Cooperative Linux utilizes the concept of a Cooperative Virtual Machine (CVM). In contrast to traditional VMs, the CVM shares resources that already exist in the host OS. In traditional (host) VMs, resources are virtualized for every (guest) OS. The CVM gives both OSs complete control of the host machine while the traditional VM sets every guest OS in an unprivileged state to access the real machine.Unlike in other Linux virtualization solutions such as User Mode Linux (or the forementioned VMware), special driver software on the host operating system is used to execute the coLinux kernel in a privileged mode (known as ring 0 or supervisor mode). By constantly switching the machine's state between the host OS state and and the coLinux kernel state, coLinux is given full control of the physical machine's MMU (i.e, paging and protection) in its own specially allocated address space, and is able to act just like a native kernel, achieving almost the same performance and functionality that can be expected from a regular Linux which could have ran on the same machine standalone. Since coLinux uses the same binary format for user-space executables as native Linux, coLinux can load and run an existing unmodified Linux distribution concurrently with the host OS.


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http://en.wikipediawiki/Cooperative_Linux
http://colinux