What is an MMU?
A Memory Management Unit (MMU) is part of complex processor CPUs and controls the access of the operating system (kernel) and applications to the main memory. The term "memory" refers to all locations that can be addressed by the CPU and may comprise Read Only Memory (ROM), Random Access Memory (RAM) as well as I/O memory. The MMU provides virtual memory that gives the illusion of a large address space, dedicated to a single process. Those address spaces are usually unique to each process, however their address ranges are typically overlapping. Each chunk of virtual memory (called page) can be mapped to a page in the linear physical memory. While two process may have overlapping virtual memory areas, their mapping to the physical address space is usually distinct. This allows the implementation of complex operating systems with multiple independent processes.
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What is an MPU?
In less complex CPUs you will often find a Memory Protection Unit (MPU). This significantly simplifies the chip design, however complex operating systems (such as Linux) cannot be executed. In this case there is exactly one linear physical address space visible to the operating systems as well as all applications and as a consequence, all processes share the same address space. However, read/write access can be explicitly granted to processes by means of MPU regions. The amount of memory regions is limited. One region can cover a contiguous memory range, depending on the architecture the start address and the size of a region might have different alignment restrictions, for example on arm_v8r it must be aligned to 64 bytes, for arm_v7r it must be naturally aligned to power of two.
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