Memory Management
S:1
OS/SSS
Background
• A compiler creates an executable code
• An executable code must be brought into
main memory to run
• Operating system maps the executable
code onto the main memory
• Hardware accesses the memory locations
in the course of the execution
S:2
OS/SSS
Background
• Applications must request memory
through OS calls (or user libs
calling certain SVC like page(x))
p=malloc(x)
then releases them :free(p)
S:3
OS/SSS
Memory management
• Fetching: When ?
– On Demand
– Anticipatory
• Pre-Allocation: Before request
• Post-Allocation: Just When
reading/writing
• Placement: From where? How?
S:4
OS/SSS
Processes in RAM
Fixed Partition Multiprogramming
0000:
0010: 01 77 77 00 10 … ; PCB 0
0020: 01 88 88 00 20 … ; PCB 1
S:5
1000:a1 40 10
1003:40
1004:a3 40 10
;x=[1040]
;mov AX, x
;inc AX
;mov x, AX
1040: 00 00
;add [BX+SI],AL?
2000:b8 0800
2003:a3 40 02
;y=[2040]
;mov AX, 8
;mov y,AX
2040: 00 00
;add [BX+SI],AL?
PCB 0
St: 1 ;ready
AX: 7777
PC: 1000
PCB 1
St: 1 ;ready
AX: 8888
PC: 2000
OS/SSS
Processes in RAM
Fixed Partition Multiprogramming
0000:
0010: 01 77 77 00 10 … ; PCB 0
0020: 01 88 88 00 20 … ; PCB 1
S:6
1000:a1 40 10
1003:40
1004:a3 40 10
;x=[1040]
;mov AX, x
;inc AX
;mov x, AX
1040: 00 00
;add [BX+SI],AL?
2000:b8 0800
2003:a3 40 02
;y=[2040]
;mov AX, 8
;mov y,AX
2040: 00 00
;add [BX+SI],AL?
Fixed Partitions
AND
Absolute
translation and
PCB 0
loading
St: 1 ;ready Bad – what a
AX: 7777
waste
PC: 1000
PCB 1
St: 1 ;ready
AX: 8888
PC: 2000
OS/SSS
Fixed Partitions
Absolute translation and loading
• Limits # processes
• Memory Misuse
• Internal/External
Fragments
• Site Dependent
Compilation/Linking
Partition0
Proc 7
Partition 1
Free
Partition 0
Proc 7
• Limited possibility of run
time dynamic allocation
S:7
OS/SSS
Memory Management Requirements
• Protection: Processes and OS from
malicious references
– Processes must be unable to reference
addresses of other processes or those
of the operating system
– Each memory access must be checked
for validity/Enforced by hardware
S:8
OS/SSS
Processes in RAM
Fixed Partition Multiprogramming
0000:
0010: 01 77 77 00 10 … ; PCB 0
0020: 01 88 88 00 20 … ; PCB 1
S:9
1000:a1 40 10
1003:40
1004:a3 40 10
;x=[1040]
;mov AX, x
;inc AX
;mov x, AX
1040: 00 00
;add [BX+SI],AL?
2000:b8 0800
2003:a3 40 02
;y=[2040]
;mov AX, 8
;mov y,AX
2040: 00 00
;add [BX+SI],AL?
PCB 0
St: 1 ;ready
AX: 7777
PC: 1000
LB: 0990
UP: 1100
PCB 1
An Exception will
be triggered if
any
address
outside
[9901100]
is
referenced for
read or write
St: 1 ;ready
AX: 8888
PC: 2000
LB: 2000
UP: 3000
OS/SSS
Memory Management Requirements
• Protection
• Relocation: To relax that unacceptable
absolute binding !
– Compiler generated addresses are relative
– A process can be loaded at different
memory location, swapped in/out, Actual
physical mapping is not known at compile time
S:10
OS/SSS
Processes in RAM
Dynamic Partition Multiprogramming
0000:
0010: 01 77 77 00 10 … ; PCB 0
0020: 01 88 88 00 20 … ; PCB 1
S:11
1000:a1 40 10
1003:40
1004:a3 40 10
;x=[1040]
;mov AX, x
;inc AX
;mov x, AX
1040: 00 00
;add [BX+SI],AL?
2000:b8 0800
2003:a3 40 02
;y=[2040]
;mov AX, 8
;mov y,AX
2040: 00 00
;add [BX+SI],AL?
PCB 0
St: 1 ;ready
AX: 7777
PC: 1000
LB: 0990
UP: 1100
How Hiding from
Proc 0 the later
fact that it will
load at 990
And P1 at 2000
PCB 1
St: 1 ;ready
AX: 8888
PC: 2000
LB: 2000
UP: 3000
OS/SSS
Processes in RAM
Dynamic Partition Multiprogramming
S:12
0000:
PCB 0
0010: 01 77 77 00 10 … ; PCB 0
0020: 01 88 88 00 20 … ; PCB 1
St: 1 ;ready
AX: 7777
PC: 1000
LB: 0990
UP: 1100
RR: 1000
1000:a1 40 00
1003:40
1004:a3 40 00
;x=[0040]
;mov AX, x
;inc AX
;mov x, AX
1040: 00 00
;add [BX+SI],AL?
2000:b8 0800
2003:a3 40 00
;y=[0040]
;mov AX, 8
;mov y,AX
2040: 00 00
;add [BX+SI],AL?
PCB 1
St: 1 ;ready
AX: 8888
PC: 2000
LB: 2000
UP: 3000
RR: 2000
OS/SSS
Processes in RAM
Dynamic Partition Multiprogramming
0000:
PCB 0
0010: 01 77 77 00 10 … ; PCB 0
0020: 01 88 88 00 20 … ; PCB 1
St: 1 ;ready
AX: 7777
PC: 1000
LB: 0990
UP: 1100
RR: 1000
+
S:13
1000:a1 40 00
1003:40
1004:a3 40 00
;x=[0040]
;mov AX, x
;inc AX
;mov x, AX
1040: 00 00
;add [BX+SI],AL?
40
2000:b8 0800
2003:a3 40 00
;y=[0040]
;mov AX, 8
;mov y,AX
2040: 00 00
;add [BX+SI],AL?
PCB 1
St: 1 ;ready
AX: 8888
PC: 2000
LB: 2000
UP: 3000
RR: 2000
OS/SSS
Processes in RAM
Dynamic Partition Multiprogramming
0000:
0010: 01 77 77 00 10 … ; PCB 0
0020: 01 88 88 00 20 … ; PCB 1
+
S:14
1000:a1 40 00
1003:40
1004:a3 40 00
;x=[0040]
;mov AX, x
;inc AX
;mov x, AX
1040: 00 00
;add [BX+SI],AL?
40
2000:b8 0800
2003:a3 40 00
;y=[0040]
;mov AX, 8
;mov y,AX
2040: 00 00
;add [BX+SI],AL?
User (Process)
address
PCB 0space is
disassociated
from
St:physical
1 ;ready
space
AX: 7777
ButPC:
contiguity
is
1000
stillLB:
preserved
0990
UP: 1100
RR: 1000
PCB 1
St: 1 ;ready
AX: 8888
PC: 2000
LB: 2000
UP: 3000
RR: 2000
OS/SSS
Processes in RAM
Dynamic Partition Multiprogramming
User (Process)
address space is
PCB 0
disassociated
from physical St: 1 ;ready
space
AX: 7777
But contiguity is PC: 1000
still preserved
+
LB: 0990
0000:
0010: 01 77 77 00 10 … ; PCB 0
0020: 01 88 88 00 20 … ; PCB 1
S:15
1000:a1 40 00
1003:40
1004:a3 40 00
;x=[0040]
;mov AX, x
;inc AX
;mov x, AX
1040: 00 00
;add [BX+SI],AL?
2000:b8 0800
2003:a3 40 00
;y=[0040]
;mov AX, 8
;mov y,AX
2040: 00 00
;add [BX+SI],AL?
40
UP: 1100
RR: 1000
Each process has
its own address
space
PCB 1
[0…something]
St:However
1 ;ready
Separate
AX: 8888 Ranges
PC:{[m..n],[k..l]
2000
LB:needs
2000complex
!
UP:management
3000
RR: 2000
OS/SSS
The Placement problem
• Where to allocate:
– Fixed Partitions
– Dynamic placement algorithms
• First Fit
• Best Fit
• Worst Fit
– Buddy Systems
S:16
OS/SSS
Dynamic partitioning example
0
128
OS 128
P0 320
448
P1 224
672
P0 224
896
1K
PCB0
128
P0 Requests 320 units [h0]
P1 Requests 224 units [h1]
P0 Requests 224 units [h2]
128
….
672
224
PCB1
448
224
null
Free
896
S:17
320
128
null
OS/SSS
null
Dynamic partitioning example
0
128
PCB0
OS 128
P0 320
448
224
P0 Requests 320 units [h0]
P1 Requests 224 units [h1]
P0 Requests 224 units [h2]
P1 exits [releases h1]
128
320
….
672
224
128
...
672
P0 224
896
1K
Free
128
896
S:18
448
224
null
OS/SSS
null
Dynamic partitioning example
0
128
OS 128
P0 320
448
224
PCB0
P0 Requests 320 units [h0]
P1 Requests 224 units [h1]
P0 Requests 224 units [h2]
P1 exits [releases h1]
Now
P2 Requests 120 ? WF !
128
320
….
672
224
128
...
672
P0 224
896
1K
S:19
128
From Here?
First Fit
Best Fit
Or
Worst fit
Free
896
448
224
null
OS/SSS
null
Dynamic partitioning example
0
128
OS 128
P0 320
448
568
672
P2 120
104
PCB0
P0 Requests 320 units [h0]
P1 Requests 224 units [h1]
P0 Requests 288 units [h2]
P1 exits [releases h1]
P2 request 120 [h3] WF
896
….
224
PCB2
120
null
Free
128
896
S:20
320
672
448
P0 224
1K
128
568
104
128
...
null
OS/SSS
null
Dynamic partitioning example
0
128
OS 128
320
448
568
672
P2 120
104
PCB0
P0 Requests 320 units [h0]
P1 Requests 224 units [h1]
P0 Requests 288 units [h2]
P1 exits [releases h1]
P2 request 128 [h3] WF
P0 releases h0
S:21
448
Free
128
896
128
224
null
PCB2
P0 224
896
1K
672
568
104
320
null
120
128
null
...
…
OS/SSS
Dynamic partitioning example
0
128
PCB0
OS 128
P0 Requests 320 units [h0]
P1 Requests 224 units [h1]
P0 Requests 288 units [h2]
P1 exits [releases h1]
P2 request 128 [h3] WF
P0 releases h0
P2 Requests 100 !
320
448
568
672
P2 120
104
P0 224
First Fit
128
null
PCB2
448
120
896
Best Fit
S:22
224
null
Free
896
1K
672
Worst Fit
128
568
104
320
null
128
...
…
OS/SSS
Dynamic partitioning example
0
128
PCB0
OS 128
P0 Requests 320 units [h0]
P1 Requests 224 units [h1]
P0 Requests 288 units [h2]
P1 exits [releases h1]
P2 request 128 [h3] WF
P0 releases h0
P2 Requests 100 !
320
448
568
672
P2 120
104
P0 224
First Fit
128
null
PCB2
448
120
896
Best Fit
S:23
224
null
Free
896
1K
672
Worst Fit
128
568
104
320
null
128
...
…
OS/SSS
Dynamic partitioning example
0
128
OS 128
320
448
568
672
P2 120
P2 100
PCB0
P0 Requests 320 units [h0]
P1 Requests 224 units [h1]
P0 Requests 288 units [h2]
P1 exits [releases h1]
P2 request 128 [h3] WF
P0 releases h0
P2 Requests 100 ! FF
S:24
448
Free
128
896
Who wants that
128
224
null
120
…
PCB2
P0 224
896
1K
672
668
4
320
null
568
128
100
...
…
OS/SSS
null
Dynamic partitioning example
0
128
OS 128
320
448
568
672
P2 120
P2 104
P0 224
896
1K
128
PCB0
P0 Requests 320 units [h0]
P1 Requests 224 units [h1]
P0 Requests 288 units [h2]
P1 exits [releases h1]
P2 request 128 [h3] WF
P0 releases h0
P2 Requests 100 ! FF
224
null
120
…
PCB2
448
568
Free
Internal Fragment
Why ?
896
128
S:25
672
320
128
104
...
null
OS/SSS
null
Dynamic partitioning example
0
128
OS 128
P2 exits
Someone wants 541 unit
568
672
fragments
120
896
128
128
448
568
placement
128
320
120
104
...
896
…
…
null
668
128
448
568
128
4
…
320
120
100
…
…
null
Needs Coalescing
S:26
null
Free
896
104
224
Case of exact
Free
P0 224
1K
672
Case of internal
320
448
PCB0
OS/SSS
...
Dynamic partitioning example
PCB0
0
128
Someone wants 541 unit – ok
OS 128
672
Free
896
External Fragments
P0 224
128
554
128
null
896
1K
128
Needs Garbage
Collection
S:27
null
But Someone Requests 600 !
External Fragments
and Garbage collection
554
672
224
OS/SSS
...
Dynamic partitioning example
0
128
OS 128
512
1K
Going BF
S:28
0
WorstFit
BestFit
10
1
1
20
1
2
40
3
3
60
6
3
80
8
2
100
4
1
120
2
2
140
1
2
160
1
6
2090
2090
Free
128
OS 128
512
1K
Going WF
OS/SSS
Dynamic partitioning example
0
128
0
OS 128
128
OS 128
10
512
512
5
0
0
1K
Going BF
S:29
50
100
Worst-Fit
150
Best-Fit
200
1K
Going WF
OS/SSS
Dynamic partitioning example
0
128
OS 128
512
0
• Preserves
• More
large
Normally
blocks for
distributed
future
• More
• External
128
OS 128
512
predictable
fragments
1K
1K
Going BF
Going WF
Random FF
S:30
OS/SSS
Buddy System
•Entire space available is treated as a single block of 2U
•If a request of size s such that 2U-1 < s <= 2U, entire
block is allocated
Otherwise block is split into two equal buddies
Process continues until smallest block greater than or equal
to s is generated
H0->128
128
256
512
H1Req 240
H0->128
128
H1->256
512
H2Req 60
H0->128
H2->64
64
H1->256
512
H3Req 256
H0->128
H2->64
64
H1->256
H3->256
256
Release H1
H0->128
H2->64
64
256
H3->256
256
Release H0
128
H2->64
64
256
H3->256
256
H4Req 100
H4->128
H2->64
64
256
H3->256
256
Release H2
H4->128
256
H3->256
256
H3->256
256
Release H3
Release H3
S:31
1M
H0Req 100
128
512
1M
OS/SSS
Buddy System
1M
H0Req 100
H0->128
128
256
512
H1Req 240
H0->128
128
H1->256
512
H2Req 60
H0->128
H2->64
64
H1->256
512
H3Req 256
H0->128
H2->64
64
H1->256
H3->256
256
Release H1
H0->128
H2->64
64
256
H3->256
256
Release H0
128
H2->64
64
256
H3->256
256
H4Req 100
H4->128
H2->64
64
256
H3->256
256
Release H2
H4->128
256
H3->256
256
H3->256
256
Release H3
Release H3
S:32
128
512
1M
OS/SSS
Buddy System
1M
H0Req 100
H0->128
128
256
512
H1Req 240
H0->128
128
H1->256
512
H2Req 60
H0->128
H2->64
64
H1->256
512
H3Req 256
H0->128
H2->64
64
H1->256
H3->256
256
Release H1
H0->128
H2->64
64
256
H3->256
256
Release H0
128
H2->64
64
256
H3->256
256
H4Req 100
H4->128
H2->64
64
256
H3->256
256
Release H2
H4->128
256
H3->256
256
H3->256
256
256/256
256
128
Release H3
512
Release H3
1M
100/128
S:33
60/64
64
256
OS/SSS
Buddy System
•Internal Fragments !
1M
H0Req 100
H0->128
128
256
512
H1Req 240
H0->128
128
H1->256
512
H2Req 60
H0->128
H2->64
64
H1->256
512
H3Req 256
H0->128
H2->64
64
H1->256
H3->256
256
Release H1
H0->128
H2->64
64
256
H3->256
256
Release H0
128
H2->64
64
256
H3->256
256
H4Req 100
H4->128
H2->64
64
256
H3->256
256
Release H2
H4->128
256
H3->256
256
H3->256
256
Release H3
Release H3
128
512
1M
•Overcomes limited processes [Fixed partitioning] and
overhead imposed by Dynamic allocation.
•Useful in special memory managers such as kernel
memory allocation, parallel systems, …etc.
S:34
OS/SSS
Memory Management Requirements
• Protection
• Relocation
• External Fragments management
• Sharing
• Post Allocation ? Why?
• Managing Requests greater then
physically available
S:35
OS/SSS
Memory Management Requirements
• Protection
• Relocation
• External Fragments management
• Sharing
• Post Allocation ? Why?
• Managing Requests greater then
physically available
S:36
OS/SSS
Thanks
&& comments
S:37
OS/SSS