VMS Help MACRO, /MIGRATION, Qualifiers, /PRESERVE  *Conan The Librarian (sorry for the slow response - running on an old VAX)

       /PRESERVE[=(option[,...])]
       /NOPRESERVE (default)

    Directs the compiler to generate special OpenVMS Alpha assembly
    code throughout a module for all VAX MACRO instructions that
    rely on VAX guarantees of operation atomicity or granularity. The
    options are:

    Option         Description

    GRANULARITY    Preserves the rules of VAX granularity of writes.
                   Specifying /PRESERVE=GRANULARITY causes the
                   compiler to use Alpha Load-locked and Store-
                   conditional instruction sequences in code it
                   generates for VAX instructions that perform byte,
                   word, or unaligned longword writes.
    ATOMICITY      Preserves atomicity of VAX modify operations.
                   Specifying /PRESERVE=ATOMICITY causes the compiler
                   to use Load...Locked and Store...Conditional
                   instruction sequences in code it generates for
                   instructions with modify operands.

    /PRESERVE and /PRESERVE=(GRANULARITY,ATOMICITY) are equivalent.
    When preservation of both granularity and atomicity is enabled,
    and the compiler encounters a VAX coding construct that requires
    both granularity and atomicity guarantees, it enforces atomicity
    over granularity.

    If you are aware of specific sections of VAX MACRO code that
    require VAX granularity and atomicity guarantees, you can forego
    compiler enforcement of these guarantees for the entire module
    and use the .PRESERVE and .NOPRESERVE directives to indicate
    those sections. Therefore, if you can isolate that code where
    granularity and atomicity must apply, these directives allow you
    to optimize the generated code by preventing the compiler from
    generating expanded Alpha code unnecessarily.

    Atomicity is guaranteed on multiprocessing systems as well as
    uniprocessing systems when you specify /PRESERVE=ATOMICITY.

    When the /PRESERVE qualifier is present, you can control the
    number of times compiler-generated code retries a granular or
    atomic update by specifying the /RETRY_COUNT qualifier.

                                 WARNING

       If /PRESERVE=ATOMICITY is turned on, any unaligned data
       references will result in a fatal reserved operand fault.
       If /PRESERVE=GRANULARITY is turned on, unaligned word
       references to addresses assumed aligned will also cause a
       fatal reserved operand fault.