This document provides an overview of principal (main) memory management techniques in operating systems. It discusses contiguous memory allocation, fragmentation, paging, page tables, and swapping. Contiguous memory allocation allocates each process to a single contiguous block, while paging allows non-contiguous allocation by dividing memory into pages. Page tables map logical to physical addresses. Swapping moves entire processes or individual pages between memory and disk.

1. COURSE NAME : OPERATING SYSTEM
COURSE CODE : 381CCS-3
CHAPTER 6 : PRINCIPAL (MAIN) MEMORY MANAGEMENT
SUBJECT COORDINATOR: DR. ANAND DEVADURAI
SUBJECT TEACHER: MRS. MARIYAM AYSHA BIVI
DEPARTMENT OF COMPUTER SCIENCE
KING KHALID UNIVERSITY
ABHA, KSA.
REFERENCE DETAIL FOR CHAPTER 6
Topic Text Reference Chapter No. Page No
Main(Principal)
Memory
Management
“Operating System
Concepts”, 10th Edition,
Abraham SilberSchatz,
Peter Baer Galvin,
Greg Gagne, Wiley, 2018
Chapter 9 349-378

2. 2
 Background
 Contiguous Memory Allocation
 Memory Protection
 Memory Allocation - Variable Partition (Multiple-partition) allocation
 Fragmentation
 Paging
 Structure of the Page Table
 Swapping
 Standard Swapping
 Swapping with Paging
 Swapping on Mobile Systems
CHAPTER 6 : MAIN (PRINCIPAL) MEMORY MANAGEMENT

3. 6.1 BACKGROUND
3
 Program must be brought (from disk) into memory and placed within a
process for it to be run
 Main memory and registers are only storage CPU can access directly Register
access in one CPU clock (or less)
 Main memory can take many cycles
 Cache sits between main memory and CPU registers Protection of memory
required to ensure correct operation
LOGICAL VS PHYSICAL ADDRESS SPACE
 Logical address (Virtual Address) – generated by the CPU
 Physical address – address seen by the memory unit
 Logical and physical addresses are
 Same in compile-time and load-time address-binding schemes;
 Differ in execution-time address-binding scheme
 Logical address space is the set of all logical addresses generated by a
program
 Physical address space is the set of all physical addresses generated by a
program

4. 6.2 CONTIGUOUS MEMORY ALLOCATION
4
 Main memory must support both OS and user processes.
 Limited resource, must allocate efficiently, Contiguous allocation is one method
 Main memory usually into two partitions:
• Resident operating system, usually held in low memory User processes then
held in high memory
• Each process contained in single contiguous section of memory
6.2.1 MEMORY PROTECTION
 Relocation registers(Base Register) used
to protect user processes from each other,
and from changing operating-system code
and data
 Relocation register: contains value of
smallest physical address
 Limit register: contains range of logical
addresses, each logical address must be
less than the limit register
 MMU maps logical address dynamically Figure 6.1 Hardware support for
relocation and limit registers

5. 6.2 CONTIGUOUS MEMORY ALLOCATION(CONT…)
5
6.2.2 MEMORY ALLOCATION - Variable Partition (Multiple-partition) allocation
 Hole: Block of available memory of various size are scattered throughout memory.
 When a process arrives, it is allocated memory from a hole large enough to
accommodate it
 Process exiting frees its partition, adjacent free partitions combined
 Operating system maintains information about:
a) allocated partitions b) free partitions (hole)
How to satisfy a request of size n from a list of free holes?
 First-fit: Allocate the first hole that is big enough
 Best-fit: Allocate the smallest hole that is big enough; must search entire list,
unless ordered by size
- Produces the smallest leftover hole
 Worst-fit: Allocate the largest hole; must also search entire list
- Produces the largest leftover hole
 First-fit and best-fit better than worst-fit in terms of speed and storage utilization
Figure 6.2 Variable Partition

6. 6.2 CONTIGUOUS MEMORY ALLOCATION(CONT…)
6
6.2.3 FRAGMENTATION
 External Fragmentation – total memory space exists to satisfy a request, but it is
not contiguous
 Internal Fragmentation – allocated memory may be slightly larger than
requested memory; this size difference is memory internal to a partition, but not
being used
 First fit analysis reveals that given N blocks allocated, 0.5 N blocks lost to
fragmentation
 Reduce external fragmentation by
 Compaction:
• Shuffle memory contents to place all free memory together in one large
block
• Compaction is possible only if relocation is dynamic, done at execution time
• The simplest compaction algorithm is to move all processes toward one end of
memory; all holes move in the other direction, producing one large hole of
available memory. This scheme can be expensive
 Paging
• Permit the logical address space of processes to be noncontiguous,
thus allowing a process to be allocated physical memory wherever
such memory is available.

7. 6.3 PAGING
7
 Divide physical memory into fixed-sized blocks called frames
- Size is power of 2, between 512 bytes and 16 Mbytes
 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
 Set up a page table to translate logical to physical addresses
Figure 6.3 Paging model of
logical and physical memory
Figure 6.4 Paging Example
Logical address: n = 2 and m = 4.
Using a page size of 4 bytes and a
physical memory of 32 bytes (8 pages)

8. 6.3 PAGING(CONT..)
8
 Address Translation Scheme
• Page number (p) – used as an index into a page table which contains base address of
each page in physical memory
• Page offset (d) – combined with base address to define the physical memory address
that is sent to the memory unit
 For given logical address space 2m and page size 2n
Figure 6.5 Paging Hardware
page number page offset
p d
m -n n

9. 6.4 STRUCTURE OF THE PAGE TABLE
9
6.4.1 HIERARCHICAL PAGE TABLE
 Break up the logical address space into multiple page tables
 We then page the page table
Figure 6.6 A Two level Page- table Scheme
A) Two-Level Paging
Example
 A logical address 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:
 p1 is an index into the outer page table,
p2 is the displacement within the page of
the inner page table
 Known as forward-mapped page table Figure 6.7 Address Translation Scheme

10. 6.4 STRUCTURE OF THE PAGE TABLE
10
6.4.1 HIERARCHICAL PAGE TABLE
Figure 6.8 Three level Paging Scheme
B) Three Level Paging

11. 6.4 STRUCTURE OF THE PAGE TABLE
11
6.4.2 HASHED PAGE TABLE
Figure 6.9 Hashed Page Table
 Common in address spaces > 32 bits
 The virtual page number is hashed into a page table
• This page table contains a chain of elements hashing to the same location
 Each element contains (1) the virtual page number (2) the value of the mapped
page frame (3) a pointer to the next element
 Virtual page numbers are compared in this chain searching for a match
• If a match is found, the corresponding physical frame is extracted

12. 6.4 STRUCTURE OF THE PAGE TABLE
12
6.4.3 INVERTED PAGE TABLE
Figure 6.10 Inverted Page Table
 Rather than each process having a page table and keeping track of all possible
logical pages, track all physical pages,
 One entry for each real page 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
 Decreases memory needed to store each page table, but increases time
needed to search the table when a page reference occurs

13. 6.5 SWAPPING
13
Figure 6.11 Standard swapping of two
processes using a disk as a backing store
 A process can be swapped temporarily out of memory to a backing store, and then
brought back into memory for continued execution
• Total physical memory space of processes can exceed physical memory
6.5.1 STANDARD SWAPPING
 Backing store – fast disk large enough to accommodate copies of all memory images for
all users; must provide direct access to these memory images
 Roll out, roll in – swapping variant used for priority-based scheduling algorithms; lower-
priority process is swapped out so higher-priority process can be loaded and executed
 Major part of swap time is transfer time; total transfer time is directly proportional to the
amount of memory swapped
 System maintains a ready queue of ready-to-run processes which have memory images
on disk

14. 6.5 SWAPPING
14
Figure 6.12 Swapping with Paging
6.5.2 SWAPPING WITH PAGING
 Linux and Windows use a variation of swapping in which pages of a process—
rather than an entire process—can be swapped.
 The term swapping refers to standard swapping.
 Paging refers to swapping with paging.
 A page out operation moves a page from memory to the backing store; the
reverse process is known as a page in

15. 6.5 SWAPPING
15
6.5.2 SWAPPING ON MOBILE SYSTEMS
 Not typically supported
• Flash memory based
 Small amount of space
 Limited number of write cycles
 Poor throughput between flash memory and CPU on mobile
platform
 Instead of Swapping it use other methods to free memory if low
• iOS asks apps to voluntarily relinquish allocated memory
 Read-only data thrown out and reloaded from flash if needed
 Failure to free can result in termination
• Android terminates apps if low free memory, but first writes
application state to flash for fast restart