Embedded hypervisors, targeting embedded systems and certain real-time operating system (RTOS) environments, are designed with different requirements when compared to desktop and enterprise systems, including robustness, security and real-time capabilities. The resource-constrained nature of many embedded systems, especially battery-powered mobile systems, imposes a further requirement for small memory-size and low overhead. Finally, in contrast to the ubiquity of the x86 architecture in the PC world, the embedded world uses a wider variety of architectures and less standardized environments. Support for virtualization requires memory protection (in the form of a memory management unit or at least a memory protection unit) and a distinction between user mode and privileged mode, which rules out most microcontrollers. This still leaves x86, MIPS, ARM and PowerPC as widely deployed architectures on medium- to high-end embedded systems.