Chapter 7 Physical Memory

• Preparing a program for execution

• Memory partitioning schemes

• Allocation strategies for variable partitions

• Managing insufficient memory

7.1 preparing a program for execution

• Program transformations compilation/assembly linking loading

Source module1

Object module1

Source module2

Object module2

Source module3

Object module3

. . .

Load module (in secondary

memory)

Load module (in main memory)

translation linking loading execution

• Logical-to-physical address binding

notations: binding( relocation) static binding dynamic binding (dynamically) relocatable

Fun1(){ … fun2(); …}

Fun2(){ …}

… call m2,fun2 …

//fun2 …

… Call Ladd1 …Ladd1: //fun2 …

… Call Fadd1 …Fadd1: //fun2 …

• Static binding programming-time binding compile-time binding link-time binding load-time binding

int i;

……

i= …;

……

f();

……

……

Store 20

……

Branch f

i20

0Function f

……

……

Store 120

……

Branch 0

i

100

0

120

Function f

……

……

Store 1120

……

Branch 1000

i

1100

1000

1120

• Dynamic binding

the binding occurs at runtime, i.e., immediately preceding each memory reference. This delays the binding of the program to the machine until the latest possible moment.

Physical_address=address_map(logical_address)

Processor Main memory

Address_map

Logical

address

Physicaladdress

Data transfers(R/W)

Processor Main memory

Relocation register

Logical

addressPhysicaladdress

Data transfers(R/W)

+

Physical_address=logical_address+ RR

int i;

……

i= …;

……

f();

……

……

Store 20

……

Branch f

i20

0Function f

……

……

Store 120

……

Branch 0

i

100

0

120

Function f

……

……

Store 120

……

Branch 0

i

1100

1000

1120

Address map

7.2 Memory partitioning schemes

• Fixed partitions

properties: the sizes of partitions are determined statically the sizes of partitions cannot be changed at runtime partitions have different sizes to accommodate

the different programs

Schemes to schedule for the partitions: using a separate queue for each partition using a common queue

processes

OS

queues

processes

OS

queues

• Variable partitions

properties: the sizes of partitions are determined at

runtime the sizes of partitions equal the exact amount of

space requested memory consist of variable size blocks,

alternating between occupied blocks and free blocks

A B C D E

memory management: requests to allocate free memory areas

AB C

A’B C

AB

B C’

AB C

C

AB C

C

B’ B’

• Linked list implementation

freesize A

occupied

size B

occupied

size Cfreesize E

occupied

size D

• Bitmap implementation

00011111 11100000 …

1-KB block is represented by one bit( A, B, C, D, E: 3KB, 2KB, 5KB, 1KB, 5KB)

Releasing block:

Allocating block:

Searching block:

B [i]=b [i] & ‘11011111’

B [i]=b [i] | ‘11000000’

B [0]=b [0] & ‘10000000’

B [0]=b [0] & ‘01000000’

……

• The buddy system assumption:

fixed number of possible hole sizes, and each of which is a power of 2 buddies:

any hole of size 2i can be divided into two holes of size 2i-1, these two holes are called buddies implementation:

0 4 6 8 12 14

H

01234 .....

allocation( a request for n unit)find the smallest hole size such that n <=2i;

if list I is not empty

remove a hole form the list&allocate;

else

find a larger hole list( not empty) than list I;

remove and divide into two holes(l&r) of half of size;

place r on the next-lower list;

if hole l is the smallest one for the request

allocate;

else

divide repeatedly;

release( for the block of size 2i)If the buddy of the block is occupied

the new hole is added to the list i;

Else

remove the buddy from the list I;

coalesce the two holes into a hole of size 2i+1;

repeat until the largest possible hole is created;

7.3 Allocation strategies for variable partitions

• First-Fit:finding the first hole large enough to accommodate the given request.

• Next_Fit( rotating-first-fit)starting each search at the point where the previous search stopped.

• Best_Fitselecting the hole with the closest fit.

• Worst_fitusing the largest currently available hole for any given request.

• Measures of memory utilization

Equilibrium state:

Prob(release)==Prob(request)

Prob(n++)==Prob(n--)

AB C AB

AB C AB C

C

a b

c d

m=a+b+c+d (7.1)

n= (7.2)

N=d+b (7.3)

Prob(n++)=Prob(release)*a/m (7.4)

Prob(n--)=Prob(release)*d/m+Prob(request)*(1-p) (7.5)

A=d+(1-p)m (7.6)

22 cbd

)1( pmd

ma

m=d+(1-p)m+b+c+d

=(1-p)m+2b+2d

=(1-p)m+2n

Conclusion1:n=0.5pm

The fraction of memory occupied by holes:

f =?

Assumption:

average hole size: h

average occupied block size: b

k=

M=nh+mb

=0.5kmb+mb

=mb(0.5k+1)

Conclusion2:

bh

2)15.0(5.0

kk

kmbkmh

Mnh

f

7.4 Managing insufficient memory

• Memory compaction

～～ ～～

2

3

4

2

5

5

9 ～～ ～～

3

2

5

20

～～ ～～

3

2

11

5

9 ～～ ～～

3

2

11

5

9

p1

p2

p3

p1

p2

p3

p3

p1

p2

p2

p1

p3

• Swapping

creating new space by selecting one of the resident processes and temporarily evicting it to secondary storage.

properties: can always evict as many resident

processes as is necessary; affects only one or a small number of

processes; requires accesses to secondary

memory.

• Overlays

different portions of the program replace( overlay) each other in memory as execution proceeds.

A

B C

D E

0A

B C

A

D

A

C

E

FAQs

• 3.1 m.A() x=11,y=9;m.A() x=12,y=10;m.B() x=11,y=10;m.B() x=10,y=10;m.B() x=10,y=10;m.B() x=10,y=10;m.A() x=10,y=8;m.A() x=10,y=8;

• 3.3monitor stack{

char array[MAX];

UINT bottom=MAX-1,top=MAX-1;

condition c;

void push(char x){

if(bottom-top<MAX)

array[top--]=x;

if(bottom-top==1)

c.signal;

}

void pop(char &x){

if(bottom==top)

c.wait;

x=array[++top];

}

}

• 4.8Pb(s){

do{

R=0;

SWAP(R,S);

while(!R);

}

Vb(s){

s=1;

}

• 4.13f(x){

P(mutex);if(x){

condcnt_c1++;if(urgentcnt)

V(urgent);else

V(mutex);P(condsem_c1);condcnt_c1--;

}x++;////////////////if(condcnt_c2){

urgentcnt++;V(condsem_c2);P(urgent);urgentcnt--;

}////////////////x=0;if(urgentcnt)

V(urgent);else

V(mutex);

• 4.19

500 20Hardware timers

Wall-clock countdown

Timer queue TQ p1 20 p2 125 p3 50 p4 15

520 70Hardware timers

Timer queue TQ p1 70 p2 55 p3 50 p4 15

590 55Hardware timers

Timer queue TQ p1 55 p2 15 p3 35 p4 15