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Re: [DotGNU]Register Based VM's Q.

From: Scott Lanham
Subject: Re: [DotGNU]Register Based VM's Q.
Date: Tue, 14 Aug 2001 20:58:24 +0000

So is a VM assumed to have a flat memory space?

The reason I ask this is that, as an example and from my understanding, 386 protected mode allows for the isolation of tasks to their own memory 
space and putting an abitrary pointer in a register then dereferencing it has 
no effect other than to either map the pointer to the tasks own memory space 
or cause an exception. Very specific "outside contact" points can be then 
defined and the task can only use them if it wants to use system resources.

I guess there must be some issue with using this kind of model with a VM? I 
always did like the 386's protection mechanisms :-P

> All examples that I've seen to date use the same
> registers for storing both integer and pointer quantities.
> When this happens, it becomes possible to put an
> arbitrary integer value in a register, and then dereference
> it as a pointer.  Presto, chango: we've just accessed
> memory that we shouldn't have.
> Theoretically, I suppose it would be possible to build
> a typed register-based VM.  However, you still have
> problems with RTL: it sort of assumes that integers
> and pointers are interchangeable.  The "Pmode" register
> type is typically #define'd to either "SImode" (int)
> or "DImode" (double int).  I don't think anyone has
> yet tried defining "Pmode" to be something else.
> Cheers,
> Rhys.

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