What is MMU?#

The Memory Management Unit (MMU) is a crucial component of a computer's architecture that handles memory management tasks. It acts as an interface between the CPU and the main memory (RAM), ensuring efficient and secure access to memory resources. The MMU is responsible for translating virtual addresses used by programs into physical addresses in RAM, enabling processes to run in their own isolated memory spaces.

Address Translation#

The MMU performs address translation, converting virtual addresses generated by the CPU into physical addresses in RAM. This allows each process to have its own virtual address space, preventing memory conflicts between processes.

// Example of address translation
#include <stdio.h>
int main() {
    int virtual_address = 0x1234; // Example virtual address
    int physical_address = translate_address(virtual_address); // MMU translates to physical address
    printf("Physical Address: %x\n", physical_address);
    return 0;
}

Memory Protection#

The MMU enforces memory protection by ensuring that processes can only access their allocated memory regions. This prevents unauthorized access to memory areas used by other processes, enhancing system stability and security.

// Example of memory protection
#include <stdio.h>
int main() {
    int *ptr = (int *)0x5678; // Attempt to access a protected memory address
    if (is_protected(ptr)) {
        printf("Access denied: Protected memory area.\n");
    } else {
        printf("Access granted: Safe memory area.\n");
    }
    return 0;
}

Memory Management#

The MMU manages memory allocation and deallocation for processes. It keeps track of free and used memory blocks, ensuring efficient utilization of RAM.

// Example of memory management
#include <stdio.h>
#include <stdlib.h>
int main() {
    int *ptr = (int *)malloc(sizeof(int) * 10); // Allocate memory for an array of 10 integers
    if (ptr == NULL) {
        printf("Memory allocation failed.\n");
        return 1;
    }
    // Use the allocated memory
    for (int i = 0; i < 10; i++) {
        ptr[i] = i;
    }
    // Free the allocated memory
    free(ptr);
    return 0;
}

Paging#

Paging is a memory management scheme that eliminates the need for contiguous allocation of physical memory. It divides the virtual address space into fixed-size pages and maps them to physical memory frames. This allows processes to use non-contiguous memory, improving memory utilization and reducing fragmentation.

// Example of paging
#include <stdio.h>
#include <stdlib.h>
 
int main() {
    int page_size = 4096; // Example page size (4 KB)
    int num_pages = 10; // Number of pages to allocate
    int *pages = (int *)malloc(page_size * num_pages); // Allocate memory for pages
    if (pages == NULL) {
        printf("Memory allocation failed.\n");
        return 1;
    }
    // Use the allocated pages
    for (int i = 0; i < num_pages; i++) {
        pages[i] = i;
    }
    // Free the allocated pages
    free(pages);
    return 0;
}

Translation Lookaside Buffer (TLB)#

The Translation Lookaside Buffer (TLB) is a cache used by the MMU to speed up address translation. It stores recently used virtual-to-physical address mappings, reducing the time required for address translation.

The MMU is a fundamental component of modern computer systems, playing a vital role in memory management, address translation, and ensuring the security and stability of the operating system. Understanding how the MMU works is essential for anyone interested in computer architecture and operating systems.

Memory-Mapped I/O#

Memory-mapped I/O is a method used by the MMU to map hardware device registers into the address space of a process. This allows the CPU to interact with hardware devices using standard memory access instructions.

// Example of memory-mapped I/O
#include <stdio.h>
#include <sys/mman.h>
#include <fcntl.h>
#include <unistd.h>
int main() {
    int fd = open("/dev/mem", O_RDWR | O_SYNC);
    if (fd < 0) {
        perror("Failed to open /dev/mem");
        return 1;
    }
 
    // Map a physical address to a virtual address
    void *mapped_base = mmap(NULL, getpagesize(), PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0x10000000);
    if (mapped_base == MAP_FAILED) {
        perror("Failed to map memory");
        close(fd);
        return 1;
    }
 
    // Access the mapped memory
    volatile unsigned int *reg = (volatile unsigned int *)mapped_base;
    *reg = 0x12345678; // Write to the hardware register
 
    // Unmap the memory
    munmap(mapped_base, getpagesize());
    close(fd);
    return 0;
}

Conclusion#

The Memory Management Unit (MMU) is a critical component of modern computer systems, responsible for managing memory efficiently and securely. It enables address translation, memory protection, and memory management, ensuring that processes can run smoothly without interfering with each other. Understanding the MMU is essential for anyone interested in computer architecture and operating systems.

What is MMU?