3.6.3 Hardware system area 3.6.4 Virtual Flash Memory (FC 0643) Main storage can be accessed by all processors, but cannot be shared between LPARs. Any  system image (LPAR) must include a defined main storage size. This defined main storage is  allocated exclusively to the LPAR during partition activation. The HSA is a non-addressable storage area that contains system LIC and  configuration-dependent control blocks. On z15 servers, the HSA has a fixed size of  256 GB and is not part of the purchased memory that you order and install. The fixed size of the HSA eliminates planning for future expansion of the HSA because the  hardware configuration definition (HCD)/input/output configuration program (IOCP) always  reserves space for the following items: The HSA features sufficient reserved space to allow for dynamic I/O reconfiguration changes  to the maximum capability of the processor. IBM Virtual Flash Memory (VFM, FC 0643) is the replacement for the Flash Express features  that were available on the IBM zEC12 and IBM z13. No application changes are required to  change from IBM Flash Express to VFM. IBM VFM provides up to 6.0 TB of virtual flash memory in 512 GB increments. The minimum  is 0, while the maximum is 12 features. The number of VFM features ordered reduces the  maximum orderable memory for the z15 server. GLYPH<SM590000> Communication with and control of optional expanded storage GLYPH<SM590000> Error checking and correction GLYPH<SM590000> Six channel subsystems (CSSs) GLYPH<SM590000> A total of 15 LPARs in CSSs 1 through 5, and 10 LPARs for the sixth CSS for a total of 85  LPARs GLYPH<SM590000> Subchannel set 0 with 63.75-K devices in each CSS GLYPH<SM590000> Subchannel set 1 with 64-K devices in each CSS GLYPH<SM590000> Subchannel set 2 with 64-K devices in each CSS GLYPH<SM590000> Subchannel set 3 with 64-K devices in each CSS Chapter 3. Central processor complex design  131