Re: Virtual Memory

[Stephan Haslbeck <haslbecs@informatik.tu-muenchen.de>]

On Tue, 14 Feb 1995, C.P.Briscoe-Smith wrote:

> >I think it will be very diffcult (if not even impossible) only to implement 
> >some simple way of swapping on 68000s. It's just the missing MMU. All 
> >programs would have to know that their pointers might change and some 
> >memory they just accessed is not valid any more. Maybe there is a way of 
> >putting this into the MiNTLibs. But it will be hard work, no question. 
> >And porting programs will be even harder, because most programs rely on 
> >non changing pointers.
> 
> You're thinking of paging.  Paging does indeed require a PMMU to be
> implemented properly.
> 
> Swapping involves picking some process which is not running, and
> copying it in its entirity into a swap file.  Then its address space
> can be given to another process; either one which is newly started or
> one from the swap file.  The process has to be loaded in again before
> it can be restarted, which may make response times horribly slow, but
> it would work for existing binaries.  The location where the process
> is stored will not change during its life-time, but it may not be
> physically in memory all the time.

And what if the space is blocked by another program that won't stop for a 
long time (or never)? Only programs that are expected to exit quickly 
after their start can run under such conditions. I think there would be 
more restrictions than without any VM.
 
Ciao, 
Stephan


+------------------------------+-----------------------------+
|      Stephan Haslbeck	       |    Fachschaft Informatik    |
|       Agricolastr. 61	       |   Technische Universitaet   |
|      D-80686 Muenchen	       |      Muenchen, Bayern	     |
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