Re: [f-cpu] More Dark and Dusty Corners

[Juergen Goeritz <goeritz@oekomm.de>]

On Tue, 4 Sep 2001, Michael Riepe wrote:

> On Tue, Sep 04, 2001 at 08:38:57AM +0200, Reichelt wrote:
> [...]
> > > I said "register", so I guess I meant hardware ;)  Whether user mode
> > > programs are allowed to read that register is currently unspecified.
> > NO they should NOT be allowed to read that register.
> [...]
> > A virtual machine works by keeping the CPU in user mode while sometimes tricking 
> > it into believing it is in supervisor mode. Instructions which would blow the 
> > masquerade are handle in the trap handler.
> 
> For a VM, read access should be turned off, right.  But there may be
> other situations where limited (read-only) access to selected special
> registers is useful.

Maybe from the debugging interface?

> In a simple privilege model (user/supervisor only), the register should
> be invisible.  In the capabilities model, there might be a bit that
> turns access privileges on and off independently.

Why not add another mode for access that may be switched 
on from 'outside' to go into hardware debugging mode?

JG

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