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Windows 8 Client Hyper-V Hides Interesting Architectural Details

When is a PC not really a PC? When it’s a virtual machine (VM), of course. In that case, software is used to emulate a physical machine of sorts, as well as the operating system and applications it supports. Burning with curiosity this morning, I fired up DriverAgent to see its takes on what goes into the virtual innards of a Windows 8 VM in the company’s latest and greatest desktop environment.

An interesting grab-bag of generic PC parts.

An interesting grab-bag of generic PC parts.

The first thing I noticed about the listing was the lack of hardware details — such as, for example, no CPU (no Processor entry), no monitor, no physical storage devices, and so forth. The next thing is the presence of a number of numerous entries based on the old Intel 810 Chipset and its related 82371AB/EB controllers — including a PCI Bus Master IDE controller, a PCI to ISA bridge (ISA mode) controller, and a Pentium II Processor to PCI bridge. All of this points to an older and very generic Pentium II virtual machine architecture for Hyper-V desktop OSes. This makes sense, because a large number of virtualized device drivers are available for this virtual platform, and makes the rest of the plumbing for such VMs easy to define and hook up, including other no-name system elements like the COM1 communications port, the PCI bus, CMSO/real time clock, DMA controller, motherboard resources, numeric data processor, a programmable interrupt controller, and so forth. And when there’s IO to be done we see virtualized drivers including an Msft Virtual CD/ROM ATA device, and a Virtual HD ATA device.

All very simple if not also entirely rudimentary. Intel’s had emulators for all these parts kicking around for years. I’m pretty sure that where at least the baseline virtual PC elements in this architecture originate. It also gets interesting if you visit DriverAgent and right-click on the itty-bitty icons at the left of each entry that appears in the preceding screencap. Normally, anything with a “real driver” attaches to a link where you can find alternate drivers you might wish to substitute for what you’re currently using (at least on a non-virtual PC). As I click through the list, only the following elements have links attached:

  • Intel 82371AB PCI Bus Master IDE Controllers PCI Drivers
  • Microsoft Hyper-V S3 Cap (but the corresponding driver page is empty)
  • Generic PnP Monitor (driver page empty)
  • Intel 82371AB/EB PCI to ISA bridge (ISA mode)
  • Intel 82443BX Pentium(r) II Processor to PCI Bridge PCI Drivers
  • Microsoft Shared Fax Driver (driver page is empty)
  • Microsoft XPS Document Writer v4 (driver page is empty)

Everything else is completely virtualized,  probably integrated into the runtime virtual machine image that supports desktop operating systems inside Hyper-V v3. In pondering why Microsoft chose such simple, basic elements and such a rudimentary infrastructure for the virtual architecture, I’m struck by how far back in time the hardware upon which the virtual machine is modeled actually goes. It’s old enough to accommodate even “ancient” Windows versions (a lot of this stuff goes back to the 1999-2001 period) yet capable enough to provide useful PC emulation even for modern desktop operating systems like Windows 7 and 8. When I run either of those OSes on a modestly equipped host PC (i7 2600K with plenty of RAM) they are surprisingly fast and snappy, not much slower than the host OS itself, running on the same hardware.

Given that updating drivers for a virtual machine is fraught with peril, I’m glad that DriverAgent didn’t find any drivers it thinks need to be updated on the test VM where I captured the screenshot. I have to image that if and when such driver updates occur, they’ll be handling as part of a rebuild of the baseline VM images, and simply be passed down the line for ordinary users to test and deploy as they see fit.

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