# Virtual Memory Implementation Virtual memory is an abstraction of the main memory. It allows programs to be executed in a space larger than the actual physical memory, or RAM, by using disk storage as a backup. ## Paging The first step in implementing virtual memory is to divide physical memory into fixed-size blocks, called frames, and virtual memory into blocks of the same size, called pages. When a program needs to access a memory location, it references the virtual address. The operating system translates this to a physical address. ```c unsigned long virtual_address = 0x12345; unsigned long page_number = virtual_address / PAGE_SIZE; unsigned long offset = virtual_address % PAGE_SIZE; ``` ## Page Tables A page table is used to map virtual pages to physical frames. Each entry in the table contains the frame number associated with a page and flags to control access to the page (e.g., whether it's readable, writable, executable, etc.). ```c struct page_table_entry { unsigned long frame_number; unsigned int flags; }; ``` ## Page Faults If the page table entry for a virtual page is not in physical memory (a condition known as a page fault), the operating system must swap it in from disk. This involves finding a free frame, reading the page from disk into that frame, and updating the page table. ```c void handle_page_fault(unsigned long page_number) { struct page_table_entry *pte = &page_table[page_number]; unsigned long frame_number = find_free_frame(); read_page_from_disk(pte->frame_number, frame_number); pte->frame_number = frame_number; pte->flags |= PAGE_PRESENT; } ``` ## TLB To speed up the translation from virtual addresses to physical addresses, a Translation Lookaside Buffer (TLB) is used. This is a small cache that stores recent translations and can be accessed much faster than the main page table. ```c unsigned long translate_virtual_address(unsigned long virtual_address) { unsigned long page_number = virtual_address / PAGE_SIZE; unsigned long offset = virtual_address % PAGE_SIZE; struct tlb_entry *entry = lookup_tlb(page_number); if (entry) { return (entry->frame_number * PAGE_SIZE) + offset; } else { struct page_table_entry *pte = &page_table[page_number]; add_to_tlb(pte->frame_number, page_number); return (pte->frame_number * PAGE_SIZE) + offset; } } ``` This is a brief overview of how virtual memory is implemented at a high level. It's a complex system that involves many other components, such as the CPU, the operating system, and the disk subsystem.