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From: williams@umaxc.weeg.uiowa.edu (Kent Williams)
Newsgroups: comp.os.msdos.programmer
Subject: Re: Interrupt Driven Communication
Message-ID: <3746@ns-mx.uiowa.edu>
Date: 28 Dec 90 18:39:35 GMT
References: <53025@eerie.acsu.Buffalo.EDU>
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Reply-To: williams@umaxc.weeg.uiowa.edu.UUCP (Kent Williams)
Organization: U of Iowa, Iowa City, IA
Lines: 16

38,400 BPS is too fast on an XT unless you do a hard polling loop in
assembly language.

38,400 BPS = 3,840 CPS (at 8 bits per byte).  That means an interrupt
roughly every 275 microseconds.  You have roughly one instruction per micro-
second on an XT (and that's being generous).  I would estimate you couldn't
do a bare bones interrupt service in under 50 instructions, so you would tie
up the CPU 1/5 of the time in interrupt services for the serial port.  Then,
consider how many instructions it takes to effect a task switch from the timer
tick interrupt.  What do you get? Missed serial port interrupts!


--
             Kent Williams --- williams@umaxc.weeg.uiowa.edu 
"'Is this heaven?' --- 'No, this is Iowa'" - from the movie "Field of Dreams"
"This isn't heaven, ... this is Cleveland" - Harry Allard, in "The Stupids Die"