Operating System - Memory Management Part 3 - Notes, Study notes of Operating Systems

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MEMORY MANAGEMENT

Memory Protection

• (^) Memory protection implemented by associating protection bit with each frame
• (^) Valid-invalid bit attached 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

Hierarchical Page Tables

• (^) Break up the logical address space into multiple

page tables

• (^) A simple technique is a two-level page table

Two-Level Paging Example

• (^) A logical address (on 32-bit machine with 4K page size) is divided into:
  • (^) a page number consisting of 20 bits
  • (^) a page offset consisting of 12 bits
• (^) Since 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 pi 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 pi p 2 d 10 10 12

• Address-translation scheme

for a two-level 32-bit paging

architecture

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

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

User’s

View of a

Program

Segmentation Architecture

• (^) Logical address consists of a two tuple: <segment-number, offset>,
• (^) Segment table – maps two-dimensional physical addresses; each table entry has: - (^) base – contains the starting physical address where the segments reside in memory - (^) limit – specifies the length of the segment
• (^) Segment-table base register (STBR) points to the segment table’s location in memory
• (^) Segment-table length register (STLR) indicates number of segments used by a program; segment number s is legal if s < STLR

Segmentation Architecture

(Cont.)

• (^) Relocation.
  • (^) dynamic
  • (^) by segment table
• (^) Sharing.
  • (^) shared segments
  • (^) same segment number
• (^) Allocation.
  • (^) first fit/best fit
  • (^) external fragmentation

Address

Translation

Architecture

Example of

Segmentation

Virtual Memory

• (^) Background
• (^) Demand Paging
• (^) Process Creation
• (^) Page Replacement
• (^) Allocation of Frames
• (^) Thrashing
• (^) Demand Segmentation
• (^) Operating System Examples

Background

• (^) Virtual memory – separation of user logical memory from physical memory. - (^) Only part of the program needs to be in memory for execution. - (^) Logical address space can therefore be much larger than physical address space. - (^) Allows address spaces to be shared by several processes. - (^) Allows for more efficient process creation.