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    CheckIO -- get the status of an IORequest

    result = CheckIO(iORequest)
    D0               A1

    BOOL CheckIO(struct IORequest *);

    This function determines the current state of an I/O request and
    returns FALSE if the I/O has not yet completed.  This function
    effectively hides the internals of the I/O completion mechanism.

    CheckIO() will NOT remove the returned IORequest from the reply port.
    This is best performed with WaitIO(). If the request has already
    completed, WaitIO() will return quickly. Use of the Remove()
    function is dangerous, since other tasks may still be adding things
    to your message port; a Disable() would be required.

    This function should NOT be used to busy loop (looping until IO is
    complete).  WaitIO() is provided for that purpose.

    iORequest - pointer to an I/O request block

    result - NULL if I/O is still in progress.  Otherwise
             D0 points to the IORequest block.

    CheckIO can hang if called on an IORequest that has never been used.
    This occurs if LN_TYPE of the IORequest is set to "NT_MESSAGE".
    Instead simply set LN_TYPE to 0.

    DoIO, SendIO, WaitIO, AbortIO

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