Security, Persistence,

Chris Bitmead uid(x22068)
Fri, 09 May 1997 17:49:49 +1000

>>>MMUs are very expensive beasts that cost a lot in silicon,
>> How will you implement POS without a MMU? How will you implement a
>> virtual memory system without MMU?
>Ever heard about *software*?
>About the fact that anything that can be done in hardware can be
>done in software [known since 1936]?

Sure. You can emulate a 386 chip complete with MMU on a 286 chip. I
can guarantee that you will not be satisfied with the performance.

Of course the reason for MMU is that otherwise every pointer access
needs to be checked. Doing this in software is a massive overhead.

I'm not a LispVM expert, but since everything in Lisp looks like a
pointer, I doubt this is viable.

>About the fact that doing things in hardware
>costs more and is more expensive than doing them in software
>[one of the lessons of RISC and MISC]?
>Of course, when the architecture already has an MMU,
>the cost is already paid, so we can as well use the MMU if useful.

>> Yes we need all objects to have an immutable flag that can't be
>> changed.
>As I see things, the Right Thing(tm) is to do like ML,
>and separate values from references.
>Authorization to execute would be associated to a value, not to a reference.

I never would have imagined otherwise.