On OpenVMS I64 systems, the compiler provides two sets of built-
    ins:

    o  Alpha instruction built-ins that are used to generate Itanium
       instructions for which there are no VAX equivalents.

    o  Itanium instruction built-ins that are used to access Itanium
       instructions for which there are no VAX equivalents.

    On OpenVMS I64 systems, all of the Alpha PALcode built-ins are
    emulated by means of macros provided by the system.

    Both sets of built-ins are presented in tables. The second column
    of each table specifies the operands the built-in expects, where:

       WL = write longword
       ML = modify longword
       AL = address of longword
       WQ = write quadword
       RQ = read quadword
       MQ = modify quadword
       AQ = address of quadword
       AB = address of byte
       AW = address of word
       WB = write byte
       WW = write word

                                   NOTE

       Be careful when mixing built-ins with VAX MACRO instructions
       on the same registers. The code generated by the compiler
       expects registers to contain 32-bit sign-extended values,
       but it is possible to create 64-bit register values that are
       not in this format. Subsequent longword operations on these
       registers could produce incorrect results.

       Therefore, make sure to return registers to 32-bit sign-
       extended format before using them in VAX MACRO instructions
       as source operands. Note that loading the register with
       a new value using a VAX MACRO instruction (such as MOVL)
       returns it to this format.