Windows 8 on ARM: building a common Windows platform

Microsoft is talking in detail about Windows 8 on ARM, or "WOA" as the company …

Ars Staff – Feb 9, 2012 2:34 pm | 49

Microsoft has started to talk in detail about Windows on ARM: what it will do, what it won’t do, and how it has been put together in its latest post on its Building Windows 8 blog. The focus of the lengthy post is the work Microsoft has done to bring Windows to ARM: building a common ARM platform that works the same way, whether using a processor from NVIDIA, Texas Instruments, or Qualcomm. This was a substantial undertaking: unlike desktop PCs, ARM systems are all wildly different.

Windows on ARM, or “WOA” as Microsoft is calling it, is substantially identical to Windows on x86/x64. The difference between ARM systems and x86 systems is more than just the instruction set of the processor. x86/x64 systems are almost all built in the same way. The system uses BIOS or UEFI to initialize hardware and hand over control to the operating system, they use ACPI for enumerating hardware and power management, major system devices like the video card and storage controllers are connected to PCI or PCIe with other peripherals attached to USB, and storage is either SATA or SCSI.

This idea of a common system platform dates back to the original IBM PC: being PC-compatible meant fitting into IBM’s idea of how the system’s components should be interconnected. The connections and technology have changed (with the BIOS being the lone hold-out against progress, though even that is finally making way for UEFI), but the basic concept of a standard platform has not.

ARM has no standard platform. ARM system-on-chips might attach devices with USB or PCIe, but they might not; those devices might be hardwired to particularly memory addresses, or they might use low-power and/or low-speed busses rarely found on regular PCs such as I²C or SD I/O. They might use SATA for their mass storage, but they often use eMMC. (Incidentally, some of Intel’s x86 SoCs also discard x86 platform conventions; as a result, they can’t currently run Windows.)

Until now, users of ARM platforms have mostly tolerated this lack of conformity. There are some exceptions—Google required a specific configuration and design for its initial Android tablets—but overall the ecosystem is diverse and varied. This complexity led Intel to criticize Microsoft’s ARM plans in May last year, claiming that Microsoft would have to produce not one ARM version but four.

Intel was wrong, because Microsoft is building an ARM platform. All ARM systems will use UEFI and ACPI. Booting and hardware detection will all work in the same way, regardless of whether a WOA system uses a Texas Instruments, Qualcomm, or NVIDIA chip. All systems must have USB and Bluetooth 4 (or better). Mass storage must use SATA, eMMC, or USB. All WOA systems will include a Trusted Platform Module. The result is that the WOA systems will be similar (though not identical) in all the important aspects, and the operating system won’t have to support a vast array of different system designs. Microsoft will ship one version of Windows that runs on ARM.

The differences between the ARM and x86/x64 operating systems will be substantially reserved to the very lowest level of Windows; its Hardware Abstraction Layer and a few kernel-mode drivers. Everything at a higher level will be common to both. This has resulted in some cross-pollination. Windows 8’s mobile broadband support was primarily motivated by the demands of WOA—but the feature is available on all processor architectures.

In spite of the platformization of ARM, WOA will not ship as a standalone product, and we won’t be able to build our own ARM systems from components. It will be offered solely as an OEM pre-install. This also means that the chances of seeing WOA on any currently shipping ARM tablets are slim to non-existent; the software won’t be readily available, and existing tablets don’t conform to Microsoft’s system spec.

The lack of extant hardware has complicated Microsoft’s testing efforts; the company has had to build custom racks of ARM SoCs for its labs. By March, it plans to have more than 4,200 such systems. It’s a problem for developers, too: the upcoming Consumer Preview will only be available for x86 and x64. Microsoft intends to seed a small number of pre-release ARM devices to developers ahead of the operating system’s release.