What is memory-mapped I/O and how does it work?

Memory-mapped I/O assigns peripheral registers to specific memory addresses, allowing access using standard load/store instructions. No special I/O instructions needed; the same instructions read memory and peripherals. Example: writing to address 0x40021000 may control GPIO. Advantages: simple programming model, works with C pointers, and compiler optimization applicable. Considerations: Use volatile keyword to prevent compiler optimization, consider memory barriers for correct ordering, and understand bus architecture. ARM uses memory-mapped I/O; contrast with port-mapped I/O (x86 in/out instructions).