Memory Mapped IO in ARM: What It Is and How It Works

In ARM architecture, memory mapped IO is a method where hardware devices are controlled by reading and writing to specific memory addresses. These addresses correspond to device registers, allowing the CPU to interact with peripherals as if they were normal memory locations.

How It Works

Memory mapped IO works by assigning certain memory addresses to hardware devices instead of regular RAM. When the ARM processor reads from or writes to these addresses, it is actually communicating with the device connected to that address.

Think of it like a mailbox system: each device has a special mailbox (memory address). When the CPU wants to send a message or get information, it puts or takes data from that mailbox. This way, the CPU uses the same instructions it uses for memory to control devices, making the process simple and efficient.

Example

This example shows how an ARM processor might turn on an LED connected to a device register at a specific memory address.

c

volatile unsigned int *LED_CONTROL = (volatile unsigned int *)0x40021018;

void turn_on_led() {
    *LED_CONTROL = 1;  // Write 1 to the device register to turn on the LED
}

int main() {
    turn_on_led();
    return 0;
}

Output

No output on console; LED turns on physically

When to Use

Memory mapped IO is used when the CPU needs to control or get data from hardware devices like sensors, displays, or communication ports. It is common in embedded systems, microcontrollers, and ARM-based devices where direct and fast hardware access is required.

For example, an ARM processor in a smartphone uses memory mapped IO to control the touchscreen, camera, or network chip by reading and writing to their registers.

Key Points

• Memory mapped IO uses normal memory addresses to access hardware devices.
• The CPU reads/writes device registers as if they were memory locations.
• This method simplifies hardware control by using standard instructions.
• Commonly used in embedded ARM systems for fast and direct device communication.

Key Takeaways

Memory mapped IO lets ARM CPUs control devices by reading and writing to special memory addresses.

It treats device registers like normal memory, simplifying hardware communication.

This method is widely used in embedded systems for efficient and direct device access.

Using memory mapped IO allows fast control of peripherals like LEDs, sensors, and communication modules.