efn

    OpenVMS usage:integer
    type:         longword bit mask (unsigned)
    access:       read
    mechanism:    by value
    Number of the event flag to be set when the IO_FASTPATH(W)
    operation completes. The efn argument is a longword containing
    the number of the event flag.

 cpu_mask

    OpenVMS usage:bitmap
    type:         quadword bitmap
    access:       read
    mechanism:    by 32- or 64-bit reference
    The cpu_mask argument specifies a set of CPUs to be operated
    upon.

 function_code

    OpenVMS usage:integer
    type:         longword (unsigned)
    access:       read
    mechanism:    by value
    The function_code specifies the operation to be performed. Note
    that there is currently only one function code:

    FP$K_BALANCE_PORTS - Distribute Fast Path ports across CPUs.

 mask_length

    OpenVMS usage:integer
    type:         longword length
    access:       read
    mechanism:    by value
    The mask_length specifies the length of the cpu_mask bitmap in
    bytes. If the mask_length is not supplied or is specified as
    zero, a length of 4 bytes is used.