I/O space vs memory-mapped

The x86_64 architecture has two distinct address spaces:

The processor asserts a hardware line to the external bus to indicate which address space is being referenced. The x86 has special instructions to deal with I/O space (e.g., IN AL, DX vs MOV AL,  address). Common hardware design on an x86 indicates that the control ports for peripherals live in the I/O address space. On non-x86 platforms, this requirement doesn't exist—all peripheral devices are mapped into various locations within the same address space as the instruction and code memory.