Types of Loader
Types
of Loader :-
There are
eight(8) general loader schemes available. But generally main loader scheme is four(1,2,3,4).
Those are –
1. Absolute Loader.
2. Relocating Loader.
3. Direct Linking Loader.
4. Dynamic Loader.
5. Assemble – and – go or Compile – and
– go loader.
6. Boot Strap Loader.
7. Linking Loader.
8. Relocation Loader.
Absolute Loader :-
It is a sim placed
type of loader scheme. It this scheme the loader simply accepts the machine
language code produced by assembler and place it into main memory at the
location specified by the assembler. The task of an absolute loader is virtually
trivial. Absolute loader is simply to implemented but it has several
disadvantage –
First
: The
programmer must specify to the assembler the address in main memory where the
program is to be loaded.
Second
: The programmer must
remember the address of multiple sub – programs and there performance.
Fig - Absolute Loader
Relocating Loader :-
To avoid possible reassembling of all subroutines , When a single
subroutines is changed and to performed the task of allocation and linking of
the programmer. The general class of relocating loader was introduced.
An example of relocating loader is Binary Symbolic
Subroutine (BSS) loader, IBM 7294, IBM 1130, GE635 are the BSS loader.
Output of a relocating loader is the object program and
information about all other programs its
references. In addition ,there is relocating
information has two location in this program that need to be changed. If
it is to be loaded in an arbitrary location in memory.
Direct Linking Loader :-
Direct linking loader is a general relocatable loader and perhaps the
most popular loading scan presently used. It has the advantage
of allowing the programmer multiple procedure segments and multiple data segments. It has also an advantage that has given to the programmer
complete freedom in referencing data or instructions content in other segments.
This provides flexibility of inter segment referencing and accessing while at
the same time allowing independent translation of programs.
Dynamic loader :- Dynamic loader is the Persian of
loader that actually intersect the “calls” and loads the necessary procedure is
called over lay super visor or simply flipper. This over all scheme is called Dynamic Loading or Load
on Call.
An
advantage id Dynamic Loader is that, no over
head is in corrected unless the procedure to be called or referenced is
actually used. Also the system can be dynamically re – configured.
The major
draw back is occurred due to the fact that we here postponed most of the
binding process until execution time.
Assemble – and – go or
Compile – and – go loader :-
Any method of performing the loader function is to have the
assembler run in one of the part of
memory and place the machine instructions in data directly into assigned
memory location. This is a simply solution, involving mode any extra procedure.
It is used by the WATFOR FORTAN compiler and other several language processor
(PL/I). Such a loading scheme is commonly called compile
– and – go and assemble – and – go
loader.
Boot Strap Loader :- When a computer is first turned on
or restarted a special type of absolute loader is executed , called Boot Strap Loader. It loads the operation
system into the main memory and executes
the related programs. It is added to the beginning of all object programs that
are to be loaded into and empty ideal system.
Linking Loader :- The need for linking a program with
other programs assign because a program
written by a programmer or its translated version is really of a stand alone
nature. That is a program generally cannot executed on its own. Without
requiring the presents of some other programs is computer memory. The standard
function must reside into the main memory. The linking function next address of
program known to each other, So that such transfers can take place during the
execution.
Relocation Loader :- Another
function commonly performed by a loader is that of program re – location. Relocation is simply moving a
program from one area to another in the
storage. It referred to adjustment of address field and not to movement of a
program. The task of relocation is to add some constant value to each
relative address in the segment the part
of a loader which performed relocation is called re – location loader.
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