One key feature of Tock is the ability to load and run multiple applications simultaneously. In a modern computer, the OS uses the Memory Management Unit (MMU) to provide a virtual address space for each process. The code for each process is written assuming that it is the only code in the world and that it can place memory at any address it pleases. The MMU handles translating these virtual addresses into a physical address in real memory, which is shared between all processes. Unfortunately, in the world of embedded systems MMUs are not available. Processors like the ARM Cortex-M series omit them since they are power and area-hungry.