The remaining sections describe the z14 system structure, showing a logical representation  of the data flow from PUs, caches, memory cards, and various interconnect capabilities. 3.3 CPC drawer design A z14 system can have up to four CPC drawers in a full configuration, up to 170 PUs that can  be characterized for customer use, and up to 32 TB of customer usable memory capacity.  Each CPC drawer is logically divided in two clusters to improve the processor and memory  affinity and availability.  The following types of drawer configurations are available for z14: For serviceability, the CP SCM spare is supplied as a universal field spare. The z14 has up to 20 memory controller units (MCUs) (five MCUs per CPC drawer). The  configuration uses five-channel redundant array of independent memory (RAIM) protection,  with dual inline memory modules (DIMM) bus cyclic redundancy check (CRC) error retry.  The cache hierarchy (L1, L2, L3, and L4) is implemented with embedded dynamic random  access memory (eDRAM) caches. Until recently, eDRAM was considered to be too slow for  this use. However, a breakthrough in technology that was made by IBM eliminated that  limitation. In addition, eDRAM offers higher density, less power utilization, fewer soft errors,  and better performance. Concurrent maintenance allows dynamic central processing complex  (CPAC) drawer add and repair. 2 z14 servers use CMOS Silicon-on-Insulator (SOI) 14 nm chip technology, with advanced low  latency pipeline design, which creates high-speed yet power-efficient circuit designs. The PU  SCM has a dense packaging, which allows closed water loop cooling. The heat exchange  from the closed loop is air-cooled by a radiator unit (RU) or optionally, water-cooled by a  water-cooling unit (WCU). The water-cooling option can lower the total power consumption of  the system. This benefit is significant for larger configurations. For more information, see  2.8.1, “Power and cooling” on page 77. GLYPH<SM590000> Five CP SCMs populated: Modes M01, M02, M03, and M04 GLYPH<SM590000> Six CP SCMs populated: Model M05 90  IBM z14 (3906) Technical Guide 2 For configurations with two or more CPC drawers installed