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Re: [Xen-devel] Multiple Address versus single AS Operating System

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Subject: Re: [Xen-devel] Multiple Address versus single AS Operating System
From: Mark Williamson <mark.williamson@xxxxxxxxxxxx>
Date: Tue, 21 Jun 2005 23:57:54 +0100
Cc: VISHAL MALIK <sunnymak04@xxxxxxxxx>
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> I was wanted to confirm and also find out what people think is the main
> difference between multiple address space Operating systems and single
> address space operating systems.
> Intel is a MAS platform as the cache's are physically indexed whereas
> most of the RISC processors have Single Address space Operating system
> architecture due to the fact that the caches are virtually indexed.

Actually, this is a separate issue to whether the OS itself presents a MAS or 
SAS model.  All widely used general purpose OSes are MAS (eg. UNIX, Linux, 

When the cache is virtually indexed, you can still use a physical "tag".  This 
means that there's no ambiguity between address spaces when doing a lookup, 
*except* when sharing pages between address spaces (in which case a MAS OS 
must be careful to avoid the same data being stored in two different cache 

> I have been trying to ask if the above assertion is correct or we can have
> a MAS-OS on RISC platforms too? And if yes, then from a hardware
> perspective, what is the defining criteria to have a SAS-OS or MAS-OS.

The hardware just has to have an MMU so that we can do virtual memory.  
Exactly how the hardware behaves is orthogonal to whether the OS is SAS or 
MAS - you could implement either.

In a SAS OS, all processes coexist within one linear address space, starting 
at 0 and going up to 2^n -1 (for platform with n bit addresses).  All they 
exist within the same address space, the MMU is still used to provide the 
usual protection between them.

In a MAS OS, every process sees its own private linear address space from 0 to 
2^n -1.  This is the familiar model we get with Unix and other systems.


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