3. Virtual machine

Operating systems provide a virtualization layer between hardware and applications, but since an operating system does not virtualize the processor, a traditional application can only run on a single processor and a single system. The rise of distributed applications requires code to interact in a heterogeneous environment, and many applications require code to be migrated from one machine to another, be they web applications such as applets or javascript, mobile agent-based applications, or distributed applications that need to load call stubs for distributed objects. Since compiled code can only run on a single processor and a single system, it is impossible to migrate this code to a heterogeneous environment. To solve this problem, a processor virtualization layer is introduced. This can emulate a real machine, as in the case of concrete virtual machines (Qemu, Bochs, VirtualBox or VirtualPC),...