The Question is:
VAX - How do VBSS (virtual balance set slots) work?
The Answer is :
VBSS works quite nicely.
At its simplest, VBSS permits more balance set slots
than work normally fit into the available S0 and S1
system address space -- VAX systems are constrained
by the capacity of system address space. Effectively,
VBSS is a way to virtualize the process data structures.
OpenVMS Alpha solves this through the use of 64-bit
(S2) space, and particularly moving the larger data
structures from S0 and S1 space into S2 space and
(in the case of the page tables) into page table
For technical and internal details, please see the
OpenVMS Internals and Data Structures Manuals.