Explain memory-mapped I/O and how peripheral registers are accessed.

Memory-mapped I/O maps peripheral registers to specific memory addresses, allowing software to control hardware through regular memory operations. Implementation: Peripheral registers appear at fixed addresses in memory map. Read/write operations use pointers cast to register addresses. Volatile keyword prevents compiler optimization. Structure overlays provide convenient register access. Example: #define GPIO_BASE 0x40020000. volatile uint32_t *GPIO_MODER = (uint32_t*)(GPIO_BASE + 0x00); Better approach using structures: typedef struct { volatile uint32_t MODER; volatile uint32_t OTYPER; ... } GPIO_TypeDef; GPIO_TypeDef *GPIOA = (GPIO_TypeDef*)GPIO_BASE; Advantages: Simple, uniform access method. No special instructions needed. Compiler can optimize nearby accesses. Note: Some architectures use port-mapped I/O with special instructions (x86 IN/OUT).