The superscalar processor implements third-generation Simultaneous Multi-Threading (SMT)$^{4}$. It also implements redesigned OoO, augmented caches and translation lookaside buffer (TLB), optimized pipeline, and better branch prediction. 

Also featured is an expanded instruction set with Vector Packed Decimal Facility, Guarded Storage Facility, Vector Facility enhancements, Semaphore Assist Facility, Order Preserving Compression, Entropy Encoding for Co-processor Compression for better performance in several different areas, and the IBM Integrated Accelerator for zEnterprise Data Compression (on-chip compression accelerator). 

Depending on the model, the z15 server can support 512 GB - 40 TB of usable memory, with up to 8 TB of usable memory per CPC drawer. In addition, a fixed amount of 256 GB is reserved for the hardware system area (HSA) and is not part of customer-purchased memory. Memory is implemented as a redundant array of independent memory (RAIM) and uses extra physical memory as spare memory. The RAIM function accounts for 20% of the physical installed memory in each CPC drawer. 

New with z15, Virtual Flash Memory (VFM) feature is offered from the main memory capacity in 0.5 TB units (versus 1.5 TB per feature on z14) increasing granularity for the feature. VFM provides much simpler management and better performance by eliminating the I/O to the adapters in the PCIe+ I/O drawers. VFM does not require any application changes when moving from IBM zFlash Express (previously available on z13fi and zEC12). 

## 1.2.2 Capacity and performance 

The z15 server provides increased processing and enhanced I/O capabilities over its predecessor, the z14 system. This capacity is achieved by increasing the performance of the individual PUs and the number of PUs per system, redesigning the system cache, increasing the amount of memory, and introducing new I/O technologies. 

The increased performance and the total system capacity available (with potential energy savings) allows consolidation of diverse applications on a single platform with significant financial savings. The introduction of new technologies and an expanded and enhanced instruction set ensure that the z15 server is a high-performance, reliable, and rich-security platform. The z15 server is designed to maximize the use of resources and allows you to integrate and consolidate applications and data across the enterprise IT infrastructure. 

z15 server is offered in one model (T01) with five maximum processor features, with 1 - 190 configurable PUs. The processor features Max34, Max71, Max108, and Max145 have one, two, three, respective four CPC drawers with 41 active PUs per CPC drawer. The high-capacity feature (Max190) has five processor (CPC) drawers with 43 PUs per drawer. 

The z15 T01 feature Max190 is estimated to provide up to 25% more total system capacity than the z14 Model M05, with the same amount of memory and power requirements. With up to 40 TB of main storage and enhanced SMT, the performance of the z15 processors deliver considerable improvement. Uniprocessor performance also increased significantly. A z15 Model 701 offers average performance improvements of up to 14%$^{5}$ over the z14 Model 701. 

The Integrated Facility for Linux (IFL) and IBM Z Integrated Information Processor (zIIP) processor units on the z15 server can be configured to run two simultaneous threads per clock cycle in a single processor (SMT). This feature increases the capacity of these processors with 25% in average$^{5}$ over processors that are running single thread. SMT is also enabled by default on System Assist Processors (SAPs). 

