Memory management in Operating system . Reference by Operating System Galvin

1. Memory Management
UMESHCHANDRA YADAV
ROLL NO.1783910909
2nd
year CSE
Department of Computer Science & Engineering
RAJKIYA ENGINEERING COLLEGE ,KANNAUJ
U.P. 209732

2. CONTENT
1.INTRODUCTION
2.MEMORY REALLOCATION
3.MEMORY MANGEMENT UNIT
4.SAWPPING
5.PAGGING
6.FRAGMENTATION
7.SEGMENTATION

3. INTRODUCTION
Memory management refers to management of main
memory address.
An Operating System activities for memory management
1.In multiprogramming, the OS decides which process will get
memory when and how much.
2.Allocates memory when a process requests .
3.De-allocates memory when a process no longer needs it or
has been terminated.

4. A pair of base and limit registers define the logical address
space
Base and Limit Registers

5. Multistep Processing of a User Program

6. Compile time
Hardwire physical location at compile time(absolute code)
Problem
• Each program needs to be written with others in mind
• Not really transparent
Load time
Compiled code relocatable (relocatable code)
How? All addresses relative to a start address. Change start address
to relocate
Problem
• Once loaded, can’t change or move
Execution time
Can move during execution
This is what’s generally done, but need hardware support
When to Relocate?

7. Logical address – generated by the CPU; also
referred to as virtual address
Physical address – address seen by the memory
unit
Logical and physical addresses are the same in
compile-time and load-time address-binding
schemes.
Logical and physical addresses differ in execution-
time address-binding scheme
Logical vs. Physical Address Space

8. Contiguous Allocation
Multiple-partition allocation
Hole – block of available memory; holes of various size are scattered
throughout memory
When a process arrives, it is allocated memory from a hole large enough
to accommodate it
Operating system maintains information about:
a) allocated partitions b) free partitions (hole)
OS
process 5
process 8
process 2
OS
process 5
process 2
OS
process 5
process 2
OS
process 5
process 9
process 2
process 9
process 10

9. Dynamic Storage-Allocation Problem
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
How to satisfy a request of size n from a list of free holes

10. It’s hardware device that maps virtual
to physical address
In MMU scheme, the value in the
relocation register is added to every
address generated by a user process
at the time it is sent to memory
The user program deals with logical
addresses; it never sees the real
physical addresses
Memory-Management Unit (MMU)

11. SWAPPING
Swapping is a mechanism in which a process can be swapped
temporarily out of main memory (or move) to secondary storage and
make that memory available to other processes.

12. Paging
Logical address space of a process can be
noncontiguous; process is allocated physical memory
whenever the latter is available
Divide physical memory into fixed-sized blocks called
frames (size is power of 2, between 512 bytes and 8,192
bytes)
Divide logical memory into blocks of same size called
pages

13. Paging Hardware

14. 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
Reduce external fragmentation by compaction
Compaction is possible only if relocation is dynamic, and is done at
execution time

15. 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

16. Logical View of Segmentation
1
3
2
4
1
4
2
3
user space
physical memory space

17. 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;

18. Segmentation Hardware

19. THANK YOU