I/O (Input Output)

In order to control peripheral devices from software, software needs to access I/O area. As for accessing I/O area, CPU use either memmory mapped I/O and port mapped I/O.

Memory Mapped I/O	Port Mapped I/O
Memory and Device share same address bus	Different address space
Memory Read/Write to access device	Special instruction to access device(IN/OUT)
Most widely used I/O method	was popular x86 old system

source code of RPi.GPIO (c_gpio.c)

Sample of memory mapped I/O.

void output_gpio(int gpio, int value)
{
    int offset, shift;

    if (value) // value == HIGH
        offset = SET_OFFSET + (gpio/32);
    else       // value == LOW
       offset = CLR_OFFSET + (gpio/32);

    shift = (gpio%32);

    *(gpio_map+offset) = 1 << shift;
}

int input_gpio(int gpio)
{
   int offset, value, mask;

   offset = PINLEVEL_OFFSET + (gpio/32);
   mask = (1 << gpio%32);
   value = *(gpio_map+offset) & mask;
   return value;
}