MMU vs MPU in ARM: Key Differences and Usage Guide

In ARM architecture, a MMU (Memory Management Unit) manages virtual memory and supports complex features like paging and address translation, while a MPU (Memory Protection Unit) provides simpler memory protection without virtual memory support. The MMU enables operating systems to use virtual memory, whereas the MPU is used mainly for basic memory access control in embedded systems.

Quick Comparison

This table summarizes the main differences between MMU and MPU in ARM processors.

Feature	MMU (Memory Management Unit)	MPU (Memory Protection Unit)
Memory Management	Supports virtual memory with address translation	No virtual memory support, works with physical addresses
Protection Granularity	Page-level protection (e.g., 4KB pages)	Region-based protection with configurable sizes
Complexity	Complex hardware, supports paging and caching	Simpler hardware, focused on access permissions
Use Case	Used in systems running full OS like Linux	Used in embedded systems without OS or with RTOS
Address Translation	Translates virtual addresses to physical addresses	No address translation, only access control
Performance Impact	Can slow down due to translation overhead	Minimal overhead, faster access checks

Key Differences

The MMU in ARM processors is designed to handle virtual memory by translating virtual addresses used by software into physical addresses used by hardware. This allows operating systems to provide features like memory paging, swapping, and isolation between processes. The MMU supports complex memory management schemes and caching policies, making it essential for running full-featured operating systems such as Linux or Windows.

On the other hand, the MPU provides a simpler form of memory protection without virtual memory. It divides the physical memory into regions and sets access permissions like read, write, or execute for each region. This helps prevent accidental or malicious access to critical memory areas in embedded systems or real-time operating systems (RTOS) that do not require virtual memory.

In summary, the MMU is about managing and translating memory addresses for multitasking OS environments, while the MPU focuses on protecting memory regions in simpler or resource-constrained systems.

Code Comparison

Example of configuring an MMU region in ARM assembly-like pseudocode to enable virtual memory mapping:

c

/* MMU region setup pseudocode */
#define REGION_SIZE_4KB 0x1000

void configure_mmu_region(unsigned int virtual_addr, unsigned int physical_addr) {
    // Set translation table entry
    translation_table[virtual_addr >> 12] = (physical_addr & 0xFFFFF000) | ACCESS_FLAGS;
    // Enable MMU
    enable_mmu();
}

// Usage
configure_mmu_region(0x80000000, 0x00000000);

Output

MMU enabled with virtual address 0x80000000 mapped to physical address 0x00000000

MPU Equivalent

Example of configuring an MPU region in ARM Cortex-M style C code to protect a memory region:

c

/* MPU region setup example for ARM Cortex-M */
#include "core_cm4.h"  // CMSIS header for MPU

void configure_mpu_region(void) {
    MPU->RNR = 0; // Select region 0
    MPU->RBAR = 0x20000000; // Base address of region
    MPU->RASR = (0x04 << 1) | // Size = 32 bytes (example)
                 (0x03 << 24) | // Full access permissions
                 (1 << 0);     // Enable region
    MPU->CTRL = 1; // Enable MPU
}

// Usage
configure_mpu_region();

Output

MPU enabled with region 0 protecting address 0x20000000 with full access

When to Use Which

Choose MMU when you need virtual memory support, complex memory management, and running full operating systems like Linux or Android. It is essential for multitasking, memory isolation, and advanced caching.

Choose MPU when working with embedded systems or real-time operating systems that do not require virtual memory but need basic memory protection to prevent accidental access or corruption. MPU is simpler, faster, and uses less power, making it ideal for microcontrollers.

Key Takeaways

MMU supports virtual memory and address translation; MPU only provides memory protection without translation.

Use MMU for full OS environments needing complex memory management.

Use MPU for embedded or real-time systems requiring simple access control.

MMU is more complex and can impact performance; MPU is simpler and faster.

MPU protects physical memory regions; MMU manages virtual to physical address mapping.