System Software - Unit 2

1. Define the basic functions of assembler.

 * Translating mnemonic operation codes to their machine language equivalents.

* Assigning machine addresses to symbolic labels used by the programmer.

 2. What is meant by assembler directives? 

Give example. These are the statements that are not translated into machine instructions, but they provide instructions to assembler itself. 

example START,END,BYTE,WORD,RESW and RESB

3. What are forward references? 

It is a reference to a label that is defined later in a program.

 Consider the statement 

10 1000 STL RETADR 

. . . . . . . . 

80 1036 RETADR RESW 1 

The first instruction contains a forward reference RETADR. If we attempt to translate the program line by line, we will unable to process the statement in line10 because we do not know the address that will be assigned to RETADR .The address is assigned later(in line 80) in the program. 

4. What are the three different records used in object program? 

The header record, text record and the end record are the three different records used in object program.

 (i)The header record contains the program name, starting address and length of the program. 

(ii)Text record contains the translated instructions and data of the program. 

(iii) End record marks the end of the object program and specifies the address in the program where execution is to begin.

5. What is the need of SYMTAB (symbol table) in assembler? 

The symbol table includes the name and value for each symbol in the source program, together with flags to indicate error conditions. Some times it may contain details about the data area. SYMTAB is usually organized as a hash table for efficiency of insertion and retrieval.

6. What is the need of OPTAB (operation code table) in assembler? 

The operation code table contains the mnemonic operation code and its machine language equivalent. Some assemblers it may also contain information about instruction format and length. OPTAB is usually organized as a hash table, with mnemonic operation code as the key.

7. What are the symbol defining statements generally used in assemblers? 

„EQU‟-it allows the programmer to define symbols and specify their values directly. 

The general format is 

Symbol EQU value „ORG‟-it is used to indirectly assign values to symbols. When this statement is encountered the assembler resets its location counter to the specified value. The general format is ORG value In the above two statements value is a constant or an expression involving constants and previously defined symbols.

8. Define relocatable program. 

An object program that contains the information necessary to perform required modification in the object code depends on the starting location of the program during load time is known as relocatable program. 

9. Differentiate absolute expression and relative expression. 

If the result of the expression is an absolute value (constant) then it is known as absolute expression. Eg: BUFEND – BUFFER If the result of the expression is relative to the beginning of the program then it is known as relative expression. Label on instructions and data areas and references to the location counter values are relative terms. Eg: BUFEND + BUFFER

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 address associated 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.