Implementing a multitasking OS on top of UNIX

[Craig Robinson]

In article <1991May9.040020.26194 at ux1.cso.uiuc.edu> phil at ux1.cso.uiuc.edu (Phil Howard KA9WGN) writes:
>craig at 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 at ux1.cso.uiuc.edu   |  Guns don't aim guns at  \
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Craig
-- 
craig at acns.nwu.edu