func

    OpenVMS usage:function_code
    type:         longword
    access:       read only
    mechanism:    by value
    I/O function code. Must be one of the following:

    o  IO$_READVBLK

    o  IO$_WRITEVBLK

    o  IO$_READLBLK

    o  IO$_WRITELBLK

    Various function modifiers are supported, depending on the
    device and driver. Disk drivers support IO$M_NOVCACHE and
    IO$M_DATACHECK. Some tape devices support IO$M_REVERSE. Illegal
    modifiers are detected by the $IO_PERFORM(W) service.

 bufobj

    OpenVMS usage:buffer object
    type:         quadword (unsigned)
    access:       read only
    mechanism:    by 32- or 64-bit reference
    Handle describing the buffer object that contains the user's
    buffer. This identifier cannot be 0.

 iosobj

    OpenVMS usage:object handle
    type:         vector longword (unsigned)
    access:       read only
    mechanism:    by 32- or 64-bit reference
    Buffer object handle describing the buffer object that contains
    the I/O Status Area (IOSA). This might or might not be the same
    identifier as the bufobj argument. This identifier cannot be 0.

 astadr

    OpenVMS usage:ast_procedure
    type:         procedure value
    access:       read only
    mechanism:    by 32- or 64-bit reference
    Completion AST routine address (0, if none). There is no
    AST parameter argument. When the AST routine is called, the AST
    parameter will be the address of the IOSA for the operation.
    Applications can store data in the IOSA at offset IOSA$IH_
    CONTEXT.

 flags

    OpenVMS usage:mask_longword
    type:         64-bit integer (unsigned)
    access:       read only
    mechanism:    by value
    Flag mask. The flags argument is a bit vector in which each bit
    corresponds to a flag. Flags are defined in the module IOSADEF.

    The following table describes the flags that are valid for the
    $IO_SETUP service:

    Flag            Description

    FIO$M_          This is a high priority I/O; that is, it is
    EXPEDITE        to be given preferential treatment by the I/O
                    subsystem. Use of this bit requires ALTPRI or
                    PHY_IO privilege.

    FIO$M_AST_      The AST procedure does not use, or call any
    NOFLOAT         procedure that uses, any floating-point
                    registers. This is a performance option. If
                    set, AST delivery will neither save nor restore
                    floating-point registers. Caution: Use of
                    floating-point registers when FIO$M_AST_NOFLOAT
                    has been specified can cause unpredictable,
                    difficult to detect, error conditions.

    All other bits in the flags argument are reserved for future use
    by HP and should be specified as 0.

 return_fandle

    OpenVMS usage:fandle
    type:         64-bit integer (unsigned)
    access:       write only
    mechanism:    by 32- or 64-bit reference
    Address of an aligned quadword to receive the fandle for this I/O
    operation.