Newsgroups: comp.unix.internals
Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!casbah.acns.nwu.edu!craig
From: craig@casbah.acns.nwu.edu (Craig Robinson)
Subject: Re: Implementing a multitasking OS on top of UNIX
Message-ID: <1991May9.055804.6550@casbah.acns.nwu.edu>
Organization: Northwestern University
References: <1991May9.015124.20638@casbah.acns.nwu.edu> <1991May9.040020.26194@ux1.cso.uiuc.edu>
Date: Thu, 9 May 1991 05:58:04 GMT
Lines: 56

In article <1991May9.040020.26194@ux1.cso.uiuc.edu> phil@ux1.cso.uiuc.edu (Phil Howard KA9WGN) writes:
>craig@casbah.acns.nwu.edu (Craig Robinson) writes:
>
>>For my operating systems class we are required to do the following project:
>>"Groups of three students will design and develop a priority based
>>multitasking kernel that provides for process control, process
>>synchronization, and process communication.  The kernel will be developed in
>>the C programming language and will run on top of the UNIX OS."
>
>>This is the exact project specification as stated in our syllabus.  No
>>more, no less is required.
>
>>1)  In what way would we be able to implement a quantum driven system?   
>>How can we simulate clock interrupts?
>
>Do you actually need clocks?  Was that a part of the assignment spec?
>

Well, I lied a little bit when I said that the above was the exact project
specification.  Since we were given this assignment the Prof. has asked
that we try to implement a quantum driven system. 

>Actually it should not be hard.  I assume you have some sort of way to
>identify your tasks and a means to send them a message.  Have a signal
>handler (which is the equavalent of an interrupt handler here) send a
>message to the designated task when a signal comes in for a clock tick.
>The schedule the next one and return.

Good idea, I like it.

>>2) In UNIX, what happens *at the CPU level* when a process makes a system
>>call?  How does the processor know to switch from user to kernel mode?  How
>>can we simulate this on top of UNIX?
>
>Again, is this part of your assignment?  You don't need to distinguish
>between user and kernel mode.  Everything can be in user mode.  You should
>not be trying to build a whole UNIX system.

You know, you're right.  I never thought about that.  I have been so caught
up in trying to understand how UNIX internals work, that I have been trying
to build a system like it, and was feeling overwhelmed by the task.  But
that is not at all what I need to do.  Thanks for the insight Phil.

Just for my own edification though, what does happen at the CPU level when
a process makes a system call?


> /***************************************************************************\
>/ Phil Howard -- KA9WGN -- phil@ux1.cso.uiuc.edu   |  Guns don't aim guns at  \
>\ Lietuva laisva -- Brivu Latviju -- Eesti vabaks  |  people; CRIMINALS do!!  /
> \***************************************************************************/


Craig
-- 
craig@acns.nwu.edu