# ACPI SSDT _UID generation

According to the ACPI spec:

> The _UID must be unique across all devices with either a common _HID or _CID.

When generating SSDTs in coreboot the independent drivers don’t know which _UID is already in use for a specific _HID or _CID. To generate unique _UIDs the ACPI device’s path is hashed and used as ID. As every ACPI device has a different path, the hash will be also different for every device.

Windows 10 verifies all devices with the same _HID or _CID and makes sure that no _UID is duplicated. If it is it’ll BSOD.