int nvme_init(char * rsdp)
{
    ...

    /* get and store ECAM base address and starting and ending pcie bus number */
    pcie_ecam = (uint64_t *) *(mcfg + 44);
    start_bus_num = (uint8_t) *((char *) mcfg + 44 + 10);
    end_bus_num = (uint8_t) *((char *) mcfg + 44 + 11);

    /* enumerate pcie bus */
    printk("@@@debug\n");
    check_all_buses(start_bus_num, end_bus_num);
    printk("@@@debug\n");

    return 0;
}

uint64_t get_vendor_id(uint8_t bus, uint8_t start, uint8_t device, uint8_t function)
{
    uint64_t *phy_addr;

    phy_addr = pcie_ecam + ((bus - start) << 20 | device << 15 | function << 12);

    return *phy_addr; /* hangs here */
}

void check_device(uint8_t bus, uint8_t start, uint8_t device)
{
    uint8_t function = 0;
    uint16_t vendor_id;

    vendor_id = (uint16_t) get_vendor_id(bus, start, device, function);
    printk("@@@vendor_id={d}\n", (int) vendor_id);
    if (vendor_id == 0xFFFF)    /* device doesn't exist */
        return;

    printk("device #{d} exists!\n", (int) device);

    /* TODO: check for class and subclass */
}

void check_all_buses(uint8_t start, uint8_t end)
{
     uint8_t bus;
     uint8_t device;

     if(end == -1)
         end = start;

     for (bus = start; bus <= end; bus++) {
         for (device = 0; device < 32; device++) {
             check_device(bus, start, device);
         }
     }
}