Path: utzoo!utgpu!watserv1!maytag!xenitec!zswamp!root
From: root@zswamp.fidonet.org (Geoffrey Welsh)
Newsgroups: comp.dcom.modems
Subject: Re: UARTS for speed...
Message-ID: <5717.27549621@zswamp.fidonet.org>
Date: Wed, 28 Nov 90 14:50:37 EST
Organization: Izot's Swamp BBS - Kitchener, Ontario

John Temples (john@jwt.UUCP ) wrote:

 >(Geoffrey Welsh) writes:
>*IF* the software is configured to do it, installing a 16550 
>*can* reduce CPU overhead slightly.
>   However, you won't feel the difference.

 >Depending on the environment, I would say the reduction is more than
 >"slight."  At one time, I had both a 16450 and a 16550 on my dumb
 >Digiboard.  I was switching my 38.4 kbps terminal between the two
 >ports to see what difference it made.  CPU utilization (as measured
 >by u386mon) went from over 40% on the 16450 to under 10% on the
 >16550.  This was measured on a 386/33 running UNIX, with serial
 >drivers that took advantage of the 16550.  I imagine the difference
 >would be even more noticeable on a slower machine.

   The difference is in the overhead of firing up the driver evrey time a 
character appears; if the incoming data stream is fairly heavy, a 16550 can cut 
down on the number of times you've got to do that context switch. I am 
beginning to gain an appreciation for how much overhead 386 Unix 
implementations have in this situation.

>   Here's a quick DOS Turbo Pascal program I hacked together to enable the 
>FIFOs on a 16550 and set the trigger level to 1 byte

 >It should probably be mentioned that enabling FIFO mode can make some
 >software quit working.  I've seen the extra bits that are set in the
 >IIR when FIFOs are enabled mess up some interrupt handlers that
 >weren't expecting them.

   It should also be noted that people who experience overhead decreases after 
installing 16550s but NOT enabling their FIFOs should get their heads 
examined. A 16550 without FIFOs is a 16450 (one interrupt per byte).
 

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