Virtualize The Operating System

Need to run more than one operating system at a time? Doing so can be very handy for supporting backward compatibility. It's also a useful way to isolate new platforms while running an existing platform on the same hardware.

Two approaches are available for running multiple operating systems. The first is the virtual machine manager (VMM). The second is the user-mode operating system. It's possible to mix and match these approaches, though the typical installation is much more limited (see the figure).

VMMs virtualize the host processor. This enables the host operating system, which is optional, and the VMM to run simultaneously with the client operating systems. The VMM traps the client operating systems' access to the hardware.

Designers should take notice of two commercial VMMs, Microsoft's Virtual PC and VMware's namesake, which support standard operating systems such as Windows XP. Using these VMMs, developers can run their host development platform alongside the target platform, assuming the target runs on the same processor as the host.

Virtual PC does a great job hosting Windows operating systems, while VMware tackles workstation and high-end IT infrastructures. VMware's high-end management is designed to tackle large, multiprocessing environments. Then there's Xen, an open-source VMM supported by the University of Cambridge. Xen definitely is not in the same class as Virtual PC or VMware, but it can be a useful tool.

Embedded designers looking for non-x86 alternatives can check out platforms like Green Hills Software's Integrity Padded Cell for its Integrity operating system and Jaluna's OSware. These VMMs run on a variety of processors, but they typically require more customization than embedded designers are familiar with.

The second option for running multiple operating systems, the user mode (UM), requires modification of the client operating systems. The advantage is that the operating system now looks like an application to the host operating system removing the hardware virtualization requirement. VMM overhead is small, allowing a user-mode implementation to be even more efficient.

User-Mode Linux (UML) is an open-source project designed to run Linux as an application. Its initial target was the x86 architecture. LynuxWorks BlueCat Linux UML was just released for the Power PC G5 processor architecture. The PowerPC is very popular in embedded applications. It is the first commercial UML product for non-x86 platforms.

Virtualization is an invaluable tool for development and deployment. Designers definitely should check out the options.