250+ TOP MCQs on Instruction Set of 8086/8088 and Answers

Microprocessors Questions and Answers for Freshers on “Instruction Set of 8086/8088 – 3”.

1. The mnemonic that is placed before the arithmetic operation is performed is
a) AAA
b) AAS
c) AAM
d) AAD
Answer: d
Clarification: The AAD instruction converts two unpacked BCD digits in AH and AL to the equivalent binary number in AL.

2. The Carry flag is undefined after performing the operation
a) AAA
b) ADC
c) AAM
d) AAD
Answer: d
Clarification: Since the operation, AAD is performed before division operation is performed, the carry flag, auxiliary flag and overflow flag are undefined.

3. The instruction that performs logical AND operation and the result of the operation is not available is
a) AAA
b) AND
c) TEST
d) XOR
Answer: c
Clarification: In the TEST instruction, the logical AND operation is performed and the result is not stored but flags are affected.

4. In the RCL instruction, the contents of the destination operand undergo function as
a) carry flag is pushed into LSB & MSB is pushed into the carry flag
b) carry flag is pushed into MSB & LSB is pushed into the carry flag
c) auxiliary flag is pushed into LSB & MSB is pushed into the carry flag
d) parity flag is pushed into MSB & LSB is pushed into the carry flag
Answer: a
Clarification: In RCL(Rotate right through carry), for each operation, the carry flag is pushed into LSB and the MSB of the operand is pushed into carry flag.

5. The instruction that is used as prefix to an instruction to execute it repeatedly until the CX register becomes zero is
a) SCAS
b) REP
c) CMPS
d) STOS
Answer: b
Clarification: The instruction to which the REP is prefix, is executed repeatedly until CX register becomes zero. When CX becomes zero, the execution proceeds to the next instruction in sequence.

6. Match the following

A) MOvSB/SW       1) loads AL/AX register by content of a string
B) CMPS           2) moves a string of bytes stored in source to destination
C) SCAS           3) compares two strings of bytes or words whose length is stored in CX register
D) LODS           4) scans a string of bytes or words

a) A-3,B-4,C-2,D-1
b) A-2,B-1,C-4,D-3
c) A-2,B-3,C-1,D-4
d) A-2,B-3,C-4,D-1
Answer: d
Clarification: By using the string instructions, the operations on strings can be performed.

7. The instructions that are used to call a subroutine from the main program and return to the main program after execution of called function are
a) CALL, JMP
b) JMP, IRET
c) CALL, RET
d) JMP, RET
Answer: c
Clarification: At each CALL instruction, the IP and CS of the next instruction are pushed onto the stack, before the control is transferred to the procedure. At the end of the procedure, the RET instruction must be executed to retrieve the stored contents of IP & CS registers from a stack.

8. The instruction that unconditionally transfers the control of execution to the specified address is
a) CALL
b) JMP
c) RET
d) IRET
Answer: b
Clarification: In this the control transfers to the address specified in the instruction and flags are not affected by this instruction.

9. Which instruction cannot force the 8086 processor out of ‘halt’ state?
a) Interrupt request
b) Reset
c) Both interrupt request and reset
d) Hold
Answer: d
Clarification: Only an interrupt request or Reset will force the 8086 processor to come out of the ‘halt’ state.

10. NOP instruction introduces
a) Address
b) Delay
c) Memory location
d) None of the mentioned
Answer: b
Clarification: NOP is the No operation. It means that the processor performs no operation for the clock cycle and thus there exists a delay.

11. Which of the following is not a machine controlled instruction?
a) HLT
b) CLC
c) LOCK
d) ESC
Answer: b
Clarification: Since CLC is a flag manipulation instruction where CLC stands for Clear Carry Flag.