Operating Systems

1
Operating Systems

• Memory management

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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

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• 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

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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
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linkage editor
Other object modules
load module
load time
system library
loader
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dynamically loaded system library
in-memory binary memory image
execution time (run time)
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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

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Raw Memory Model

• The raw memory provides no services
• and gives the programmer complete
• control.

0
Figure 3 Raw Machine Model
1 MB
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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.

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Single User Contiguous Memory(continued)
0
Monitor
fence register
User
Figure 4 Single User Contiguous Memory
32K
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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.

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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.

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Figure 6 Overlay Management Example 2
symbol table
20K
common routines
30K
overlay driver
10K
70K
pass 1
pass 2
80K
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Protection

• It is undesirable to permit user programs
• (accidentally or intentionally) to accesses
• memory outside of their partition.

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Protection (continued)
0
Monitor
fence register
Figure 7 Protection in Resident Monitor Model
User
32K
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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.

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Nonuniform Sized Fixed Partitions

• Fixed Partitions may have differing sizes.

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Operating system
Figure 8 Nonuniform fixed Partitions 3
New Process
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Uniformly Sized Fixed Partitions

• Memory is frequently partitioned into
• uniformly sized regions.

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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
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Simple Paging

• Paging provides relocation, and splits
• memory into fixed length partitions
• called frames.

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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
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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
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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
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Benefits of Simple Paging

• Simple paging allows discontiguous
• storage for memory objects exceeding
• the page frame size.

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. . .
Contiguous virtual storage locations
. . .
Address mapping mechanism
Real storage
Virtual memory
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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.

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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
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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.

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Associative Memory Lookup
Page
Displacement
Virtual

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

Address r
p1
d
p
p1
Frame
Displacement
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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.

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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)
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Dynamic Partitions

• Dynamically partitioned memory allows
• placement of relocatable code in variable
• size contigous memory regions.

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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
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Fragmentation

• Fragmentation makes available memory
• useless by breaking it into discontiguous
• pieces too small to use.

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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.

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Internal Fragmentation

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

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Logical Address Page1,Offset478
Page 2
000001 01110111110
478
Page 1
Figure 16 Internal Fragmentation 3
Internal fragmentation
Page 0
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External Fragmentation

• External fragmentation happens when
• dynamic partitions are released. The
• fragments are frequently called holes.

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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
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Coalescing Holes

• Adjacent holes in dynamic partitions
• should be coalesced into a single larger
• hole.

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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
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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.

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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
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Dynamic Partition Placement
(a) First-Fit Strategy Place job in first
storage hole on free storage place list in which
it will fit
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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
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(b) Best-Fit Strategy Place job in the smallest
possible hole in which it will fit.
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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
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(c) Worst-Fit Strategy Place job in the largest
possible hole in which it will fit.
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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
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Simple Segmentation

• Segmentation provides relocation, and
• supports contiguous variable length
• partitions.
• Segmentation often provides
• protection (counterexample Intel 8086).

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limit
base
memory
Logical address
CPU
yes
lt

no
Trap address error
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Memory Layout of A C Programs

• Traditional Unix/C memory images of
• programs use segments.

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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
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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).

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In Use List
user data
user data
user data
Free List
user data
Key
user data
Memory management Information
user data
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sbrk

• A user program can exhaust its default
• heap space allocation.
• The Unix sbrk system call increases data
• segment allocation at run time.

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application code
user process
Memory allocation function malloc
sbrk system call
kernel
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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.

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