I/O-mapped memory refers to memory addresses that are beyond the end of main memory and are instead mapped to I/O controllers. Each controller is assigned a specific range of addresses, typically configured at boot time.

When the CPU performs load or store instructions to I/O-mapped memory addresses, these operations are translated into messages sent across the I/O bus to the corresponding controller. This mechanism allows the CPU to communicate with I/O devices using the same instructions it uses for regular memory access.

For example, an address like 0x80000000 might be mapped to a disk controller's control registers, allowing the CPU to send commands by writing to that address.