Re: Ada's Slide To Oblivion ...

["Marin David Condic" <dont.bother.mcondic.auntie.spam@[acm.org>]

IIRC, the MC680x0 had pins coming out of it for Supervisor/User and
Code/Data access so that a little bit of silicon could figure out that
User/Code/Write access would not be allowed. A little hardware design would
mean you didn't actually need the MMU to protect the space. Of course if you
could get into Supervisor mode, you'd be handed the keys to the gates of
paradise and all bets are off. But typically the only way to do that would
be a) if your program got loaded at bootstrap or b) whatever program *did*
get loaded at bootstrap had poor design.

MDC
--
Marin David Condic
Senior Software Engineer
Pace Micro Technology Americas    www.pacemicro.com
Enabling the digital revolution
e-Mail:    marin.condic@pacemicro.com
Web:      http://www.mcondic.com/


"Ian Wild" <ian@cfmu.eurocontrol.be> wrote in message
news:3C63CB16.645FE25E@cfmu.eurocontrol.be...
>
> (sigh)
>
> Let's take the example of a 68000 'cos I've got the data sheet
> readily available.  There are three lines coming out of the
> processor which distinguish various spaces.  Of interest are:
>
>   supervisor code
>   supervisor data
>   user code
>   user data
>
> If the OS is clever, and many of them were, it's set up the MMU
> tables such that 'user code' exists at some point in 'supervisor
> data', at least long enough to load the executable from disc.  At
> some later point it may re-map things so that 'user data' exists
> at that same address in 'supervisor data', letting it initialise
> the bss.  At some late stage it initialises the MMU so that
> 'user code' starts at address 0, 'user data' starts at address 0
> (but with the first page unmapped to catch NULL pointers), and
> does a mode switch.  There is then NO WAY from user mode to write
> to the 'user code' space, and only limited (ie PC relative) access
> for reading.
>
> Oh - before anyone suggest it, no, the MMU is not visible from
> user mode either.
>
> Is this really rocket science?  I would've though the above
> sufficiently obvious that I could have got a patent on it.