Page 118 - DCAP507_SYSTEM_SOFTWARE

System Software

Notes  It is possible to write source program with numerous programs and multiple languages,
since the source programs are first transformed to object programs always, and loader
accepts these object modules to translate it to executable form.

8.1.3 Absolute Loaders

Absolute loader is a type of loader in which relocated object files are produced, loader accepts
these files and positions them at particular locations in the memory. This type of loader is
known as absolute since no relocation information is required; rather it is attained from the
programmer or assembler. The beginning address of every module is recognized to the
programmer, this corresponding beginning address is accumulated in the object file, then task
of loader turns out to be very simple and that is to simply position the executable form of the
machine instructions at the locations declared in the object file. In this scheme, the programmer
or assembler should have information of memory management. The resolution of external
references or linking of dissimilar subroutines are the concerns which require to be managed by
the programmer. The programmer should pay attention to two things: first thing is specification
of beginning address of each module to be utilized. If some alteration is performed in some
module then the length of that module may differ. This causes a variation in the starting address
of instant next modules, its then the programmer's duty to make essential variations in the
starting addresses of respective modules. Second thing is while branching from one division to
another the absolute starting address of individual module is to be recognized by the programmer
so that such address can be specified at individual JMP instruction.

Example:

This example includes two segments, which are inter-reliant. At line number 1 the assembler
directive START states the physical starting address that can be used throughout the execution of
the first segment MAIN. Then at line number 15 the JMP instruction is provided which states the
physical starting address that can be utilized by the second segment. The assembler makes the
object codes for these two segments by taking into account the stating addresses of these two
segments. During the execution, the first segment will be loaded at address 1000 and second
segment will be loaded at address 5000 as mentioned by the programmer. Thus, the problem of

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