Silberschatz, Galvin and Gagne  Operating System Concepts Segmentation Memory-management scheme that supports user view of memory. A program is a collection of segments. A segment is a logical unit such as: main program, procedure, function, method, object, local variables, global variables, common block, stack, symbol table, arrays

Silberschatz, Galvin and Gagne  Operating System Concepts Logical View of Segmentation user spacephysical memory space

Silberschatz, Galvin and Gagne  Operating System Concepts Segment Tables Load segments separately into memory Each process has a segment table in memory that maps segment-numbers (table index) to physical addresses. Each entry has:  base – contains the starting physical address where the segments reside in memory.  limit – specifies the length of the segment.

Silberschatz, Galvin and Gagne  Operating System Concepts Segment Table Example

Silberschatz, Galvin and Gagne  Operating System Concepts Segmentation Addressing Logical addresses are a segment number and an offset within the segment

Silberschatz, Galvin and Gagne  Operating System Concepts Segmentation Registers Each process has a segment-table base register (STBR) and a segment-table length register (STLR) that are loaded into the MMU. STBR points to the segment table’s location in memory STLR indicates number of segments used by a program  Segment number s is legal if s < STLR

Silberschatz, Galvin and Gagne  Operating System Concepts Considerations Since segments vary in length, memory allocation is a dynamic storage-allocation problem. Allocation.  first fit/best fit  external fragmentation  (Back to the old problems :-) Sharing.  shared segments  same segment number

Silberschatz, Galvin and Gagne  Operating System Concepts Memory Protection Protection. With each entry in segment table associate:  validation bit = 0  illegal segment  read/write/execute privileges Protection bits associated with segments; code sharing occurs at segment level.

Silberschatz, Galvin and Gagne  Operating System Concepts Dynamic Linking Linking postponed until execution time. Small piece of code, stub, used to locate the appropriate memory-resident library routine. Stub replaces itself with the address of the routine, and executes the routine. Dynamic linking is particularly useful for libraries, which may be loaded in advance, or on demand  Also known as shared libraries  Permits update of system libraries without relinking I NEED TO LEARN MORE ABOUT THIS

Silberschatz, Galvin and Gagne  Operating System Concepts Dynamic Loading Routine is not loaded until it is called When loaded, address tables in the resident portion are updated Better memory-space utilization; unused routine is never loaded. Useful when large amounts of code are needed to handle infrequently occurring cases.

Silberschatz, Galvin and Gagne  Operating System Concepts Paging Divide physical memory into fixed-sized blocks called frames (size is power of 2, between 512 bytes and 8192 bytes). Divide logical memory into blocks of same size called pages. Keep track of all free frames. To run a program of size n pages, need to find n free frames and load program (for now, assume all pages are loaded).

Silberschatz, Galvin and Gagne  Operating System Concepts Logical View of Paging

Silberschatz, Galvin and Gagne  Operating System Concepts Page Tables Each process has a page table in memory that maps page numbers (table index) to physical addresses.

Silberschatz, Galvin and Gagne  Operating System Concepts Paging Addressing Logical addresses are a page number and an offset within the segment

Silberschatz, Galvin and Gagne  Operating System Concepts Page Table Example

Silberschatz, Galvin and Gagne  Operating System Concepts Paging Registers Each process has a page-table base register (PTBR) and a page-table length register (PTLR) that are loaded into the MMU PTBR points to the page table in memory. PTLR indicates number of page table entries

Silberschatz, Galvin and Gagne  Operating System Concepts Free Frames Before allocation After allocation

Silberschatz, Galvin and Gagne  Operating System Concepts Real Paging Examples Example from Geoff’s notes x 2  Multiply offsets by page table width = address width  WORK OUT DETAILS

Silberschatz, Galvin and Gagne  Operating System Concepts Memory Protection Can access only inside frames anyway PTLR protects against accessing non-existent, or not-in- use, parts of a page table. An alternative is to attach a valid-invalid bit to each entry in the page table:  “valid” indicates that the associated page is in the process’ logical address space, and is thus a legal page.  “invalid” indicates that the page is not in the process’ logical address space. Finer grained protection is achieved using rwx bits for each page

Silberschatz, Galvin and Gagne  Operating System Concepts Valid (v) or Invalid (i) Bit In A Page Table

Silberschatz, Galvin and Gagne  Operating System Concepts Relocation and Swapping Relocation requires update of page table Swapping is an extension of this

Silberschatz, Galvin and Gagne  Operating System Concepts TLBs In simple paging every data/instruction access requires two memory accesses. One for the page table and one for the data/instruction. The two memory access problem can be solved by the use of a special fast-lookup hardware cache called associative memory or translation look-aside buffers (TLBs) Associative memory – parallel search Address translation (A´, A´´)  If A´ is in associative register, get frame # out.  Otherwise get frame # from page table in memory Fast cache is an alternative Page #Frame #

Silberschatz, Galvin and Gagne  Operating System Concepts Paging Hardware With TLB

Silberschatz, Galvin and Gagne  Operating System Concepts TLB Performance Effective access time  Assume memory cycle time is X time unit  Associative lookup =  time unit  Hit ratio  – percentage of times that a page number is found in the associative registers  Effective Access Time (EAT) EAT = (X +  )  + (2X +  )(1 –  ) = 2X +  –  X  OK if  is small and  is large, e.g., 0.2 and 95% TLB Reach  The amount of memory accessible from the TLB.  TLB Reach = (TLB Size) X (Page Size)  Ideally, the working set of each process is stored in the TLB Examples  entry TLB  register TLB, claims 98% hit rate

Silberschatz, Galvin and Gagne  Operating System Concepts Considerations Fragmentation vs Page table size and TLB hits Page size selection  internal fragmentation => smaller pages  table size => larger pages (32 bit address = 4GB, 4KB frames => bytes = 4MB table!)  i K pages  up to 32K pages  Newer hardware tending to larger page sizes - to 16MB Multiple Page Sizes. This allows applications that require larger page sizes the opportunity to use them without an increase in fragmentation.  UltraSparc supports 8K, 64K, 512K, and 4M  Requires OS support

Silberschatz, Galvin and Gagne  Operating System Concepts Two-Level Paging Example A logical address (on 32-bit machine (4GB addresses) with 4K page size (12 bit offsets)) is divided into:  a page number consisting of 20 bits.  a page offset consisting of 12 bits pages, 4 bytes per entry => 4MB page table The page table is paged, the page number is further divided into:  a 10-bit page number.  a 10-bit page offset. Thus, a logical address is as follows: where p i is an index into the outer page table, and p 2 is the displacement within the page of the outer page table. page number page offset pipi p2p2 d 10 12

Silberschatz, Galvin and Gagne  Operating System Concepts Address-Translation Scheme Address-translation scheme for a two-level paging architecture  p 1 and p 2 may be found in a TLB Example from Geoff’s notes Examples  SPARC - 3 level hierarchy (32 bit)  level hierarchy (32 bit) Multiple memory accesses, up to 7 for 64 bit addressing

Silberschatz, Galvin and Gagne  Operating System Concepts Hashed Page Tables Common in address spaces > 32 bits. The page number is hashed into a page table. This page table contains a chain of elements hashing to the same location. Page numbers are compared in this chain More efficient than multi-level for small processes

Silberschatz, Galvin and Gagne  Operating System Concepts Inverted Page Table For very large virtual address spaces (see VM) One entry for each frame of memory. Entry consists of the virtual address of the page stored in that real memory location, with information about the process that owns that page, e.g., PID.  Virtual address may be duplicated, but PIDs are unique Decreases memory needed to store each page table, but increases time needed to search the table when a page reference occurs. Use hash table to limit the search to one — or at most a few — page-table entries. Used by IBM RT and PowerPC

Silberschatz, Galvin and Gagne  Operating System Concepts Inverted Page Table Architecture

Silberschatz, Galvin and Gagne  Operating System Concepts Shared Pages Shared code  One copy of read-only (reentrant) code shared among processes (i.e., text editors, compilers, window systems).  Shared code must appear in same location in the logical address space of all processes. Private code and data  Each process keeps a separate copy of the code and data.  The pages for the private code and data can appear anywhere in the logical address space.

Silberschatz, Galvin and Gagne  Operating System Concepts Shared Pages Example

Silberschatz, Galvin and Gagne  Operating System Concepts Segmentation with Paging – MULTICS The MULTICS system solved problems of external fragmentation and lengthy search times by paging the segments. Solution differs from pure segmentation in that the segment-table entry contains not the base address of the segment, but rather the base address of a page table for this segment.

Silberschatz, Galvin and Gagne  Operating System Concepts MULTICS Address Translation Scheme

Silberschatz, Galvin and Gagne  Operating System Concepts Intel Address Translation