On Tue, 20 Jul 1999 13:14:36 -0400 (EDT), "Richard B. Johnson"
<root@chaos.analogic.com> said:
> One thing that helps(ed) VAX/VMS is that the kernel has many aspects of a
> process. In other words, the WSMAX, WSMIN, NPAGEDYN, etc., can be set
> separately for the kernel. This has some drawbacks since a call for system
> services does not execute in the context of the current process (as in
> Unix). However, it allows the kernel to survive any user-mode attempts to
> kill it.
Getting off-topic, but it's still interesting stuff...
Huh? This is really not true at all. A system call on VMS executes in
the context of the calling process, except it is in kernel mode instead
of user mode. It's very much part of the same process, even though it
is running with privileges.
VMS extends this in a very useful manner. There is a sys$cmkrnl()
system call which calls a user function with kernel privileges. It lets
arbitrary program code run in kernel mode. The linker even has ways of
letting that code link directly against certain key system services.
Being able to build and run arbitrary kernel code as a user application
is a very, very handy thing to be able to do. Of course, you need the
appropriate privilege to be able to do this, and normal mortals don't
get given that privilege every day. :)
--Stephen
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