Нашел ответ в руководстве пользователя, а не в спецификации.
Expansion bus specifications PCI Express expansion slots Specification Slot 1 One full-height, half-length x8 PCIe Gen3 card slot connected to processor Slot 2 One full-height, half-length x16 PCIe Gen3 card slot connected to processor Slot 3 One full-height, half-length x1 PCIe Gen3 card slot connected to Platform Controller Hub (PCH) Slot 4 One full-height, half-length x8 PCIe Gen3 card slot connected to PCH