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