void setup() {
  GPIOA->MODER = (GPIOA->MODER & ~0xFFFF) | 0x5555;
  GPIOB->MODER &= ~0xFFFF;
  GPIOB->PUPDR = 0xAAAA;
}

void loop() {
  GPIOA->ODR = GPIOB->IDR & 0xFF;
}