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Operating%20Systems

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Title: Operating%20Systems


1
Operating Systems
  • Memory management

2
Memory Management
  • List of Topics
  • 1. Memory Management
  • 2. Memory In Systems Design
  • 3. Binding Times
  • 4. Introduction to Memory Management
  • 5. Raw Memory Model
  • 6. Single User Contiguous Memory
  • 7. Relocation - Why and How
  • 8. Overlay Management

3
  • Topics (continued)
  • 9. Protection
  • 10. Fixed Partitions
  • 11. Nonuniform Sized Fixed Partitions
  • 12. Uniformly Sized Fixed Partitions
  • 13. Simple Paging
  • 14. Benefits of Simple Paging
  • 15. Page Tables
  • 16. Translation Lookaside Buffers
  • 17. Hierarchical Address Caching

4
  • Topics (continued)
  • 18. Dynamic Partitions
  • 19. Fragmentation
  • 20. Internal Fragmentation
  • 21. External Fragmentation
  • 22. Coalescing Holes
  • 23. Compaction
  • 24. Dynamic Partition Placement
  • 25. Simple Segmentation
  • 26. Memory Layout of A C Program

5
  • Topics (continued)
  • 27. malloc
  • 28. sbrk
  • 29. Memory Hierarchy

6
Memory In Systems design
I/O System
CPU
memory
Figure 1 memory Connects CPU and Peripherals
7
Binding Times
source program
compiler or assembler
compile time
object model
8
linkage editor
Other object modules
load module
load time
system library
loader
9
dynamically loaded system library
in-memory binary memory image
execution time (run time)
10
Introduction to Memory Management
  • Memory management issues
  • 1. Relocation
  • 2. Protection
  • 3. Sharing
  • 4. Logical Organization
  • 5. Physical Organization
  • Consider a series of solutions starting
  • with the most primitive first

11
Raw Memory Model
  • The raw memory provides no services
  • and gives the programmer complete
  • control.

0
Figure 3 Raw Machine Model
1 MB
12
Single User Contiguous Memory
  • Primitive operating systems (such as MS-
  • DOS and CP/M) provide some interfaces
  • to the hardware but not much else in the
  • way of services.

13
Single User Contiguous Memory(continued)
0
Monitor
fence register
User
Figure 4 Single User Contiguous Memory
32K
14
Relocation - Why and How
  • Relocation refers to the ability to store a
  • program at an arbitrary base memory
  • address.
  • Actual memory locations have physical
  • or absolute addresses, user program's
  • access these locations using logical
  • addresses.

15
Relocation - Why and How (continued)
Base register
14000
Logical address 346
Physical address 14346

CPU
memory
Figure 5 Address Translation
16
Overlay Management
  • Overlays have gone out of fashion with
  • cheaper memory, users (and compilers)
  • determine which code to swap in and out.

17
Figure 6 Overlay Management Example 2
symbol table
20K
common routines
30K
overlay driver
10K
70K
pass 1
pass 2
80K
18
Protection
  • It is undesirable to permit user programs
  • (accidentally or intentionally) to accesses
  • memory outside of their partition.

19
Protection (continued)
0
Monitor
fence register
Figure 7 Protection in Resident Monitor Model
User
32K
20
Fixed Partitions
  • Fixed partitioning refers to memory being
  • split into contiguous non overlapping
  • regions of precomputed sizes.
  • Fixed sized partitions make the selection
  • of a partition for a job easy.

21
Nonuniform Sized Fixed Partitions
  • Fixed Partitions may have differing sizes.

22
Operating system
Figure 8 Nonuniform fixed Partitions 3
New Process
23
Uniformly Sized Fixed Partitions
  • Memory is frequently partitioned into
  • uniformly sized regions.

24
Figure 9 Uniform Fixed Partition Allocation 3
Operating system 512 K
512 K
512 K
512 K
512 K
512 K
512 K
512 K
25
Simple Paging
  • Paging provides relocation, and splits
  • memory into fixed length partitions
  • called frames.

26
0 p-1
Page frame 0
p 2p-1
Page frame 1
Figure 10 Simple Paging 1
2p 3p-1
Page frame 2
3p 4p-1
Page frame 3
27
4p 5p-1
Page frame 4
5p 6p-1
Page frame 5
Figure 10 Simple Paging (continued) 1
6p 7p-1
Page frame 6
7p 8p-1
Page frame 7
28
Page frame size p p p p p p p p
Range of real storage addresses 0
p-1 p 2p-1 2p 3p-1 3p 4p-1 4p 5p-1 5p
6p-1 6p 7p-1 7p 8p-1
Page frame number 0 1 2 3 4 5 6 7
29
Benefits of Simple Paging
  • Simple paging allows discontiguous
  • storage for memory objects exceeding
  • the page frame size.

30
. . .
Contiguous virtual storage locations
. . .
Address mapping mechanism
Real storage
Virtual memory
31
Page Tables
  • One simple mechanism is to allocate
  • some real memory space for a table, and
  • hash page addresses using the high
  • order address bits as pointers into the
  • page table. There are 2 real memory
  • accesses per virtual memory access.

32
Page Tables (continued)

b
b
Page
Displacement
p
Virtual address

d
p
v(p,d)
Page
bp
Map Table
b
p
Real

p1
d
address
p1
33
Translation Lookaside Buffers
  • Translation lookaside buffers (TLB)
  • eliminate one physical memory reference
  • using special associative memory, which
  • addressed by its contents in O(1) parallel
  • search time.

34
Associative Memory Lookup
Page
Displacement
Virtual

d
address
p
V(p,d)
Associative map
Real

Address r
p1
d
p
p1
Frame
Displacement
35
Hierarchical Address Caching
  • Rather than placing all addresses in the
  • TLB recently/frequently used addresses
  • are stored in associative memory, with
  • misses being serviced by the page table.

36
Hierarchical Address Caching (continued)
Page table
Virtual address v(p,d)
origin register
Displacement
Page
p
Address of
d
Page table b
Try this first
Partial associative
Map (most active pages only

p
p1
p
Only if no match in associative map
p1
Only if match in associative map
Performed only if no match in associative map bp
Displacement
Frame
d
p1
Real address
Direct map (all pages)
37
Dynamic Partitions
  • Dynamically partitioned memory allows
  • placement of relocatable code in variable
  • size contigous memory regions.

38
Dynamic Partitions (continued)
user
needs
I
9K.
H
18K.
G
11K.
Operating
Operating
Operating
Operating
F
32K.
system
system
system
system
E
14K.
User
User
User
User
D
25K.
A 15K
A 15K
A 15K
A 15K
User
User
User
C
10K.
B 20K
B 20K
B 20K
B
20K.
User
User
Free
C 10K
C 10K
A
15K.
User
Free
D 25K
Free
Free
39
Fragmentation
  • Fragmentation makes available memory
  • useless by breaking it into discontiguous
  • pieces too small to use.

40
Fragmentation (continued)
  • There are two categories of memory
  • fragmentation
  • 1. Internal Fragmentation --- A fixed partition
    contains more memory than required by the user,
    and some is wasted.
  • 2. External Fragmentation --- Results from the
    holes left by dynamic partitions.

41
Internal Fragmentation
  • Internal fragmentation occurs when fixed
  • size partitions are too large.

42
Logical Address Page1,Offset478
Page 2
000001 01110111110
478
Page 1
Figure 16 Internal Fragmentation 3
Internal fragmentation
Page 0
43
External Fragmentation
  • External fragmentation happens when
  • dynamic partitions are released. The
  • fragments are frequently called holes.

44
operating system
operating system
operating system
User A
User A
User A
User B finishes and frees its storage.
User B
Hole
Hole
User C
User C
User C
User D finishes and frees its storage.
User D
User D
Hole
User E
User E
User E
Hole
Hole
Hole
45
Coalescing Holes
  • Adjacent holes in dynamic partitions
  • should be coalesced into a single larger
  • hole.

46
operating system
operating system
operating system
other users
other users
other users
Operating system combines adjacent holes
to form a single larger hole.
2K Hole
2K Hole
7K Hole
User A finishes and frees its storage
5K Hole
5K user A
other users
other users
other users
47
Compaction
  • If the amount of memory available in the
  • holes is large enough to service a
  • request, the holes may made contiguous
  • by compacting storage.

48
operating systems
operating systems
In use
In use
Operating system places all in use blocks
together leaving free storage as a single,
large hole.
In use
Free
In use
In use
Free
Free
In use
Free
49
Dynamic Partition Placement
(a) First-Fit Strategy Place job in first
storage hole on free storage place list in which
it will fit
50
0
Operating systems
(kept in storage address order, or something in
random order.)
a
16K hole
b
In use
c
Free Storage List
14K hole
Request for 13K
Start Address
d
In use
Length
e
5K hole
a 16K c 14K e 5K g 30K
f
In use
g
. . .
30K hole
h
51
(b) Best-Fit Strategy Place job in the smallest
possible hole in which it will fit.
52
0
Operating systems
(Kept in ascending order by hole size.)
a
16K hole
b
In use
c
Free Storage List
14K hole
Request for 13K
d
Start Address
In use
Length
e
5K hole
e 5K c 14K a 16K g 30K
f
In use
g
. . .
30K hole
h
53
(c) Worst-Fit Strategy Place job in the largest
possible hole in which it will fit.
54
0
Operating systems
(Kept in descending order by hole size.)
a
16K hole
b
In use
c
Free Storage List
14K hole
Request for 13K
d
Start Address
In use
Length
e
5K hole
g 30K a 16K c 14K e 5K
f
In use
g
. . .
30K hole
h
55
Simple Segmentation
  • Segmentation provides relocation, and
  • supports contiguous variable length
  • partitions.
  • Segmentation often provides
  • protection (counterexample Intel 8086).

56
limit
base
memory
Logical address
CPU
yes
lt

no
Trap address error
57
Memory Layout of A C Programs
  • Traditional Unix/C memory images of
  • programs use segments.

58
high address
Command-line arguments and environment
variables
stack
heap
initialized to zero by exec
uninitialized data (bss)
Initialized data
read from program file by exec
low address
text
59
malloc
  • Programmers often want to allocate data
  • objects which persist beyond the
  • function call creating them (e.g.
  • constructors in OOP).
  • In C and C the malloc operator
  • maintains a linked list of data objects, in
  • the user program's Data segment (on the
  • Heap).

60
In Use List
user data
user data
user data
Free List
user data
Key
user data
Memory management Information
user data
61
sbrk
  • A user program can exhaust its default
  • heap space allocation.
  • The Unix sbrk system call increases data
  • segment allocation at run time.

62
application code
user process
Memory allocation function malloc
sbrk system call
kernel
63
Memory Hierarchy
  • Users want to
  • 1. Increase their address
  • space, using slow cheaper memory to
  • extend their more expensive faster
  • memory.
  • 2. Increase the speed at which they can
  • the extended memory by using small
  • amounts of expensive fast memory.

64
Memory Hierarchy
Registers
Cache
Main Memory
Magnetic Disk (Cache)
Backups Optical Juke Box Remote Access
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