Types of loaders in system software?

Answer 1

It is normally the responsibility of the Operating System to load and execute files. The part of the operating system that performs this function is called a loader. The absolute loader

There are two types of loaders, relocating and absolute. The absolute loader is the simplest and quickest of the two. The loader loads the file into memory at the location specified by the beginning portion (header) of the file, then passes control to the program. If the memory space specified by the header is currently in use, execution cannot proceed, and the user must wait until the requested memory becomes free. The relocating loader

The relocating loader will load the program anywhere in memory, altering the various addresses as required to ensure correct referencing. The decision as to where in memory the program is placed is done by the Operating System, not the programs header file. This is obviously more efficient, but introduces a slight overhead in terms of a small delay whilst all the relative offsets are calculated. The relocating loader can only relocate code that has been produced by a linker capable of producing relative code. A loader is unnecessary for interpreted languages, as the executable code is built up into the memory of the computer.

Answer 2

different types of loader in system software

• Compile-and-Go Loaders:
• A compile and go loader is one in which the assembler itself does the processes of compiling then place the assembled instruction inthe designated memory loactions.
• The assembly process is first executed and then the assembler causes a transfer to the first instruction of the program.
• E.G. WATFOR FORTRAN compiler
• This loading scheme is also called assmble-and-go.

  Advantages of Compile-and-go loaders:

• Simple and easier to implement.
• No additional routines are required to load the compiled code into the memory.

  Dis advantages of Compile-and-go loaders:

• Wastage im memory spave due to the presence of the assembler.
• There is a need to re-assemble the code every time it is to be run.
• It becomes increasingly difficult to handle large number of segments when the input code is written in a variety of HLL say one routine in pascal and one in FORTRAN and so on.
• Such loader make designing modular programs and systems near impossible.

  Absolute Loader

  The assembler generates the object code equivalent of the source program bu the output is punched on to the cads forming the object decks instead of loading in memory.

  The function of the loader is to read these cads and load them into memory specified by the assembler. The four functions as performed in and absolute loader are :

  1.Allocation

  2.Linking

  3.Relocation

  4.Loading

  Advantages of Absolute Loader:

• Simple, easy to design and implement.
• Since more core memory is available to the user there is no memory limit.

  Dis advantages of Absolute Loader:

• The programmer must specifically tell the assembler the address where the program is to be loaded.
• When subroutines are referenced, the programmer must specify their address whenever they are called.

Machine Dependent Features

Relocation loader

• Loaders that allow for program relocation are called relocating loaders/relative loaders.
• There are two methods for this purpose.
• In the first method, a modification record is used to describe each part of the object code that must be changed when the program is relocated.
• There is one modification record for each calue that must be changed during relocation.
• Each modification record specifies the starting address and length of the field whose value is to be altered.
• It then describes the modification to be performed.
• This method is not well suited for all machine architectures,
• On a machine that primarily uses direct addressing and hads a fixed instruction format, a different rechnique is used.
• The text records are same as before except that there is a relocation bit with each word of object code.
• These relocation bits are gathered together into a bit mask following a length indicator in each text record.
• If the bit is set to 1, the programs's starting address must be added to the word when program is relocated.
• If bit is 0, then no modification is necessary. The bits for unused words are also set to 0.
• The BSS loader permits the usage of multiple subroutine segments or code pats and only one data segmet, which is in common with the others.
• Each routine is assembled seperately and this transferred to the loader with appropriate informatin relating to relocation and references among the subroutine along with the actual text.
• The instructions, which depend on the address where the code is loaded, are termed address sensitive instructions.

  Program linking loader

  Let an application program 'A' consists of a set of program units. A program units interacts with another by using the addresses latter's instructions and data in its own instructions. To enable this, the program units must contain public defintions and external references. A public definition in a program unit may be reference in others. An external reference is a reference to symbol, which is not defined in the program unit containing the reference.

  EXTRN and ENTRY statements are used to deeal with the above cases. ENTRY statement lists the public definitions of program unit, whcih may be referenced in other program units. EXTRN statement lists the symbols to which the external references are made in the program units.

  Before executing the program A, for each of its program units, every external reference shoyld bound to the correct link time address. This is called "LINKING".Once it is linked, it is said to be resolved.

  A binary program is a machine language program consisting of a set of program units, such that each program unit has been relocateed to the memory area starting at its link origin and linking has bee performed for each external reference in the program units.

Answer 3

There are four types of loader in system software namely:

1: Compile and Go loader

2: Absolute Loader

3: Relocating Loader

4: Direct Linking loader

Regards

Subhash

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