#define RCC_BASE    0x40021000
#define RCC_IOPENR  *(volatile uint32_t*)(RCC_BASE + 0x34)

#define GPIOA_BASE   0x50000000
#define GPIOA_MODER  (GPIOA_BASE + 0x00)
#define GPIOA_ODR    (GPIOA_BASE + 0x14)

#define PA15 15

#define register_value(x) *(volatile uint32_t*)(x)

int main()
{
  RCC_IOPENR |= 0x1;

  register_value(GPIOA_MODER) &= ~(0x3 << (PA15 * 2));
  register_value(GPIOA_MODER) |= (0x1 << (PA15 * 2));

  register_value(GPIOA_ODR) |= (0x1 << PA15);

  return 0;
}