From:	CRDGW2::CRDGW2::MRGATE::"SMTP::CRVAX.SRI.COM::RELAY-INFO-VAX"  6-DEC-1989 20:36
To:	MRGATE::"ARISIA::EVERHART"
Subj:	RE: Help with SUPERVISOR mode code

Message-Id:  <8912070124.AA07491@crdgw1.ge.com>
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Date: Wed, 6 Dec 89 18:52 EST
From: Leichter-Jerry@CS.YALE.EDU
Subject: RE: Help with SUPERVISOR mode code
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To: zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!aplcen!haven!vrdxhq!pbs!pcrable@tut.cis.ohio-state.EDU,
        INFO-VAX@KL.SRI.COM
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 IN%"zaphod.mps.ohio-state.edu!uakari.primate.wisc.edu!aplcen!haven!vrdxhq!pbs!pcrable@tut.cis.ohio-state.EDU"

	I need some help writing supervisor mode code.

	Since there is no $CMSUPER system service, I'm stuck using the CHMS
	instruction in macro.  The macro instruction does not explain how the
	code argument is related to the address of the function that is to
	execute in supervisor mode.

	There also seems to be a catch 22 situation with declaring a change
	mode to  supervisor handler since the handler itself must be declared
	while in  supervisor mode.  Is there already a change mode to
	supervisor handler there ?

Sigh.  Another one of those "dangerous questions" - ones where you KNOW that
because the person asked the question, he is just going to get himself into a
whole bunch of trouble if you give him the answer.

In summary:

	- Of COURSE it is only possible to change the XXX-mode vector from
		XXX (or a more privileged) mode.  If not, anyone could change
		the kernel-mode vector, issue a CMKRNL instruction and take
		over any VMS system.

	- SYS$CMKERNL and SYS$CMEXEC are system services which have little
		directly to do with the underlying CMxx instructions; they
		just call you back after getting into the appropriate mode,
		IF you have the right privileges.

	- There is no SYS$CMSUPER service because supervisor mode belongs to
		CLI's.  A CLI could choose to provide such a service, but none
		of the standard ones do.

	- As a result, the only way to get to supervisor mode is to get to
		exec or kernel, and get back out from there.  The easiest
		technique is to declare a supervisor-mode AST for yourself.
		You could also construct and execute an appropriate REI.

	- Programming in supervisor mode is totally unsupported under any of
		the standard CLI's.  You MUST know what you are doing.

	- A program with access to supervisor mode can gain complete access
		to the system - while it's not obvious how, and I don't want
		to go into it, it isn't hard to get to kernel mode from super-
		visor mode.  This is a matter of the way VMS is designed, not
		the way the VAX hardware works.  This is probably the main
		reason that user-written CLI's are not supported.

							-- Jerry