From CoLinux: Cooperative Linux is the first method for optimally running Linux on Windows and other operating systems natively. It is a port of the Linux kernel and support code that allows it to run cooperatively without emulation along with another operating system. More generally, Cooperative Linux (short-named coLinux) is a port of the Linux kernel that allows it to run cooperatively alongside another operating system on a single machine. For instance, it allows one to freely run Linux on Windows 2000/XP/Vista/7, without using a commercial PC virtualization software such as VMware, in a way which is much more optimal than using any general purpose PC virtualization software.
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