Nothing to do with (local) DRAM, everything to do with PCI-e bus. Reading and writing to memory mapped I/O registers is how you send commands to peripherals, r

Nothing to do with (local) DRAM, everything to do with PCI-e bus. Reading and writing to memory mapped I/O registers is how you send commands to peripherals, read their status, etc. These registers are not really memory at all, but representation on various logic states on the peripheral. So what you write is often not what you read back. Note that I/O memory mapping has nothing to do with usermode memory mapping.