Re: Peeking in to the serial buffer
From: Forrest Kenney (forrest_dot_kenney_at_hp_dot.com)
Date: 01/05/04
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Date: Mon, 05 Jan 2004 19:20:11 GMT
JF Mezei wrote:
> Forrest Kenney, OpenVMS terminal driver historian and sometime maintainer wrote:
> > have talked about IO$M_TIMED
> > to time the I/O. What is poorly understood is that the timeout is not
> > time from when the read was
> > started but it a time from last character received.
>
> Mr Kenney, if you were a female, I'd send you big kisses for the timeout
> functionality of the terminal driver interface. This is one of the smartest
> features allowing really clean programming on VMS.
>
Wish I could take credit for that but it predates my working on the driver.
>
> Do you know offhand why this was not implemented in the TCPIP QIO interface ?
> (even if the timeout logic would have been slightly different due to the
> packet nature of TCP).
>
No clue why they did what they did or did not do.
>
> Also, in the terminal driver, If I have DMA enabled on the terminal port, does
> each character received still generate an interrupt so that the driver can
> update its timeout ? Is there a difference in performance if you enable a
> timer or not ?
There is a low cost for this when a character is being processed by the driver
it adds the current value of exe$gl_abstim to the timeout and saves it away, The
system once a second ticker takes care of handeling the timeout work.
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