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

    Directs the compiler to generate special Alpha assembly code
    for VAX MACRO instructions, within portions of the source
    module, that rely on VAX guarantees of operation atomicity or
    granularity.

    Format

      .[NO]PRESERVE  argument-list

1 - Parameters

 argument-list

    One or more of the symbolic arguments listed in the following
    table:

    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.

2 - Description

    The .PRESERVE and .NOPRESERVE directives cause the compiler to
    generate special Alpha assembly code for VAX MACRO instructions,
    within portions of the source module, that rely on VAX guarantees
    of operation atomicity or granularity.

    Use of .PRESERVE or .NOPRESERVE without specifying GRANULARITY
    or ATOMICITY will affect both options. 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.

    Alternatively, you can use the /PRESERVE and /NOPRESERVE compiler
    qualifiers to affect the atomicity and granularity in generated
    code throughout an entire MACRO source module.

    Atomicity is guaranteed for multiprocessing systems as well as
    uniprocessing systems when you specify .PRESERVE ATOMICITY.

    When the .PRESERVE directive is present, you can use the /RETRY_
    COUNT qualifier on the command line to control the number of
    times the compiler-generated code retries a granular or atomic
    update.

                                 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.

3 - Example

  INCW 1(R0)

      This instruction, when compiled with .PRESERVE GRANULARITY,
      retries the insertion of the new word value, if it is
      interrupted. However, when compiled with .PRESERVE ATOMICITY,
      it will also refetch the initial value and increment it, if
      interrupted. If both options are specified, it will do the
      latter.