RE: Memory control

> >And is it possible to
> >extend that into the program's executable code?
>
> Generally speaking, no. Data pages and program pages are well defined
> from eachother. I would imagine you'd simply get a mrmory fault if you
> attempted to change a code page.

That's what I'd expect.  I'd expect the same eventually if I defined a based
variable, started moving blocks of data to it and incrementing it past the
available memory on my AS400.  I would NOT expect that doing so would affect
anything outside my job, or even outside my program's data pages.

> >Can you move data to a
> >procedure pointer?
>
> And keep the pointer valid? Not sure, don't think so. Then again, I have
> heard of someone running PPC machine instructions out of a user space,
> so at that point it gets really interesting.

Now that sounds interesting.  Even so, I would've expected OS400 to to
protect itself from attempts to modify its memory from such code.

> >Including OS400 itself?  Using just HLL programs?  Isn't Leif's book
> mostly
> >about MI?
>
> Sure you can mod OS400 data -- but be careful! Changing the global SEPT
> is an example.

How much of all this depends on the OS400 release level, and the security
level of the system? How up-to-date is Leif's book?  I notice in the V5R1
memo to users it says

"OS/400 internal control blocks more secure In V5R1, integrity protection
for some system control blocks was improved. This may cause applications
that attempt to directly change these control blocks to fail. Check with
your application provider to determine if their applications continue to
work correctly on V5R1."

> Since most MI opcodes can be accessed from RPG I would
> think you might be able to do it in RPG, however, why don't you consider
> MI an HLL?

Now that's an interesting question.  How high is up?<g>  If it's not machine
code (possibly PPC code depending on your hardware) does that make it an
HLL?  Or if it's not microcode it's HLL?  I guess I consider languages that
are not dependent upon the machine architecture to be HLL's.  Can you run MI
on a PC?

Interesting discussion.

Peter Dow
Dow Software Services, Inc.
www.dowsoftware.com
909 793-9050 voice
909 793-4480 fax

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