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From: jonasf@kuling.UUCP (Jonas "flax" Flygare)
Newsgroups: comp.sys.apollo
Subject: Context switch on timer interrupt..
Message-ID: <1579@kuling.UUCP>
Date: 2 Jul 90 22:09:53 GMT
Sender: news@kuling.UUCP
Organization: Dep. of Computer Systems, Upsala University, Sweden
Lines: 20

I'm working on a little project that might become an assignment 
this fall... My hopes is to write a small OS-kernel using lightweight 
processes or some mutation thereof and timer interrupts.
The first step was to get 'user controlled' context switches using 
a hacked variant of setjmp/longjmp worked just fine. (I had to make sure
no check was made as to which direction I moved on the stack + allocate
space on the stack for each process + make things Look Good before exiting 
or I would have 'Stack frame corrupted etc etc..') 
I then set up a timer to generate interrupts and a scheduler to 
be installed using signal(...) and it works fine, for one turn in the 
scheduler (each process executes once) but when I go from process N to the 
first again my program dies. I suspect there is some check on the stack
in the signalhandler, but I have no idea how to get around this problem..
(I'd prefer _not_ to have to set the stack pointer manually each time
I make a context switch just to fool the system I'm not doing anything 
Bad..)
Any help appreciated.. :-)
-- 
jonasf@kuling.docs.uu.se : "Doedth eddydthig dthrike you adth dthrayge 
Jonas (flax) Flygare     :  aboud dthidth houdth?" -- Dirk Gently