In SMRT Option 2 (below the line storage) you can specify 'System Storage'.
What is it used for? Who owns this storage?
With 'System Storage' you define to TPX how many bytes TPX should leave for the operating system below the line to satisfy GETMAIN requests from e.g. VSAM, the security package and others.
This storage is not owned by TPX. Pieces of it can be requested from the operating system by programs using GETMAIN. If the storage is exhausted you will experience S80A, S878 abends.
The TPX region has REGION=0M defined, no storage limitations.
In the start phase TPX issues a conditional GETMAIN asking the operating system how many storage is available below the line.
Assume z/OS returns: I have 8.5 MB available below.
If TPX would do a GETMAIN now for 8.5 Megabytes there would be no storage left.
Any GETMAIN from e.g. operating system components would fail with S80A, S878 abends.
TPX would not come up.
So TPX checks the 'System Storage' value and subtracts this from the 8.5 MB available.
If 'System Storage' is set to 2048K TPX does a GETMAIN only for 6.5 Megabytes for its modules, Slots and DSA below.
The 2 MB left remains under the control of the operating system to satisfy GETMAIN requests below the line.
TPX itself does not do any explicate GETMAIN here except you issue VTADD commands. For every VT added with VTADD TPX will request round about 150 bytes below through GETMAIN.
Starting with TPX 5.1 we strongly recommend to set 'System Storage' to 2048K.
If you are using VTADD adjust 'System Storage' based on the number of VTs you add.