System Software - Unit III Question Bank

System Software - Unit III - Question Bank

1.      Define loader?

A loader is a system program that performs the loading function. Many loaders also support relocation and linking. Some systems have a linker to perform the linking operation and a separate loader to handle relocation and loading.

2. What is meant by loading in system software?

Loading means brings the object program into memory for execution.

3. Define relocation?

Relocation modifies the object program so that it can be loaded at an address different from the location originally specified.

4. Define linking?

Linking means combines two or more separate object programs and supplies the information needed to allow references between them.

5. What is an absolute loader?

All functions of absolute loader are performed in a single pass. The header record is checked to verify that the correct program has been presented for loading. As each Text record is read, the object code it contains is moved to the indicated address in memory. When the End record is encountered, the loader jumps to the specified address to begin execution of the loader program.

6. How the object code for STL instruction is loaded into memory?

Each pair of bytes from the object program record must be packed together into one byte during loading. For example, the machine operation code for an STL instruction would be represented by the pair of characters “1” and “4”. When the loader reads these, they will occupy two bytes of memory. However this operation code must be stored as one byte during loading.

7. Write the algorithm for an absolute loader?

Begin

Read Header record

Verify program name and length

Read first Text record

While record type 

Begin

{ if object code is in character form, convert into internal representation } move object code to specified location in memory read next object program record

        end
        jump to address specified in End record
end

8.   What is a Bootstrap loader?

When a computer is first turned on or restated, a special type of absolute loader, called a bootstrap loader, is executed. This loads the first program to be run by the computer-usually an operating system.

9.   Write a subroutine using SIC/XE to read one character from input device and convert it from ASCII code to hexadecimal and is returned in a register A.

GETC        TD INPUT
JEQ GETC
RD INPUT
COMP #4
JEQ 80
COMP #48
JLT GETC
SUB #48
COMP #10
JLT RETURN
SUB #7

RETURN RSUB
INPUT BYTE X’ F1’
END LOOP

10. What are relocating loader?

Loaders that allow for program relocation are called relocating loader or relative loaders. There are two methods for relocating. a) A modification record is used to describe each part of the object code that must be changed when the program is relocated.

b) A machine that uses direct addressing and has a fixed instruction format, it is more efficient to specify relocation. Along with the Text record a relocation bit is associated with each word of the object code.

11. Write notes on relocation bit?

A machine that uses direct addressing and has a fixed instruction format, it is more efficient to specify relocation. Along with the Text record a relocation bit is associated with each word of the object code.

If the relocating bit corresponding to a word of object code is set to 1, the programs starting address is to be added to this word when the program is relocated. A bit value of 0 indicates no relocation is needed. If a Text record contains fewer than 12 words of object code, the bits corresponding to unused words are set to 0.

12. What are the data structures used for a linking loader?

The various data structures used are

ESTAB -External symbol table b) PROGADDR – Beginning of address in memory where the linked program is to be loaded. c) CASADDR -Contain the starting address assigned to the control section currently being scanned by the loader.

13. What are the contents of ESTAB?

It is used to store the name and address of each external symbol in the set of control section being loaded. The table also often indicates in which control section the symbol is defined. A hashed organization is typically used for this table.

14. What is the content of CSADDR during pass 1?

CSADDR – It contains the starting address assigned to the control section currently being scanned by the loader. This value is added to all relative addresses within the control section to convert them to actual addresses.

15. What are the functions of pass 1 of loader?

During the first pass, the loader is concerned only with Header and Define record types in the control sections. The control section name from the Header record is entered into ESTAB, with value given by CSADDR. All external symbols appearing in the Define record for the control section are also entered into the ESTAB. When the End record is read, the control section length CSLTH is added to CSADDR.

16. What the function of pass 2 of loader?

Pass 2 performs the actual loading, relocation and linking of the program. CSADDR contains the actual starting address of the control section currently being loaded. As each Text record is read, the object code is moved to the specified address. When Modification record is encountered, the symbol whose value is to be used to modification is looked up in ESTAB. This value is added or subtracted from the indicated location in memory. The last step is to transfer the control the loaded program to begin execution.

17. Define load map?

At the end of Pass1, ESTAB contains all external symbols defined in the set of control sections together with the address assigned to each. Many loaders include as an option the ability to print a load map that shows these symbols and their addresses.

18. What is meant by automatic library call?

Many linking loaders automatically incorporate the routines from a subprogram library into the program being loaded. This feature allows the programmer to use subroutines from one or more libraries as if they were part or the programming language. On some systems, this feature is referred to as automatic library call or library search.

19. What are the commands used for the selection of alternative sources of input in loaders, delete external symbols and to change the external symbol?

One typical loader option allows the selection of alternate sources of input. Ex:-INCLUDE program-name(library-name) Might direct the loader to read the designated object program from a library and treat it as if it were part of the primary loader input Other commands allow the user to delete external symbols or entire control sections. Ex DELETE Csect-name Might instruct the loader to delete the named control section from the set of programs being loaded. The command

CHANGE name1,name2 might cause the external symbol name1 to be changed to name 2.

20. What is a linkage editor?

A linkage editor produces a linked version of the program, which is written to a file or library for later execution. The linkage editor performs relocation of all control sections relative to the start of the linked program. This means that the loading can be accomplished in one pass with no external symbol table required. The linked program produced by a linkage editor is generally in a form that is suitable for processing by a relocating loader.

21. What is a linking loader?

A linking loader performs all linking and relocation operations, including automatic library search, and loads the linked program directly into the memory for execution. A linking loader searches libraries and resolves external references every tine the program is executed.

22. Write the block diagram   of processing of an object program using a linking loader?

23. Write the block diagram of processing of an object program using a linkage editor? 

24. What is meant by dynamic linking?

This scheme postpones the linking function until execution time: a subroutine is loaded and linked to the rest of the program when it is first called. This type of function is usually called dynamic linking, dynamic loading or load on call.

25. Define binding?

When dynamic linking is used, the association of an actual address with the symbolic name of the called routine is not made until the call statement is executed. Another way of describing this is to say that the binding of the name to an actual address is delayed from load time until execution time.