8085 Microprocessor Lab - Viva Questions

8085 Microprocessor Lab Viva Questions With Answers

1. What are the important CPU registers in the 8085 microprocessor? Explain.

a. Accumulator (A): It is an 8-bit register which involves in all accumulator related instructions.

b. Program counter (PC): It is a 16-bit register which carries the address of the next instruction to be executed.

c. General purpose registers (A, B, C, D, E, H, L): These registers can he used individually or as BC, DE and HL register pairs.

d. Stack pointer (SP): A 16-bit register always points the top of the stack.

e. Flag register (F): Contains five flags which indicate the processor status. Details of the flags are given below.

Carry flag: If a carry is generated in an arithmetic operation carry flag is set. The carry flag functions as a borrow flag for subtraction.

Auxiliary carry flag: Indicates the overflow from fourth bit of Accumulator due to arithmetic operation.

Sign flag: It indicates the sign of the number in the accumulator. MSB of accumulator indicates the sign of the number made of the remaining 7 bits. After the execution of arithmetic operation, if bit D7 of accumulator is 1, the sign flag will be set.

Parity flag: It sets to 1 if the parity of the accumulator content is even.

Zero flag: It sets to 1 if the result generated by certain instruction is zero.

2.  Which are the addressing modes of 8085 ? Explain with examples.

a. Direct addressing: In this mode of addressing the address of the operand is specified in the instruction.
Example: STA 4100; Address 4100 is specified in the instruction

b. Register addressing: Operands will be in the register.
Example: MOV A, B ; Name of the register will be specified in the instruction.

c. Register indirect addressing: Address of the operand will be placed in the register.
Example: MOV A, M ; address of the memory location is kept in HL pair.

d. Immediate addressing: Operand is specified in the instruction.
Example: MVI A, 44 ; datum 44 is specified in the instruction.

3. How many data lines and address lines are in 8085 microprocessor?

8 data lines and 16 address lines.

4. What is the use of HOLD pin of the microprocessor?

It supports the Direct Memory Access (DMA). When a interfacing device needs to access the microprocessor, DMA controller places a high input on HOLD line. Microprocessor then relinquishes the control of the bus and acknowledges the receipt of the request to the DMA controller. When the DMA operation is over, HOLD line will be brought down by the DMA controller and in turn, CPU exists from the hold state.

5. What is the use of ALE pin of 8085?

A positive going pulse on ALE line indicates that the bits on AD7 — AD0 are address bits. This signal is utilized to handle the low order address from the multiplexed bus and make a separate set of eight address lines.

6. What is the use of S0 and S1 pins of 8085?

The bit combination on these lines indicates the status of the microprocessor.

7. What is the use of READY pin of 8085?

Microprocessor waits until the signal on this line is high to access data from a peripheral device. This is used to delay the microprocessor Read or Write cycles until a slow peripheral device is ready to access the data.

8. Give examples of one byte, two byte and three byte instructions.

One byte instruction: MOV B, C
Two byte instruction: MVI A, 33
Three byte instruction: LXI H, 4302H

9. Give an example each for data transfer, arithmetic, logical, branch and machine control instructions.

Data transfer: MOV A, B
Arithmetic: ADD A
Logical: ORA B
Branch: JMP 4127H
Machine control: HLT

10. What is the TRAP?

It is a non-maskable hardware interrupt. It has the highest priority among all interrupts. This interrupt is utilized to service very urgent requests such as power supply failure. It is also known as RST 4.5.

11. What is the difference between memory mapped I/O and I/O mapped I/O?

In memory mapped I/O, the address of I/O devices is in the address map of 8085 and the I/O devices are accessed by the instructions STA and LDA. In I/O mapped I/O, 8 - bit address is used to address the I/O devices. The instructions IN and OUT are used to access the I/O devices in this type.

12. How many I/O devices can be connected to the system with I/O mapped I/O? Why?

256 input ports and 256 output ports. The operand of IN and OUT instruction is 8 bit.

13. Which are the important interfacing chips of a microprocessor?

8255: Programmable peripheral interface
8253: Programmable interval timer
8251: Programmable communication interface (USART-Universal synchronous/asynchronous receiver/transmitter)
8279: Programmable keyboard/display interface
8259: Programmable interrupt controller

14. How many memory locations can be addressed by 14 address lines?

16 k

15. How the microprocessor kits display something on the display panel as soon as the RESET key is pressed?

When RESET key is pressed, the program counter holds the memory address 0000H. Then the monitor program stored in that location will be executed.

16. What is an assembler and dissembler?

Assembler is a program that translates the mnemonics into corresponding machine codes. Dissembler does the opposite process.

17. How many address and data lines are there in 8086 microprocessor?

20 address lines and 16 data lines.

18. How much is the maximum memory capacity of 8086 microprocessor?

1 Mega byte

19. What is 8087 IC ?

It is the coprocessor chip compatible with 8086 microprocessor to do numerical calculations.

20. How the carry flag can be reset to zero without affecting the Accumulator contents?

By two instructions STC and CMC.

21. What will be the status of carry and zero flags when instruction SUB A is executed?

Carry flag will be reset and the zero flag will be set.

22. Write a few examples for 16 bit microprocessors?

Intel iAPX 8086/8088 & 80186/286, Zilog Z 8001/8002, Motorola 68000 and National semiconductor NS 16000.

23. Describe briefly Intel 80486 and Pentium-1 processors.

Former is a 32-bit processor which can operate at clock frequencies from 25 MHz to 66 MHz. Latter is a 32-bit address bus and 64-bit data bus processor. It can operate at clock frequencies 60 MHz, 66 MHz and 90 MHz.

24. What is a stack and stack pointer?

Stack is a portion of RAM that can be set up by the user to store data. Data in a stack can be accessed using the instructions PUSH and POP. Stack pointer is a 16-bit register inside the microprocessor indicates the topmost location of the stack.

25. What follows the mnemonic LHLD?

Address of memory location

26. What is a microcontroller?

It is a single chip on which a micro processor, memory and I/O signal lines are fabricated using VLSI technology.

27. What is the priority order of hard ware interrupts in 8085 ?

Trap, RST 7.5, RST 6.5, RST 5.5 and INTR. Trap has the highest priority and INTR has the lowest.

28. What are the sequences of operations that occur when CALL & RET instructions are executed?

When a CALL instruction is executed, the address of next instruction will be stored in the stack and the address of the subroutine will be stored in program counter.

When RET instruction is executed, the address which was stored in the stack will move to program counter.

29. How many machine cycles are required to execute IN instruction? Explain.

It needs three machine cycles. First one for the op code fetch. Second to fetch the port address that follow the IN instruction. Third to fetch the data from the port to Accumulator.

30. How many address lines are required to address 2 k byte memory?

11 lines.

Sreejith Hrishikesan

Sreejith Hrishikesan is a ME post graduate and has been worked as an Assistant Professor in Electronics Department in KMP College of Engineering, Ernakulam. For Assignments and Projects, Whatsapp on 8289838099.

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