The Access Rights Block (ARB) compatibility option, the ARB_SUPPORT
    system parameter, is provided specifically to support products that
    have not yet been updated to use the per-thread security
    Persona Security Block (PSB) data structure instead of the ARB.
    Changing the value of ARB_SUPPORT from 2 or 3 (the default) to any
    other value can affect the operation of these products.

    The following table describes ARB_SUPPORT parameters:

    ARB_SUPPORT
    Parameter         Value    Behavior

    ISS$C_ARB_NONE    0        The obsolete kernel data cells are not
                               maintained by the system. Fields are
                               initialized to zero (or set to invalid
                               pointers) at process creation.

    ISS$C_ARB_CLEAR   1        The obsolete kernel data cells are
                               cleared (or set to invalid pointers)
                               when the code would have set up values
                               for backward compatibility.

    ISS$C_ARB_READ_   2        The obsolete cells are updated with
    ONLY                       corresponding security information
                               stored in the current PSB when a
                               $PERSONA_ASSUME is issued.

    ISS$C_ARB_FULL    3 (de-   Data is moved from the obsolete cells
                      fault)   to the currently active PSB on any
                               security-based operation.

    ARB_SUPPORT is a DYNAMIC parameter.