How will Windows 8 support ARM? [closed]

Question

I recently read this question on how Windows 8 will support ARM.

Now, my question. How? ARM isn't anything like the PC/x86.

List of things that vary across different devices:

1. MMU. Some lack them, some are more limited than others. They all vary across each ARM processor. Microsoft would have to explicitly write kernel support for each ARM processor they intend to support
2. Multi-Processing. There is no standardization that I know of for multi processing on ARM processors that are capable.
3. Memory Maps. These vary extremely widely across each device and CPU. There is no standard for "detecting" the memory map either
4. There is no "standard" bus for devices other than USB. PCI isn't common, and I've never seen any real standardization for buses. Some devices use proprietary buses or don't have any "real" ones at all.
5. Opcode support. Many of the ARM CPUs out there have conflicting opcode sets. It's very difficult if not impossible to go to a "generic" set of opcodes. Some processors include an FPU and some don't. There is no way to detect what opcodes will work and what won't and how they'll behave. There is no equivalent for CPUID

The only commonly standardized thing is the interrupt controller.

How does Microsoft expect to pull this off? A different version of Windows for each device they support? A very limited list of supported devices? Forcing manufacturers to follow some standard they create?

Answer 1

There will be different builds of Windows 8 - the same way that there are different builds of Android for different handsets with varying ARM CPU's. While they have the same feature sets and behave the same way from device to device - the kernel, memory mapping, SMP is handled appropriately for the CPU implementation.

You're not going to pick up a copy of Windows 8 off the shelf at your local electronics store to install it on your custom built ARM PC; instead, you're going to have it pre-installed from the factory. With that said, I'm sure many of the feats you described in the ARM architecture will standardize over time as we are already seeing this happening since the explosion of Android devices.

Edit / Addition: Microsoft will be selective on the ARM implementations / architectures they support in Windows 8 as well:

"Windows is growing more flexible in other ways. Microsoft said back in January that the next version of Windows would support ARM-based chips from Nvidia, Texas Instruments and Qualcomm, in addition to the traditional Windows processors from Intel and AMD. Though Sinofsky and Green used Intel-based machines for Wednesday’s demo at D, Microsoft plans to demonstrate some of the ARM-based designs later today at the Computex trade show in Taiwan."

AllThingsD

Answer 2

Not an impossible task, given that there's the Linux on ARM model to copy from. Once you construct a hardware abstraction layer, all of those HW differences (except the opcode issue, which relates more to compliers and binary distribution) essentially are compartmentalized and become manageable.

One possibility is there will be vendor and model specific versions of Win8, much like how firmware is tailored for embedded devices (ever install DD-WRT on a wireless router?). Since the blurbs mention these ARM-based devices were tablet computers, the firmware resemblance is strong, compared to the standardized PC model of SW distribution.