Understanding memory management through paging mechanisms
Paging is a memory management scheme that eliminates the need for contiguous allocation of physical memory, thereby reducing issues like fragmentation. It divides the virtual memory into blocks of physical memory called "pages," which are typically of fixed size (e.g., 4KB).
The main components involved in paging are the page table, the Translation Lookaside Buffer (TLB), and the physical memory.
Pages can be placed anywhere in physical memory
Typically 4KB in size for efficient management
Virtual addresses mapped to physical addresses
Definition: A data structure used to map virtual addresses to physical addresses.
Function: Each process has its own page table, which keeps track of the frame number corresponding to each page number.
Structure: Contains entries that include the frame number and status bits (e.g., valid/invalid bit, access permissions, dirty bit).
Definition: A cache that stores recent page table entries.
Function: Reduces the time taken to access the page table by caching recent translations of virtual addresses to physical addresses.
Structure: A small, fast memory structure within the MMU.
Definition: The hardware component responsible for handling all memory and caching operations, including paging.
Function: Translates virtual addresses to physical addresses using the page table and TLB.
Definition: The hardware where data and instructions are stored.
Function: Stores the actual data corresponding to the virtual pages.
The CPU generates a virtual address that needs to be translated to a physical address.
The MMU first checks the TLB to see if the translation is cached.
TLB Hit: If found, the physical address is quickly retrieved, and the memory access proceeds.
TLB Miss: If not found, the MMU accesses the page table.
The MMU uses the virtual page number to index into the page table and retrieve the corresponding frame number.
The virtual address is converted into a physical address using the frame number obtained from the page table.
The physical address is used to access the desired memory location.
The virtual address consists of a virtual page number (VPN) and an offset within that page.
Example: If the virtual address is 0x1234 and the page size is 4KB, the VPN might be 0x1 and the offset 0x234.
The TLB is checked for an entry matching the VPN.
TLB Hit: If an entry is found, it provides the corresponding frame number.
TLB Miss: If no entry is found, the MMU must access the page table.
The VPN is used to index into the page table.
The page table entry (PTE) contains the frame number and status bits.
Valid PTE: If the PTE is marked valid, the frame number is used for address translation.
Invalid PTE: If the PTE is invalid (e.g., page not in memory), a page fault occurs, and the operating system must handle it by loading the page from disk into memory.
The physical address is formed by combining the frame number from the PTE with the offset from the virtual address.
Example: If the frame number is 0x2 and the offset is 0x234, the physical address is 0x2234.
The physical address is used to access the desired data in the RAM.
Here is a simplified block diagram of paging hardware:
Generates virtual address (VPN + Offset)
Checks TLB for translation
Maps VPN to frame number (if TLB miss)
Accessed using physical address (Frame + Offset)
Enhances performance by caching recent address translations
Maps virtual pages to physical frames, with entries containing frame numbers and status bits
Manages the entire address translation process, utilizing the TLB and page table
Allows non-contiguous memory allocation, reducing fragmentation
Ensures processes cannot access each other's memory
TLB and page table structures improve address translation speed
| Benefit | Description |
|---|---|
| Non-contiguous Allocation | Pages can be placed anywhere in physical memory, eliminating fragmentation issues |
| Virtual Memory | Enables running applications larger than physical memory through page swapping |
| Memory Protection | Prevents unauthorized access to memory regions through permission bits |
| Efficient Translation |