The IBM z13™ delivers significant new capabilities in terms of overall system capacity provided by simultaneous multithreading (SMT), and new I/O virtualization functionality for PCI Express™ (PCIe™) adapters. In this paper, we describe how the host firmware stack was enhanced to enable these new functions. For SMT, new support was added in the firmware running on the Central Processor Complex (CPC) to represent and manage the operation of more processor cores compared with the previous z Systems™ generation, with multiple threads each. The Processor Resource/Systems Manager™ (PR/SM™) hypervisor design was modified to allow exploitation of SMT under the control of the operating system running in a logical partition. The z13 supports Single Root I/O Virtualization (SR-IOV) for PCIe adapters to allow sharing of adapters among operating system images running in separate logical partitions. The virtualization intermediary firmware runs in a new firmware execution environment, the Firmware Platform Container (FPC). The IBM internal Central Electronics Complex (CEC) SIMulation (CECSIM) environment was extended to reflect the new system structure, scale, and functions, to allow early verification of the z13 firmware designs, and to ensure excellent firmware quality for an efficient hardware bring-up and system test.