Path: utzoo!yunexus!xrtll!rsnider
From: rsnider@xrtll.UUCP (Richard Snider)
Newsgroups: comp.os.minix
Subject: Re: Trace System call for Minix
Summary: interuppts will cause problems
Message-ID: <234@xrtll.UUCP>
Date: 7 Jan 89 18:44:51 GMT
Article-I.D.: xrtll.234
References: <6255@louie.udel.EDU>
Reply-To: rsnider@xrtll.UUCP (Richard Snider)
Organization: ISOTECH Computer Industries, Toronto, Canada
Lines: 41

In article <6255@louie.udel.EDU> jnall%FSU.BITNET@cunyvm.cuny.edu
(John Nall 904-644-5241) writes:
>how the operating system works if there were some sort of trace function
>in Minix -- some way to "step through" the system.
>
>It occurs to me that this would not only be nice, it should not be very
>hard to do.  Essentially, a "trace" system call would be added, and the
>student would use it at points where he needed to stop the world and look
>at it.
>Before I reinvent the wheel (or try to get a grad student to do it), has
>anyone already done this?  Or are there some real show stopping problems
>that I'm just not seeing??
>
>Thanks for the help.
>
>John Nall (jnall@rai.fsu.edu BITNET)

The problem with allowing the system to "Stop" and then letting someone
take some time to check things out is that Minix depends heavily on various
interrupts happening to do such things as turn disk drives off, watch
the keyboard, and process switching and servicing.  The only approach
that I can suggest is that you implement some sort of processor trap
which saves the state of the machine in some known place (Maybe a special
/dev type file) where you can read it out.  Now this isn't exactally what
I would call a Trace but it at least will give you some idea of what is
going on at exactally that place in the code.  The other approach I can
see is to write some sort of processor emulator which when given the
information about contents of registers and memory (at the present time)
by some call as described above, it will then go on and appear to be the
machine to the student so that they may single step it or change things
and continue...etc.  This of course is not perfect since you will have 
to save the values at all the device io address along with the processor
state so that they may be referenced by the emulator.  But since we are
talking about a teaching tool here, it may be suffcient.

Best of luck.
                                        Richard Snider
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