mcharg

    OpenVMS usage:mechanism array
    type:         vector quadword (unsigned)
    access:       read only
    mechanism:    by 32- or 64-bit reference, array reference
    The mechanism array. The mcharg argument is the 32- or 64-bit
    address of this array, which was passed to the condition handler.
    $SIGNAL_ARRAY_64 uses this structure to determine the 64-bit
    signal array address.

 sigarg_64

    OpenVMS usage:64-bit signal array
    type:         vector quadword (unsigned)
    access:       write only
    mechanism:    by 32- or 64-bit reference, array reference
    The 32- or 64-bit address of the 64-bit signal array is returned
    in this argument.