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From: jon@gaia.UUCP (Jonathan Corbet)
Newsgroups: comp.os.vms
Subject: Re: internals
Message-ID: <323@gaia.UUCP>
Date: Wed, 1-Jul-87 15:17:05 EDT
Article-I.D.: gaia.323
Posted: Wed Jul  1 15:17:05 1987
Date-Received: Fri, 3-Jul-87 02:06:06 EDT
References: <8706281911.AA23198@ucbvax.Berkeley.EDU>
Reply-To: jon@gaia.UUCP (Jonathan Corbet)
Distribution: world
Organization: Gaia Corp., Boulder, CO
Lines: 21

WARNOCK@prism.clemson.EDU (Todd Warnock):
>It's my understanding that ANYONE can issue a CHMK instruction successfully.
>(not to be confused with a successful call to the similar system service...)
>Exactly what is the difference in the two ?  What actually requires the
>CHMKNL privilege ?

It is true that anyone can successfully execute a CHMK instruction.  The
important thing to remember is that successful execution of that instruction
causes a trap to the system CHMK handler.  Thus there is no way for any
old user to just execute a CHMK and actually be running in kernel mode --
they lose control of the processor when the instruction executes.

Thus, the existence of sys$cmkrnl, which will call a given user routine
(assuming the existence of CMKRNL [or, actually, CMEXEC] privilege) after
the trap to the CHMK handler.

jon
-- 
Jonathan Corbet
{seismo | hplabs | gatech}!hao!gaia!jon
{ucbvax | allegra | cbosgd}!nbires!gaia!jon