Part 2
10. Write the steps required to translate the source program to
object program.
Convert mnemonic operation codes to their machine language
equivalents.
Convert symbolic operands to their equivalent machine addresses
Build the machine instruction in the proper format.
Convert the data constants specified in the source program into their
internal machine representation
Write the object program and assembly listing.
11. What is the use of the variable LOCCTR (location counter) in
assembler?
This variable is used to assign addresses to the symbols. LOCCTR is initialized to
the beginning address specified in the START statement. After each source
statement is processed the length of the assembled instruction or data area to be
generated is added to LOCCTR and hence whenever we reach a label in the source
program the current value of LOCCTR gives the addressassociated with the label.
12. Define load and go assembler.
One pass assembler that generates their object code in memory for immediate
execution is known as load and go assembler. Here no object programmer is
written out and hence no need for loader.
13. What are the two different types of jump statements used in
MASM assembler?
Near jump
A near jump is a jump to a target in the same segment and it is assembled by using
a current code segment CS
Far jump
A far jump is a jump to a target in a different code segment and it is assembled by
using different segment registers .
14. What is the use of base register table in AIX assembler?
A base register table is used to remember which of the general purpose registers
are currently available as base registers and also the base addresses they contain.
USING statement causes entry to the table and .DROP statement removes the
corresponding table entry.
15. Differentiate the assembler directives RESW and RESB.
RESW –It reserves the indicated number of words for data area.
Eg: 10 1003 THREE RESW 1
In this instruction one word area (3 bytes) is reserved for the symbol THREE. If
the memory is byte addressable then the address assigned for the next symbol is
1006.
RESB –It reserves the indicated number of bytes for data area.
Eg: 10 1008 INPUT RESB 1
In this instruction one byte area is reserved for the symbol INPUT .Hence the
address assigned for the next symbol is 1009.
16. Define modification record and give its format.
This record contains the information about the modification in the object code
during program relocation. the general format is
Col 1 M
Col 2-7 Starting location of the address field to be modified relative to the
beginning of the program
Col 8-9 length of the address field to be modified in half bytes.
17. Write down the pass numbers (PASS 1/ PASS 2) of the
following activities that occur
in a two pass assembler:
a. Object code generation
b. Literals added to literal table
c. Listing printed
d. Address location of local symbols
Answer:
a. Object code generation - PASS 2
b. Literals added to literal table – PASS 1
c. Listing printed – PASS2
d. Address location of local symbols – PASS1
18. What is meant by machine independent assembler features?
The assembler features that do not depend upon the machine architecture are
known as machine independent assembler features.
Eg: program blocks, Literals.
19. How the register to register instructions are translated in
assembler?
In the case of register to register instructions the operand field contains
the register name. During the translation first the object code is converted into its
corresponding machine language equivalent with the help of OPTAB. Then the
SYMTAB is searched for the numeric equivalent of register and that value is
inserted into the operand field.
Eg: 125 1036 RDREC CLEAR X B410
B4-macine equivalent of the opcode CLEAR
10-numeric equivalent of the register X.
20. What is meant by external references?
Assembler program can be divided into many sections known as control sections
and each control section can be loaded and relocated independently of the others. If
the instruction in one control section need to refer instruction or data in another
control section .the assembler is unable to process these references in normal way.
Such references between control are called external references.
21. Define control section.
A control section is a part of the program that maintains its identity after
assembly; each control section can be loaded and relocated independently of the
others. Control sections are most often used for subroutines. The major benefit of
using control sections is to increase flexibility.
22. What is the difference between the assembler directive
EXTREF and EXTDEF.
EXTDEF names external symbols that are defined in a particular control
Section and may be used in other sections.
EXTREF names external symbols that are referred in a particular control
section and defined in another control section.
23. Give the general format of define record.
This record gives information about external symbols that are defined in a
particular control section. The format is
Col 1 D
Col 2-7 name of external symbol defined in this control section
Col 8-13 relative address of the symbol with in this control section
Col 14-73 name and relative address for other external symbols.
24. Give the use of assembler directive CSECT and USE
CSECT - used to divide the program into many control sections
USE – used to divide the program in to many blocks called program blocks
25. What is the use of the assembler directive START?
The assembler directive START gives the name and starting address of the
program. The format is PN START 1000
Here
PN – Name of the program
1000 - Starting address of the program.
LOADERS AND LINKERS
1. What are the basic functions of loaders?
Loading – brings the object program into memory for execution
Relocation – modifies the object program so that it can be loaded at an
address different from the location originally specified
Linking – combines two or more separate object programs and also
supplies the information needed to reference them.
2. Define absolute loader.
The loader, which is used only for loading, is known as absolute loader.
e.g. Bootstrap loader
3. What is meant by bootstrap loader?
This is a special type of absolute loader which loads the first program to be run by
the computer. (usually an operating system)
4. What are relative (relocative) loaders?
Loaders that allow for program relocation are called relocating (relocative) loaders.
5. What is the use of modification record?
Modification record is used for program relocation. Each modification record
specifies the starting address and the length of the field whose value is to be altered
and alsodescribes the modification to be performed.
6. What are the 2 different techniques used for relocation?
Modification record method and relocation bit method.
7. Define Relocation bit method.
If the relocation bit corresponding to a word of object code is set to 1, the
program’s starting address is to be added to this word when the program is
relocated. Bit value 0 indicates no modification is required.
8. Define bit mask.
The relocation bits are gathered together following the length indicator in each text
record and which is called as bit mask. For e.g. the bit mask FFC(111111111100)
specifies that the first 10 words of object code are to be modified during relocation.
9. What is the need of ESTAB?
It is used to store the name and address of the each external symbol. It also
indicatesin which control section the symbol is defined.
10. What is the use of the variable PROGADDR?
It gives the beginning address in memory where the linked program is to
be loaded. The starting address is obtained from the operating system.
11. Write the two passes of a linking loader.
Pass1: assigns address to all external symbols
Pass2: it performs actual loading, relocation and linking.
12. Define automatic library search.
In many linking loaders the subroutines called by the program being loaded are
automatically fetched from the library, linked with the main program and loaded.
This feature is referred to as automatic library search.
13. List the loader options INCLUDE &DELETE.
The general format of INCLUDE is INCLUDE program_name (library name) This
command direct the loader to read the designated object program from a library
and treat it as the primary loader input. The general format of DELETE command
is DELETE Csect-name It instructs the loader to delete the named control sections
from the sets of programs loaded.
14. Give the functions of the linking loader.
The linking loader performs the process of linking and relocation. It includes the
operation of automatic library search and the linked programs are directly loaded
into the memory.
15. Give the difference between linking loader and linkage editors.
Linking loader Linkage editor
1. The relocation and linking is performed each time the program is loaded
1. It produces a linked version of a program and which is written in a file for
later execution Here the loading can be accomplished in a single pass Two
passes are required
16. Define dynamic linking.
If the subroutine is loaded and linked to the program during its first call
(run time), then it is called as dynamic loading or dynamic linking.
17. Write the advantage of dynamic linking.
It has the ability to load the routine only when they are needed
The dynamic linking avoids the loading of entire library for each
execution.
18. What is meant by static executable and dynamic executable?
In static executable, all external symbols are bound and ready to run. In dynamic
executables some symbols are bound at run time.
19. What is shared and private data?
The data divided among processing element is called shared data.
If the data is not shared among processing elements then it is called private data.
20. Write the absolute loader algorithm.
Begin
Read Header record
Verify program name and length
Read first text record
While record type != ‘E’ do
Begin
Moved object code to specified location in memory
Read next object program record
End
Jump to address specified in End record.
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