1 << 3);
    // output mode, pwm generation

    TIM1_CCER |= (1 << 0); // flips the gateway switch between the pwm modulation and physical external pins

    TIM1_BDTR1 |= (1 << 0);

    TIM1_CR1 |= (1 << 0); // enable timer

    while (1) {
      for (int duty = 0; duty <= 99; duty++) {
        TIM1_CCR1 = duty;
        ms_delay(500);
      }
    } 

  }

}