We’ve already discussed the software distribution changes that are going in to the ARM versions of Windows 8; this post will focus on hardware. Windows 8 on ARM (WOA hereafter) has been built in close cooperation with Texas Instruments, Qualcomm, and Nvidia. The omission of Samsung is a bit surprising given the latter’s Exynos product line, strong tablet lineup, and the fact that Samsung built the Windows 8 x86 tablets Microsoft gave away at Build last fall. It’s not clear at this point if Samsung’s plans for Windows 8 revolve primarily around x86 products, or if the company decided to sit out on becoming a developer partner.
One of the most significant challenges Microsoft had to overcome was the degree of variation that can occur between ARM developers. In the PC world, the battle over interface standards has been over for decades — but there was once a reason why users’ referred to the PCI card installation process as “plug and pray.” In WOA, the ACPI firmware has been extended to address and understand how SoC devices are configured in order to ensure it can enumerate them properly. The Windows HAL (Hardware Abstraction Layer) that handles things like interrupts and DMA in conventional PCs has also been modified and extended to support current and future SoCs.
Standards and bus support has also received a lot of attention. The SoCs that ARM manufacturers use don’t support the same standards as conventional PCs, which required Microsoft to design a new set of low-level device drivers. WOA will support low power buses like I2C/UART, SD I/O, and eMMC. In some cases, Windows itself had to be rearchitected; Sinofsky’s post states that Windows “expects a fast disk and very high bandwidth data transfer.” The team made several performance optimi ations that result in fewer read/write cycles to ensure systems weren’t hamstrung by a lack of I/O performance.
While he doesn’t mention ARM’s big.LITTLE initiative by name, Sinofsky notes that WOA will also support offloading specific tasks on to speciali ed co-processors. The x86 version of Windows already does some of this — programs can take advantage of GPU hardware acceleration for video encode/decode — but Windows 8 may make it easier for programmers to shift tasks to lower power processors.
One of the problems Microsoft has had when it comes to creating software tools for ARM systems is the lack of currently available products. The company got around this limitation by building its own server hardware and intends to have over 3200 WOA systems on-hand (deployed in rackmount servers, pictured above) by the spring. According to Sinofsky, the tight connection between SoCs, peripherals, and firmware on the hardware side led to an unusual degree of crossover on the software teams. “This is an effort where software people on the Windows team end up debugging silicon with soldering irons, and hardware engineers end up in Visual Studio, debugging timing issues with user interface code.”
This is why buffer overflows ought to be diagnosed in software
The WOA systems that ship come launch date aren’t being assembled by vendors with light input from Microsoft; the company stresses that it’s worked with Qualcomm, Nvidia, and TI from the beginning to create reference designs that’ve been thoroughly vetted and expressly manufactured for WOA. As part of the developer seed effort, MS is going to distribute a small run of WOA systems to testers, with the understanding that they aren’t in any way representative of shipping hardware. The rest of us are advised to download the new Community Preview when it becomes available at the end of the month; Redmond stresses that any PC that carries a Windows 7 logo will run Windows 8.
Windows 8 on ARM: Rearchitecting for new hardware
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