Re: For the AdaOS folks

["Luke A. Guest" <laguest@n_o_p_o_r_k_a_n_d_h_a_m.abyss2.demon.co.uk>]

On Sun, 02 Jan 2005 22:13:47 -0500, Warren W. Gay VE3WWG wrote:

>>>>Correct. That's why a protected AmigaOS must support protection domains
>>>>not only for processes but also for libraries. That's a generalization
>>>>of the UNIX model. There you have protection domains for each process
>>>>plus one protection domain for the kernel which is mainly a huge
>>>>library. In AmigaOS every library would need its own protection domain
>>>>and context switches must be as lightweight as possible.
>> 
>> I certainly wouldn't do it like that. On hardware that has an MMU (most
>> these days), that would result in a very slow system due to the amount of
>> context switches, this is why the libraries need to be mapped into the
>> address space of the running app, i.e. shared between applications.
> 
> There is no argument about the overhead in this case. However, I
> maintain that if enough people start using operating systems
> that are implemented this way, they'll finally enhance the
> hardware to correct this problem. Until then, people may as well
> continue to say "we can't do it that way". I believe the problem
> is fixable in hardware.

Yeah but, what you were saying was that you wanted OSes to use protection
domains for libraries *as well*, this is the wrong way to go. The
libraries are (and should be) mapped into an applications address space,
libraries *should never* have their own address spaces. It is that idea
which is wrong and would be slow, not the *proper* way to do it.

>> Single address space OSes are only useful if you have loads of memory and
>> you're not using that many apps at any one time. So you need to be more
>> careful with the sizes of apps. You will run out of memory, especially
>> considering the size of apps these days.
>> 
>> Luke.
> 
> Even when you have address space to "burn", you can quickly use it
> up if you decide to map files into memory. Security is another
> potential issue (you would have to map out pages that you don't
> want others to see etc.)

Well, if you have a single address space OS, you're unlikely to have
virtual memory, that's what these OSes are used for. You can still provide
memory protection on the pages that belong to each application without any
problems.

Luke.