Register Usage Conventions for OpenVMS Alpha lists the register
    usage conventions for OpenVMS Alpha. MACRO-64 recognizes FP and
    SP as register synonyms, but does not recognize AI, RA, or PV as
    register synonyms.

    Table 11 Register Usage Conventions for OpenVMS Alpha

    R0             Int func ret value
    R1             Scratch
    R2-R15         Saved
    R16-           Argument
    R21
    R22-           Scratch
    R24
    R25     AI     Argument information
    R26     RA     Return address
    R27     PV     Procedure value
    R28            Volatile scratch
    R29     FP     Stack frame base
    R30     SP     Stack pointer
    R31            Zero
    F0             F-P function ret value
    F1             F-P complex func ret value
    F2-F9          Saved
    F10-           Scratch
    F15
    F16-           Argument
    F22
    F23-           Scratch
    F30
    F31            Zero