The PCIe I/O drawer slots numbers are shown in Figure 4-2. Figure 4-2 PCIe I/O drawer slots numbers The I/O structure in a z14 CPC is shown in Figure 4-3 on page 152. The PCIe switch card  provides the fanout from the high-speed x16 PCIe host bus to eight individual card slots. The  PCIe switch card is connected to the drawers through a single x16 PCIe Gen 3 bus from a  PCIe fanout card. In the PCIe I/O drawer, the eight I/O feature cards that directly attach to the switch card  constitute an I/O domain. The PCIe I/O drawer supports concurrent add and replace I/O  features to enable you to increase I/O capability as needed without having to plan ahead. Chapter 4. Central processor complex I/O system structure  151