efn
    OpenVMS usage:ef_number
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Event flag that $QIO sets when the I/O operation completes. The
    efn argument is a longword value containing the number of the
    event flag; however, $QIO uses only the low-order byte.
    If efn is not specified, event flag 0 is set.
    The specified event flag is set if the service terminates without
    queuing an I/O request.
 chan
    OpenVMS usage:channel
    type:         word (unsigned)
    access:       read only
    mechanism:    by value
    I/O channel that is assigned to the device to which the request
    is directed. The chan argument is a word value containing the
    number of the I/O channel.
 func
    OpenVMS usage:function_code
    type:         longword (unsigned)
    access:       read only
    mechanism:    by value
    Function codes and function modifiers specifying the operation
    to be performed. The func argument is a longword containing the
    function code.
    For information about the network pseudodevice and TELNET device
    function codes and modifiers, see Network Pseudodevice Driver
    Functions and TELNET Port Driver I/O Function Codes.
 iosb
    OpenVMS usage:io_status_block
    type:         quadword (unsigned)
    access:       write only
    mechanism:    (Alpha and I64) by 32-bit reference or 64-bit
                  reference
                  (VAX) by 32-bit reference
    I/O status block to receive the final completion status of the
    I/O operation. The iosb is the address of the quadword I/O status
    block.
    When the $QIO begins executing, it clears the event flag. The
    $QIO also clears the quadword I/O status block if the iosb
    argument is specified.
    Although the iosb argument is optional, HP strongly recommends
    that you specify it, for the following reasons:
    o  If you are using an event flag to signal the completion of
       the service, you can test the I/O status block for a condition
       value to be sure that the event flag was not set by an event
       other than service completion.
    o  If you are using the $SYNCH service to synchronize completion
       of the service, the I/O status block is a required argument
       for $SYNCH.
    o  The condition value returned in R0 and the condition value
       returned in the I/O status block provide information about
       different aspects of the call to the $QIO service. The
       condition value returned in R0 provides information about the
       success or failure of the service call itself; the condition
       values returned in the I/O status block give information on
       the success or failure of the service operation. Therefore, to
       determine the success or failure of the $QIO call, check the
       condition values returned in both the R0 and the I/O status
       block.
 astadr
    OpenVMS usage:ast_procedure
    type:         procedure value
    access:       call without stack unwinding
    mechanism:    (Alpha and I64) by 32- or 64-bit reference
                  (VAX) by 32-bit reference
    AST service routine to be executed when the I/O completes. The
    astadr argument is the address of the AST routine.
    The AST routine executes at the access mode of the caller of
    $QIO.
 astprm
    OpenVMS usage:user_arg
    type:         quadword unsigned (Alpha and I64); longword
                  unsigned (VAX)
    access:       read only
    mechanism:    (Alpha and I64) by 32- or 64-bit value
                  (VAX) by 32-bit value
    AST parameter to be passed to the AST service routine. On
    Alpha and I64 systems, the astprm argument is a quadword value
    containing the AST parameter. On VAX systems, the astprm argument
    is a longword value containing the AST parameter.
 p1 to p6
    OpenVMS usage:varying_arg
    type:         quadword unsigned (Alpha and I64); longword
                  unsigned (VAX)
    access:       read only
    mechanism:    (Alpha and I64) by 32- or 64-bit reference or by
                  64-bit value depending on the I/O function
                  (VAX) by 32-bit reference or by 32-bit value
                  depending on the I/O function
    Optional device- and function-specific I/O request arguments. The
    parameter values contained in these arguments vary according to
    the function for which they are used. See Network Pseudodevice
    Driver Functions for descriptions of the network pseudodevice
    driver I/O function codes.