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Microsoft Windows Internals, 4th Edition says:
The architecture of the Intel x86 processor defines four privilege levels, or rings, to protect system code and data from being overwritten either inadvertently or maliciously by code of lesser privilege. Windows uses privilege level 0 (or ring 0) for kernel mode and privilege level 3 (or ring 3) for user mode. The reason Windows uses only two levels is that some hardware architectures that were supported in the past (such as Compaq Alpha and Silicon Graphics MIPS) implemented only two privilege levels.
Does this mean Windows once supported Alpha and MIPS?
49It may be hard to believe, but Microsoft was one of the most "open architecture" companies in the world. The original MS-DOS was compatible with tens of different platforms and other OSes, and Windows was designed in the same vein. Excel was built on a virtual machine that run on many different platforms. This was no accident - that's how IBM PC (and clones) became the de facto standard so pervasively and quickly, and how Microsoft was the one to supply the basic software. Which do you pick - the system that can run all your applications, or the one that has you vendor locked-in? :) – Luaan – 2016-04-07T13:28:44.447
13I have myself installed Windows on both Alpha and MIPS but that was a long, long time ago. When I was a Microsoft intern one of my jobs was to run the Visual Basic test suites on the hardware that the developers did not have on their desks. – Eric Lippert – 2016-04-07T15:20:34.853
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@Luaan - not just "open architecture" but also "open OS"! How else can you explain that at one time their officially licensed Unix variant was the most common installed Unix in the world - see Xenix at Wikipedia.
– davidbak – 2016-04-07T17:45:09.4004Even today, there are versions of Windows that run on architectures other than x86/x64, viz Windows RT for ARM devices. – TigerhawkT3 – 2016-04-08T23:46:35.897
@xiaokaoy this is old version,please read Sixth Edition – AminM – 2016-04-09T06:44:14.080
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@Luaan It was also a real problem that needed solving because particularly early IBM PC clones weren't always fully IBM PC compatible. Back in the day, "100% IBM compatibility" was a selling point for hardware vendors. We take that kind of things for granted today, but in the first half of the 1980s or so, they weren't guaranteed at all. Microsoft didn't really move into the operating system retail market before MS-DOS 5.0 which hit the market in 1991. Yes, a year after the groundbreaking success story of Windows 3.0.
– a CVn – 2016-04-09T23:17:30.5501@DevSolar May I humbly point toward the IBM PS/2, which flopped massively in part due to vendor lock-in (MCA being a major killer, and OS/2 never particularly taking off)? – a CVn – 2016-04-11T14:11:42.827
@Luaan: I disagree, massively, but don't see how to get the point across without violating ToS and / or inviting you over for a beer. ;-) So I'd better delete the comment so as to not invite others to jump the bandwaggon. – DevSolar – 2016-04-11T14:14:06.187