MMIO – Memory Mapped IO. Memory is accessed through a computer’s address registers. MMIO uses one address register to access main memory and memory on peripheral controllers. In a 32-bit Windows 10 operating system, the address register can access a 4GB  memory address space. Up to 3GB of that is set aside for main memory and used for programs and data. The remaining 3GB to 4GB memory range is used for memory on controller cards. This memory is used for data transfer and peripheral processing.