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From: dpw@lemuria.usi.com (Darryl P. Wagoner)
Newsgroups: unix-pc.general,comp.sys.att
Subject: Re: Question about windows and processor time (3b1)
Message-ID: <1211@lemuria.usi.com>
Date: 7 Mar 89 21:42:33 GMT
References: <356@flatline.UUCP> <450@amanue.UUCP>
Reply-To: dpw@lemuria.UUCP (Darryl P. Wagoner)
Organization: Digital (Secure Workstation Project) Boxboro, Ma
Lines: 32

In article <450@amanue.UUCP> jr@amanue.UUCP (Jim Rosenberg) writes:
>In article <356@flatline.UUCP> erict@flatline.UUCP (j eric townsend) writes:
>>Does your hard drive *pause* while you go from one window to another?
>>Mine does.
}
}Yes, UNIX is definitely multitasking -- but only at the *USER* level.  The
}*kernel* is not multithreaded or multitasked in any way.  A UNIX process is
}a twin-headed beast:  the process executing in user mode and the process
}executing in kernel mode.  A process executing in user mode can be preempted
}by an interrupt, but it's my understanding that a process executing in kernel
}mode *IS NOT PREEMPTED* -- it must voluntarily give up the CPU.  (Obviously

There is two forms of context switchs, voluntary and involuntary.  It
is true that a involuntary is based upon a hardward interupt but it
still causes a context switch even in kernel mode.  This in most cases
doesn't happen in kernel mode because system calls will either finish
before the time slice is up or will sleep on an address which causes a
voluntary context switch.

}impacting; (2) masking out interrupts.  Yes boys and girls, this awful thing
}is true.  The kernel has no semaphores.  The kernel has no message passing.

It does have message passing.  It couldn't work without them.  The sleeping
on a address is a example of message passing.



-- 
Darryl Wagoner		(home) dpw@lemuria.uucp or wagoner@imokay.dec.com
Digital Equipment Corp; 	OS/2, Just say No!
Boxboro, Ma  			(w) 508-264-5586
UUCP:  virgin!lemuria!dpw