Aaron Giles

Programming War Stories


I started working at Microsoft in 2003
I started working at Microsoft in 2003

After the Connectix engineering staff was snapped up by Microsoft, I moved from the San Francisco Bay Area to the Pacific Northwest and started a new life.

There was no time to waste once we got there, so we started right away on rebranding Virtual PC for Windows as a Microsoft product, and followed that up with a rebrand of Virtual Server as well. Then it was on to making new stuff, so I ended up co-leading the effort to created Microsoft’s own virtualization solution, Hyper-V.

I then decided to ditch the whole management gig and got involved with efforts to port Windows NT to the ARM platform. I followed that up by taking on the task of porting Windows yet again, this time to ARM’s new 64-bit architecture.


Microsoft Virtual PC 2004

When we developed Virtual PC for Windows at Connectix, we designed it for speed, and the best way to make it scream was to take complete and utter control of the entire machine for brief periods of time without the underlying OS knowing about it.

By taking over the MMU, we could make it behave like the emulated system
By taking over the MMU, we could make it behave like the emulated system

The reason this got us great performance was that we could directly leverage the MMU on the processor to manage memory. Without that, every single memory access would involve verifying permissions and mappings, and that would have crippled performance.

In order to begin taking over the machine, we first had to gain access to kernel mode. To do this, we just identified an unused interrupt vector in the OS’s interupt vector table, and patched it to point to our own code—which lived in user mode. (Security experts are rightly freaking out at reading this.)

Once in control, we would run as long as we could, until an interrupt was signaled. When that happened, we would immediately put the OS’s page tables back and then hop to the OS’s interrupt handling code, pretending like nothing at all had happened.

Windows XP Service Pack 2 was all about security
Windows XP Service Pack 2 was all about security

A good analogy is if, right after leaving for work, someone breaks into your house, rearranges your furniture, and hangs out. Then, when they see you pull up in the driveway, they put everything back exactly the way it was and slip out the window, with you none the wiser.

The rebranded Virtual PC
The rebranded Virtual PC; isn’t it pretty?

Of course, the other thing that was happening around the time of our purchase was the release of Windows XP Service Pack 2, which was the big security-focused version of Windows XP. All code had to go through a security audit, especially code from outsiders.

Thus we were told that we needed to come up with a clean and proper way to do this switch, which involved installing a driver that could be called with proper permission checking by the OS.

Apart from security improvements, the rest of our efforts in this release went toward rebranding and minor tweaks. All told, it took a little more than a year to complete the work, and then it was time to move on to the real reason Microsoft bought Connectix: creating the next generation.

Building Hyper-V

I hated the name Hyper-V at first, but it eventually grew on me
I hated the name “Hyper-V” at first, but it eventually grew on me

By the time we began work on Hyper-V, VMWare had already made significant inroads in defining and dominating the x86 virtualization market. Microsoft wanted to make virtualization a key part of Windows Server 2008, and the Connectix engineers were at the heart of this effort.

One advantage of starting late is that both Intel and AMD were in the process of introducing virtualization extensions that formalized virtual machine support in the processor. (We eventually retrofitted that support into later versions of Virtual PC as well.)

Pretty good architectural diagram of Hyper-V
Pretty good architectural diagram of Hyper-V

The effort to develop Hyper-V was big enough that we split it into three parts: one team handled the hypervisor, one team handled VMBus and virtual devices (like networking and storage), and I led up the team that handled the rest, including the user-facing parts of the product.

Even within my third of the project, we were dealing with a very broad collection of components, including:

Hyper-V was first shipped as part of Windows Server 2008
Hyper-V was first shipped as part of Windows Server 2008

Over the course of the project, my team grew from around 6 to 12 people, and I ended up not actually writing very much code, as managing a team that size required all my attention.

As a result, after the successful launch of Hyper-V as part of Windows Server 2008, I decided to take a break from management and became an individual contributor once again. This meant finding something fun to work on.

Unfortunately, these things don’t always happen quickly. I killed time working on a couple of related projects, including system support for mounting VHD images that shipped in Windows 7. And then I found what was probably the perfect project for me.

Windows on ARM

It was in 2009 when I first heard of a small incubation effort to get Windows up and running on ARM processors. At the time I encountered it, the project was being driven by one person, who actually already had the OS booting with little official help.

By 2009 ARM chips were ready to run full Windows NT
By 2009 ARM chips were ready to run full Windows NT

But the time was right for official ARM support to become important. The dawn of multicore mobile processors was here, and Windows CE’s lack of MP support was beginning to put Microsoft at an even greater disadvantage.

Since ARM processors were getting more powerful all the time, and since NT was designed from the start to support running with multiple processors, it seemed inevitable that someone would make it an official ask soon, so why not start now?

This project, it turned out, was absolutely perfect for my skills. It involved both porting (which I had done at LucasArts and in my MAME work) and bare metal programming (which I had done throughout my career dating back to my early games).

One of the first things we did was to define the runtime environment requirements. At a minimum, we chose to standardize on modern ARMv7 processors, and to require both hardware floating-point support (VFP) and SIMD support (NEON).

Qualcomm’s Snapdragon chips were nicely-optimized ARM chips perfect for running Windows NT
Qualcomm’s Snapdragon chips were nicely-optimized ARM chips perfect for running Windows NT

Doing this meant we could ignore a lot of backwards compatibility issues and focus on just supporting the high-end fully-featured processors like the ARM Cortex A9/A15 and the Qualcomm Snapdragon class of CPUs.

Another controversial call we made was to fully embrace the Thumb-2 ISA. Since there was no legacy code running on these systems, there was no need to support two ISAs (both A32 and Thumb), so we standardized on the more compact form throughout the OS.

This ISA decision meant our compiler division needed to create new versions of the compiler optimized for Thumb-2, and also needed to support the ARM form of the NT exception unwinding model, which I ended up designing.

A lot of the work I did involved digging into other peoples’ code and removing bad assumptions. Over the course of its history, Windows NT has actually run on a lot of different CPU architectures: MIPS, Alpha, PowerPC, IA64 (aka Itanium), and both x86 and x64.

Windows Phone 8 was the first to run on top of Windows NT
Windows Phone 8 was the first to run on top of Windows NT

But over time, all except for the Intel-based ones (IA64, x86, and x64) had been dropped, and even IA64 was on the way out. So a certain myopia had inevitably creeped into some of the code, and we had to help educate people once again about RISC platforms like ARM.

The original Surface RT; I’m still bitter about its artifical limitations
The original Surface RT; I’m still bitter about its artifical limitations

All in all, it was very rewarding getting the full version of Windows up and running on a new platform. We actually had pretty much everything running smoothly before the concept of Windows RT was introduced, which put some artificial limitations into the system.

Eventually, Windows RT shipped on the Microsoft Surface and Surface 2 tablets. Shortly thereafter Windows Phone 8 shipped, using the Windows NT as its foundation instead of Windows CE.

Of course, it’s well-known that those original ARM-based Surface models floundered, and Windows Phone struggled to find marketshare. But we weren’t giving up on ARM as a viable target for Windows! Also, ARM was busy developing a 64-bit variant with even better alignment to Windows’ requirements, leading to the next chapter in my Microsoft adventure....

ARM Goes 64-Bit

More to come later!


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