Xen devel - Jun 2005 - Multiple Address versus single AS Operating System

Sorry, I sent the email to the incorrect list.
My applogies.
Sunny.

VISHAL MALIK <sunnymak04@yahoo.com> wrote:
Date: Thu, 16 Jun 2005 10:39:40 -0700 (PDT)
From: VISHAL MALIK <sunnymak04@yahoo.com>
Subject: Multiple Address versus single AS Operating System
To: xense-devel@lists.xensource.com
CC: sunnymak04@yahoo.com

All,
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.
 
(in a VI-cache, you access the cache and tlb at the same time).
 
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.
 
Thanks in advance for help.
Sunny.


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On Thu, 2005-06-16 at 10:42 -0700, VISHAL MALIK wrote: 
>         
>         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. 
>           
>         (in a VI-cache, you access the cache and tlb at the same
>         time). 
>           
>         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. 
I don''t understand your question, but I can say for PowerPC:

	icache		dcache
	index/tag	index/tag
405GP:	V/P		P/P
604:	P/P		P/P
970:	V/P		V/P

So your statement that most RISC processors are virtually indexed is a
bit of an oversimplification I think.

-- 
Hollis Blanchard
IBM Linux Technology Center


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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, 
Windows).

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 
locations).
> 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.

HTH,
Mark

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