Page 125 - DCAP507_SYSTEM_SOFTWARE

Unit 8: Introduction to Linking and Loading

The output of translator is a program known as object module. The linker processes these object Notes
modules binds with essential library routines and makes a ready to implement program. Such
a program is known as binary program. The "binary program also comprises some necessary
information regarding allocation and relocation. The loader then loads this program into memory
for implementation reason.
Figure 8.4: Process of Linking a Process

Different tasks of linker are:
1. Prepare a single load module and regulate all the addresses and subroutine references
with respect to the offset position.

2. To prepare a load module concatenate all the object modules and regulate all the operand
address references in addition to external references to the offset position.
3. At accurate locations in the load module, copy the binary machine instructions and constant
data so as to prepare ready to implement module.
The linking process is done in two passes. Two passes are essential since the linker may encounter
a forward reference before recognizing its address. So it is essential to scan all the DEFINITION
and USE table at least once. Linker then builds the Global symbol table with the help of USE and
DEFINITION table. In Global symbol table name of every externally referenced symbol is
involved along with its address relative to starting of the load module. And throughout pass 2,
the addresses of external references are substituted by attaining the addresses from global
symbol table.

8.2.3 Loaders

As we turn on the computer there is nothing significant in the main memory (RAM). A small
program is written and amassed in the ROM. This program primarily loads the operating
system from secondary storage to main memory. The operating system then takes the on the
whole control. This program which is accountable for booting up the system is known as
bootstrap loader.

!
Caution This is the program which must be implemented first when the system is first
powered on.

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