Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!sun-barr!sun!imagen!atari!portal!cup.portal.com!ts
From: ts@cup.portal.com (Tim W Smith)
Newsgroups: comp.sys.mac
Subject: Re: Apple System 7.0
Message-ID: <19720@cup.portal.com>
Date: 21 Jun 89 04:56:05 GMT
References: <17183@usc.edu> <4679@okstate.UUCP> <1925@internal.Apple.COM> <18888@cup.portal.com> <576@cbnewsm.ATT.COM>
Organization: The Portal System (TM)
Lines: 24

>> If you've got kernel sources for Unix, try hacking it to not
>> use pre-emptive multitasking.  It might be interesting to see
>> what happens.
>
>Admittedly, the last time I looked at the UNIX kernel was in graduate school
>some 15 years ago, and it was 6th edition, but UNIX multitasking has been
>based on time-slices and not on preemtption in all versions I know about.

	"A scheduling discipline is nonpreemptive if, once a process
	 has been given the CPU, the CPU cannot be take away from that
	 process.  A scheduling discipline is preemptive if the CPU
	 can be taken away."

		from "An Introduction to Operating Systems", revised
		first edition, by Harvey M. Deitel, page 252.

Unix is preemptive.  Time-slices are used to determine *when* to
preempt.  In other words, time-slices are one way to implement a
preemptive scheduler.

This is all from the point of view of user mode code, of course.
To kernel mode code, Unix is nonpreemptive.

						Tim Smith