Short Question And Answer Microprocessor Set-3

1. Define Inter segment Indirect addressing mode:
It replaces the contents of IP and CS with the contents of two consecutive words in memory that are referenced by using any of the data related addressing modes except immediate and register modes.

2. What is the use of PUSHF instruction used in 8086 ?
It pushes the flag register contents to the top of the stack. No flags are affected.

3. Define XLAT instruction used in 8086.
It translates a byte in AL using a table in memory. The offset address is calculated by adding the 8 bit contents of the AL register and the contents of BX register. BX register contains the starting offset address of the Lookup table. After execution , corresponding data memory contents of the lookup table are loaded into the AL register.

4. What is the use of ‘W’ bit in opcode ?.
W bit in op-code : If an instruction in 8086 can operate on either a byte or a word , the op-code includes a W-bit which indicates whether a byte (W =0) or a word (W =1) is being accessed.

5. What is DAS instruction ?

DAS : Decimal Adjust After Subtraction This instruction converts the binary result of a SUB or SBB instruction in AL to Packed BCD format. It operates only on AL Register.

6. What is Packed BCD Format?
Packed BCD Format: Packed BCD Numbers are stored in two digits to a byte, in 4 bit groups referred to as nibbles. ALU is capable of performing only binary addition and subtraction, but by adjusting the sum or difference the correct result in packed BCD format.

7. What is the value of ‘AX’ after executing following instructions?
Ans : AX= 0100 H

8. What is difference between DIV and IDIV instruction in 8086 ?
DIV : It operates only on unsigned number. IDIV : It operates only on signed numbers.

9. Write about the following instruction :

MOV CS:[BX],DL -It copies a byte from DL Register. Effective Address for the memory location is contained in the BX Register. Normally an effective address in BX will be added to the data segment base in DS to produce the
physical memory address. In this instruction CS: indicates that we want the BIU to add the effective address to the code segment base in CS to produce the physical address. content of(BX+(CS) * 1610) = (DL)

10. What is Programmed I/O ?
Programmed I/O : It consists of continually examining the status of an Interface and performing an I/O operation with the Interface when its status indicates that it has data to be input or its data-out buffer register is ready to receive data from the CPU.

11.What is the use of Directives in 8086 ?
Directives in 8086 give directions to the assembler during the assembly process but are not translated into machine instruction

12.What is AAS instruction in 8086 ?
AAS: Adjust Result of ASCII Subtraction This instruction adjusts the binary result of SUB or SBB instruction.

13.What is Unpacked BCD Format?
Unpacked BCD Format: In Unpacked BCD, there is only one digit per byte and because of this, unpacked multiplication and division can be done. Binary operations act on single bytes and the results are adjusted. For Division, the adjustment is done before the binary division.

14. What is the value of AL after executing the following instructions.
Ans : AL= 84

15.Define Segment Override Prefix.
Segment Override Prefix : To permit exceptions to the Segment Register usage, a special one byte instruction called a segment override prefix is 16.Write the difference between CBW and CWD instruction in 8086. CBW : Convert a byte to a word, sign extend AL Register into a AH Register. CWD : Convert a word to a double word ,sign extend the AX Register into the DX Register.

17.Write about the following instruction : LES CX, [3483H]
Copy the contents of memory at the displacement of 3483H in DS to CL, contents of 3484H in DS to CH, and copy the contents of memory at displacement of 3485H and 3486H in DS to ES Register. the shift count.

18.How the execution time of an instruction is calculated?
Execution time of an instruction can be determined by multiplying the number of clock pulses needed to execute the instruction by the clock period.

19.Define EQU directive in 8086 assembly language?
It is used to assign a name to constants used in the programs. Consider the statement „corr EQU 07H‟ , this would tell the assembler to insert the value 07H everytime that it finds the name corr in the program statement.

20.Write the use of Assume directive in 8086 assembly language using one example?
Assume CS: code_here, DS:data_here It tells the assembler that the logical segment code_here contains the instruction statements for the program and should be treated as a code segment. It also tells the assembler that it should treat the logical segment data_here as the data segment for this program.