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From: brianr@tekig4.TEK.COM (Brian Rhodefer)
Newsgroups: comp.sys.amiga
Subject: Re: How do I force a context switch?
Message-ID: <1771@tekig4.TEK.COM>
Date: Fri, 7-Aug-87 19:27:12 EDT
Article-I.D.: tekig4.1771
Posted: Fri Aug  7 19:27:12 1987
Date-Received: Sun, 9-Aug-87 08:37:06 EDT
References: <8708070015.AA10387@cogsci.berkeley.edu>
Reply-To: brianr@tekig4.UUCP (Brian Rhodefer)
Organization: Tektronix, Inc., Beaverton, OR.
Lines: 44

May I interject some questions into the "context switch"
discussion?

The Amiga's multitasking has me mystified.  I am using it
on blind faith and a prayer, with next to NO idea of what
it is really doing, and would greatly appreciate some advice.

1) If Task A, running at priority P, sends a Signal to Task B,
   which is sleeping at priority P+1, what happens?  Does "A"
   go to sleep immediately, or only after its current timeslice
   is exhausted?  Does the same go for posting a message to one
   of B's ports?

2) Where do interrupts fit in the scheme of things?  I presume
   that they are able to interrupt even the highest priority tasks;
   is this true?

(further naive questions deleted...) It occurs to me that a little
concerted fooling around with some test programs would be helpful.
If I find anything interesting or counterintuitive, I'll post.

Meanwhile, would either of the following two methods result in
forcing a context switch?

1) If "Signal"ing does indeed force a re-evaluation of the task to run,
   why not spawn a simpleminded little task, at an extremely high priority,
   such as 125 or so, which endlessly (except for some devilishly clever
   termination mechanism) "Wait"s for a signal?

   To force a switch, just send the dimwit task the signal it's waiting for.
   You get suspended, it gets awakened, and it immediately puts itself back
   to sleep.  If I read Amiga Writ aright, a fresh evaluation of all suspended
   tasks, including the one which sent the signal, should ensue.

2) Does the "Change Task Priority" (or whatever it's called) routine have
   immediate effect? (i.e., if Task A promotes sleeping task B's priority
   above that of A, does A get chloroformed?)  If so, setting one's own task's
   priority to the same value it already has might force the kind of context
   switch that's sought, without the extra clutter of a slave task.


Foolishly hoping to understand my Amiga BEFORE its EPROMs evaporate,

Brian Rhodefer