#include <stdint.h>
#include <stdbool.h>

#define RCC_AHB1ENR_BIT_GPIODEN *(volatile uint32_t*)(0x42000000 + 0x00023830 * 32 + 3 * 4)
#define GPIOD_BASE 0x40020C00
#define GPIOD_MODER *(volatile uint32_t*)(GPIOD_BASE + 0x00)
#define GPIOD_ODR   *(volatile uint32_t*)(GPIOD_BASE + 0x14)

#define STK_CTRL *(volatile uint32_t*)(0xE000E010)
#define STK_LOAD *(volatile uint32_t*)(0xE000E014)

volatile bool flag = false;

void SysTick_Handler()
{
    flag = true;
}

int main(void)
{
    // GPIO Port D Clock Enable
    RCC_AHB1ENR_BIT_GPIODEN = 1;
    // GPIO Port D Pin 15 down to 12 Push/Pull Output
    GPIOD_MODER = 0x55000000;
    // Set green and red LEDs
    GPIOD_ODR = 0x5000;

    // SysTick enable with interupt and clk source to AHB/8
	STK_CTRL = (1<<1) | 1;
    STK_LOAD = 1000000; // 0.5 sec / (16 MHz / 8)
    // Do forever:
    while (1)
    {
        // Wait a moment
        while (!flag)
        {
            __asm__("    WFI"); // sleep until SysTick
        }
        flag = false;
        // Flip all LEDs
        GPIOD_ODR ^= 0xF000;
    }
}