Correct sizing of the TLB is subject to complex statistical modeling to find the optimal tradeoff between size and performance. 

## 3.6.2 Main storage 

Main storage is addressable by programs, and storage that is not directly addressable by programs. Non-addressable storage includes the hardware system area (HSA). 

Main storage provides the following functions: 

GLYPH<SM590000> Data storage and retrieval for PUs and I/O 

GLYPH<SM590000> Communication with PUs and I/O 

GLYPH<SM590000> Communication with and control of optional expanded storage 

GLYPH<SM590000> Error checking and correction 

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. 

## 3.6.3 Hardware system area 

The HSA is a non-addressable storage area that contains system LIC and configuration-dependent control blocks. On z14 servers, the HSA has a fixed size of 192 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: 

GLYPH<SM590000> Six channel subsystems (CSSs) 

GLYPH<SM590000> A total of 15 LPARs in the 5 CSSs 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 

The HSA has sufficient reserved space to allow for dynamic I/O reconfiguration changes to the maximum capability of the processor. 

## 3.6.4 Virtual Flash Memory 

IBM Virtual Flash Memory (VFM, FC 0604) 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. 

