VMS Help System Services, $SETAST, Argument  *Conan The Librarian (sorry for the slow response - running on an old VAX)

 enbflg

    OpenVMS usage:boolean
    type:         byte (unsigned)
    access:       read only
    mechanism:    by value
    Value specifying whether ASTs are to be enabled. The enbflg
    argument is a byte containing this value. The value 1 enables
    AST delivery for the calling access mode; the value 0 disables
    AST delivery.