Static and Dynamic Memory
Allocation
Memory allocation is an aspect of a more general
action in software operation known as binding
Static allocation performed by compiler, linker, or loader
• Sizes of data structures must be known a priori
Dynamic allocation provides flexibility
• Memory allocation actions constitute an overhead during
operation
Operating Systems, by Dhananjay Dhamdhere
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Mr. D. J. Patel, AITS, Rajkot
LINKERS
Mr. D. J. Patel, AITS, Rajkot
Execution of Programs
A has to be transformed before it can be executed
Many of these transformations perform memory bindings
• Accordingly, an address is called compiled address, linked
address, etc
Operating Systems, by Dhananjay Dhamdhere
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3
Mr. D. J. Patel, AITS, Rajkot
Why Linkers ??
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Modularity
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Program can be written as a collection of smaller source files, rather than one monolithic mass.
Can build libraries of common functions
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e.g., Math library, standard C library
Efficiency
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Time:
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Change one source file, compile, and then relink.
No need to recompile other source files.
Space:
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Libraries of common functions can be aggregated into a single file...
Yet executable files and running memory images contain only code
for the functions they actually use.
Mr. D. J. Patel, AITS, Rajkot
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Linking
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The process of collecting and combining various pieces of code and data into a single file that
can be loaded (copied) into memory and executed.
Linking time
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Can be done at compile time, i.e. when the source code is translated
Or, at load time, i.e. when the program is loaded into memory
Or, even at run time.
Static Linker
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Performs the linking at compile time.
Takes a collection of relocatable object files and command line arguments and generate a fully
linked executable object file that can be loaded and run.
Performs two main tasks
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Introduction to Linkers
Symbol resolution: associate each symbol reference with exactly
one symbol definition
Relocation: relocate code and data sections and modify symbol
references to the relocated memory locations
Dynamic Linker

Performs the linking at load time or at run time.
Mr. D. J. Patel, AITS, Rajkot
Performing Relocation
relocation_factorP = l_originP – t_originP
lsymb = tsymb + relocation_factorP
Ref : 224 – D M Dhamdhere
Mr. D. J. Patel, AITS, Rajkot
Relocation
Instructions using memory addresses are addresssensitive
Relocation is needed if program is to execute correctly in
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Operating
Systems,
by
Dhananjay
Dhamdhere
some other memory area: involves changing addresses
Ref : 223 –Figure : 7.2 D M Dhamdhere
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Mr. D. J. Patel, AITS, Rajkot
Example - I
Suppose an object module contains three programs A,B and C. If they are located at following
addresses
Prog Address
A
200-250
B
302-370
C
480-533
Assume the load address is 300 for the executable A, B and C to be generated.
Find:
i) Find out size of each Program
ii) Load origin of each program
iii) Relocation factor for each program
Ref : Pg: 226 Example 7.5 – D M Dhamdhere
Mr. D. J. Patel, AITS, Rajkot
Solution - I


Size
Relocation Factor
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A = 51
RFA = 300 – 200 = 100
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B = 69
RFB = 351 – 302 = 49
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C = 54
RFC = 420 – 480 = -60
Load time Origin

A = 300
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B = 351

C = 420
Mr. D. J. Patel, AITS, Rajkot
Design of Linker
Relocation Requirement
Linking Requirement
Name Table (NTAB) – with the field of Synbols
name and Linked_address
eg. : Pg: 231 – Example : 7.9
Mr. D. J. Patel, AITS, Rajkot
Self Relocating Programs
Which program can be modified, or can modfied it self
, to execute from a given Load Origin.
Classifications :
1) Non Relocatable Program
2) Relocatable Programs
3) Self Relocating Programs
Ref : pg: 232 7.3 Dhamdhere
Mr. D. J. Patel, AITS, Rajkot
Non Relocatable Program
1) Can't Execute in Any Memory Area, Only on
Traslated Origin
2) Lack of Address Sensitive Instructions
3) e.g. Hand Coded Machine Language Program
Mr. D. J. Patel, AITS, Rajkot
Relocatable Programs
1) Can Execute in Any Desired Memory Area
2) Availability of Address Sensitve Instruction
3) e.g. Object Module
Mr. D. J. Patel, AITS, Rajkot
Self Relocating Programs
1) Can Execute in Any Memory Area
2) Availability of Own Address Sensitve Instruction
3) Relocating Logic Specified on the start of the
Program
4) Useful for Time sharing Operating System
Mr. D. J. Patel, AITS, Rajkot
Linking for Overlays
Overlay :
It is a part of a program that use same load origin as
some part of the program.
Advantages :
► Keep in memory only those instructions and data that
are needed at any given time.
► Needed when process is larger than amount of
memory allocated to it.
Mr. D. J. Patel, AITS, Rajkot
An Overlay Tree
Mr. D. J. Patel, AITS, Rajkot
Loader
An operating system utility that copies programs from
a storage device to main memory, where they can be
executed. In addition to copying a program into main
memory, the loader can also replace virtual addresses
with physical addresses.
Most loaders are transparent, i.e., you cannot directly
execute them, but the operating system uses them
when necessary.
Absolute loader can only load origin = linked Origin.
Mr. D. J. Patel, AITS, Rajkot