Chapter 3.2 : Virtual Memory What is virtual memory? Virtual memory management schemes Paging Segmentation Segmentation with paging Page table management Ceng 334 - Operating Systems

Problems with Memory Management Techniques so far 1. Unused (wasted) memory due to fragmentation 2. Memory may contain parts of program which are not used during a run (ie., some routines may not be accessed in that particular run) 3. Process size is limited with the size of physical memory Ceng 334 - Operating Systems

Virtual memory of process on disk Ceng 334 - Operating Systems Virtual Memory (VM) Virtual memory of process on disk Map (translate) virtual address to real Real memory of system Ceng 334 - Operating Systems

Ceng 334 - Operating Systems Virtual Memory (VM) VM is conceptual It is constructed on disk Size of VM is not limited (usually larger than real memory) All process addresses refer to the VM image When the process executes all VM addresses are mapped on to real memory Ceng 334 - Operating Systems

Did We Solve the “Problems”? 1. Unused (wasted) memory due to fragmentation (We’ll see!) 2. Memory may contain parts of program which are not used during a run YES! Virtual memory contents are loaded into memory on demand 3. Process size is limited with the size of physical memory YES! Process size can be larger than real memory Ceng 334 - Operating Systems

Ceng 334 - Operating Systems (Pure) Paging Virtual and real memory are divided into fixed sized pages Programs are divided into pages A process address (both in virtual & real memory) has two components Page # Offset within page Ceng 334 - Operating Systems

Interpretation of an Address 16 bits for addressing means that the memory addressed is 64K bytes (0000 -FFFF) Suppose page size is 4K bytes (12 bits) The virtual memory has 16 pages (4 bits) Real memory can have at the most 16 pages Example : address : 7 F B C 0111 1111 1011 1100 Page # Offset Ceng 334 - Operating Systems

The relation between virtual addresses and physical memory addresses 64K Virtual Memory 32K Real Memory Ceng 334 - Operating Systems

Ceng 334 - Operating Systems Paging (Cont.) When process pages are transferred from VM to real memory, page numbers must be mapped from virtual to real memory addresses This mapping is done by software & hardware When the process is started only the first page (main) is loaded. Other pages are loaded on demand Ceng 334 - Operating Systems

Virtual Memory – Memory Management Unit The position and function of the MMU Ceng 334 - Operating Systems

Ceng 334 - Operating Systems Internal operation of MMU with 16 4 KB pages Note that virtual address is 16 bits (64K virtual memory) but the physical address is 15 bits (32K real memory) Ceng 334 - Operating Systems

Ceng 334 - Operating Systems Page Tables Page Table Page frame # Page protection Reference bit Modification bit Validity bit Page Table Entry 1 8 2 3 4 5 6 7 Index of page table is the virtual page # Ceng 334 - Operating Systems

Page Table Entry Fields Validity bit is set when the page is in memory Reference bit is set set by the hardware whenever the page is referred Modified bit is set whenever the page is modified Page-protection bits set the access rights (eg., read, write restrictions) Ceng 334 - Operating Systems

Address Mapping in Paging Page tables of processes PTn PT 1 PT2 . . Page # Offset within page Page Table Register + Catenate Virtual memory address Real memory address Ceng 334 - Operating Systems

Address Mapping in Paging (Cont.) During the execution every page reference is checked against the page map table entry If the validity bit is set (ie., page is in memory) execution continues If the page is not in memory a page fault (interrupt - trap) occurs and the page is fetched into memory If the memory is full, pages are written back using a page replacement algorithm Ceng 334 - Operating Systems

Memory Management Problems : Re-visit due to paging 1. Unused (wasted) memory due to fragmentation ONLY on the last page (Page Break) 2. Memory may contain parts of program which are not used during a run Virtual memory contents are loaded into memory on demand 3. Process size is limited with the size of physical memory Process size can be larger than real memory Furthermore, program does not occupy contiguous locations in memory (virtual pages are scattered in real memory) Ceng 334 - Operating Systems

Ceng 334 - Operating Systems Segmentation Pages are fixed in size, segments are variable sized A segment can be a logical entity such as Main program Some routines Data of program File Stack Ceng 334 - Operating Systems

Ceng 334 - Operating Systems Segmentation (Cont.) Process addresses are now in the form Segment # Offset within segment Segment Map Table has one entry for each segment and each entry consist of Segment number Physical segment starting address Segment length Ceng 334 - Operating Systems

Ceng 334 - Operating Systems Segmentation (1) One-dimensional address space with growing tables One table may bump into another Ceng 334 - Operating Systems

Ceng 334 - Operating Systems Segmentation (2) Allows each table to grow or shrink, independently Ceng 334 - Operating Systems

Ceng 334 - Operating Systems Segmentation (3) Comparison of paging and segmentation Ceng 334 - Operating Systems

Implementation of Pure Segmentation (a)-(d) Development of fragmentation (e) Removal of the fragmentation by compaction Ceng 334 - Operating Systems

Problems with Segmentation Similar problems in dynamic partitioning Fragmentation in real memory Relocation is necessary for compaction Ceng 334 - Operating Systems

Segmentation with Paging Segmentation in virtual memory, paging in real memory A segment is composed of pages An address has three components Page # Offset within page Segment # The real memory contains only the demanded pages of a segment, not the full segment Ceng 334 - Operating Systems

Addressing in Segmentation with Paging Segment Table Page Tables Pages Page # Offset within page Segment # Ceng 334 - Operating Systems

Segmentation with Paging: MULTICS (1) Descriptor segment points to page tables Segment descriptor – numbers are field lengths Ceng 334 - Operating Systems

Segmentation with Paging: MULTICS (2) A 34-bit MULTICS virtual address Ceng 334 - Operating Systems

Segmentation with Paging: MULTICS (3) Conversion of a 2-part MULTICS address into a main memory address Ceng 334 - Operating Systems

Segmentation with Paging: MULTICS (4) Simplified version of the MULTICS TLB Existence of 2 page sizes makes actual TLB more complicated Ceng 334 - Operating Systems

Segmentation with Paging: Pentium (1) A Pentium selector Ceng 334 - Operating Systems

Segmentation with Paging: Pentium (2) Pentium code segment descriptor Data segments differ slightly Ceng 334 - Operating Systems

Segmentation with Paging: Pentium (3) Conversion of a (selector, offset) pair to a linear address Ceng 334 - Operating Systems

Segmentation with Paging: Pentium (4) Mapping of a linear address onto a physical address Ceng 334 - Operating Systems

Segmentation with Paging: Pentium (5) Level Protection on the Pentium Ceng 334 - Operating Systems

Ceng 334 - Operating Systems How Big is a Page Table? Consider a full 2 32 byte (4GB) address space Assume 4096 byte (2 12 byte) pages 4 bytes per page table entry The page table has 2 32/2 12 (= 2 20 ) entries (one for each page) Page table size would be 2 22 bytes (or 4 megabytes) Ceng 334 - Operating Systems

Problems with Direct Mapping? Although a page table is of variable length depending on the size of process, we can not keep them in registers Page table must be in memory for fast access Since a page table can be very large (4MB), page tables are stored in virtual memory and be subjected to paging like process pages Ceng 334 - Operating Systems

Ceng 334 - Operating Systems How to Solve? Two-level Lookup Inverted Page Tables Translation Lookaside Buffers Ceng 334 - Operating Systems

Virtual Address - 32 bits (4 GB) Two-Level Lookup 12 bits - 4096 Byte pages 10 bits - 1024 pages 10 bits - 1024 directories Virtual Address - 32 bits (4 GB) Directory Page Offset Ceng 334 - Operating Systems

Two-Level Lookup (Cont.) Dir Page Offset + Physical Address Page Directory Page Table Ceng 334 - Operating Systems

Two-Level Lookup (Cont.) A process is represented by one or more entries of the page directory (ie., several page tables) Typically a page table size is set as one page which makes swapping easier This is one of the addressing schemes used by Intel family of chips (Intel chips can use paging, segmentation or a combination of both) Ceng 334 - Operating Systems

Two-Level Lookup (Cont.) Ceng 334 - Operating Systems Second-level page tables Top-level page table 32 bit address with 2 page table fields Two-level page tables Ceng 334 - Operating Systems

Ceng 334 - Operating Systems Inverted Page Tables Page # PID Page # Offset within page Virtual Address 21 17 hash Hash Table 76 23 Page # 101 Process # 17 Hash # 2 3 213 32 Page Frame Table Ceng 334 - Operating Systems

Inverted Page Tables (Cont.) The virtual page number is hashed to point to a hash table The hash table contains a pointer to the inverted page table Inverted page table contains page table entries (one for each real memory page) Entries having the same hash codes are chained Ceng 334 - Operating Systems

Inverted Page Tables (Cont.) A fixed portion of memory is used for mapping regardless of the number of processes or virtual pages This approach is used by IBM’s AS/400 and RISC System/6000 computers Ceng 334 - Operating Systems

Translation Lookaside Buffer Frame # Offset TLB Hit TLB Miss TLB Page Table Page # Virtual Address Real Address Page Fault Ceng 334 - Operating Systems

Translation Lookaside Buffer (Cont.) Translation lookaside buffer (TLB) is a cache for page table entries TLB contains page table entries that have been most recently used Whenever the page table is referred (TLB miss), the page table entry is also copied to the TLB TLB is usually an associative memory (content addressable memory) Ceng 334 - Operating Systems

TLBs – Translation Lookaside Buffers A TLB to speed up paging Ceng 334 - Operating Systems