Operating System Questions Long
A page table is a data structure used in virtual memory management to map virtual addresses to physical addresses. It is a crucial component of the memory management unit (MMU) in an operating system.
In virtual memory management, the main goal is to provide each process with its own virtual address space, which is larger than the available physical memory. This allows multiple processes to run concurrently without the need for each process to have its own dedicated physical memory.
A page table is used to translate virtual addresses generated by a process into physical addresses. It acts as a lookup table that maps each virtual page number to its corresponding physical page frame number. The page table is typically stored in the main memory and is maintained by the operating system.
When a process generates a virtual address, the MMU uses the page table to translate it into a physical address. The virtual address is divided into a virtual page number and an offset within the page. The virtual page number is used as an index into the page table to retrieve the corresponding physical page frame number. The offset is then combined with the physical page frame number to form the final physical address.
If a process tries to access a virtual address that is not currently mapped in the page table, it results in a page fault. The operating system handles this by loading the required page from the secondary storage (such as a hard disk) into a free physical page frame and updating the page table accordingly. This process is known as demand paging and allows the operating system to efficiently manage the limited physical memory by only loading the necessary pages into memory when needed.
The page table also includes additional information for each entry, such as permission bits (read, write, execute), dirty bit (indicating if the page has been modified), and reference bit (indicating if the page has been accessed). These bits are used for various purposes, such as implementing memory protection, managing page replacement algorithms, and optimizing memory access.
In summary, a page table is a data structure used in virtual memory management to map virtual addresses to physical addresses. It allows the operating system to provide each process with its own virtual address space and efficiently manage the limited physical memory by dynamically loading pages from secondary storage when needed.