Microprocessors Interview Questions of part --13

Microprocessors Interview Questions of part --13

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.
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.
Write the use of Z bit in opcode?
It is used by REP instruction. Z bit helps to control the loop. It will be repeated the number of items indicated by the CX register or until Z bit dose not match the Zero flag whichever occurs first.
What is the use of V bit in opcode ?
It is used by shift and rotate instructions to determine the number of shifts. V bit is set to 0 if the shift count is to be 1. V bit is set to 1 if the CL register contains the shift count.
What is the use of D bit in opcode?
D bit: It is used in double operand instruction. It is used to indicate whether the register specified by REG is the source operand (D=0) or the destination operand (D=1).
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.
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
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 available . It is of the form :
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 signle bytes and the results are adjusted. For Division , the adjustment is done before the binary division.
What is AAS instruction in 8086 ?
Adjust Result of ASCII SubtractionThis instruction adjust the binary result of SUB or SBB instruction.
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.
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.
Write about the following instruction :
MOV CS:[BX], DLMOV 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.
What is difference between DIV and IDIV instruction in 8086 ?
DIV : It operates only on unsigned number.IDIV : It operates only on signed numbers.
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.