Memory-mapped I/O

Learn the memory-mapped I/O concept.

Memory-mapped I/O

Memory-mapped I/O (MMIO) maps hardware registers to memory addresses. A GPIO pin corresponds to a specific memory address; writing to that address changes the pin, reading gives the pin state.

In C this is done through a pointer: volatile unsigned int *reg = (volatile unsigned int *)0x40021000;. The volatile keyword is critical: it tells the compiler not to optimize this variable. The hardware can change the value at any time; caching would give wrong results. MMIO is the most fundamental hardware access method in embedded C.

Why is volatile important?

Without volatile, the compiler can cache a register read. For example, in while (*reg & 1) the compiler might read *reg once and store the value; even if the hardware register changes, the loop spins forever.

With volatile, the compiler re-reads the register every iteration. This is mandatory for interrupt flags and hardware status registers. In embedded C, every hardware register is declared volatile. This ensures the software always sees hardware changes. Remember: volatile does not provide thread safety; it only prevents optimization.

Exercise Simulate a GPIO register

Define static volatile unsigned int gpio_data = 0; (fake register). In int main(void) write to the register with gpio_data = 1;. Read and print with printf("GPIO: %d\n", gpio_data).

Show example solution

#include <stdio.h>

static volatile unsigned int gpio_data = 0;

int main(void) {
    gpio_data = 1;
    printf("GPIO: %d\n", gpio_data);
    return 0;
}

Test yourself

Complete the volatile keyword

____ unsigned int gpio_data = 0;

Show answer

volatile unsigned int gpio_data = 0;

Which qualifier is a hardware register declared with?
- static
- const
- volatile
- auto
Show answer

volatile — volatile is mandatory for hardware registers.
Match MMIO concept

volatile

No optimization
Register

Hardware address
Pointer

Address access
MMIO

Memory-mapped I/O

Next lesson: Interrupts

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