The 8085 Microprocessor – Question and Answer

1. How many machines cycles and T-states are needed to execute STA 4200H in 8085 processor?

To execute STA 4200H instruction, 4 machine cycles and 13 T-states are needed.

Machine Cycles         T-states
Opcode fetch                4
Memory Read               3
Memory Road               3
Memory Write              3

2. List out the function of accumulator?
  1. Accumulator holds one of the data to be processed by ALU.
  2. It also stores the result of the operation.
3. What do you mean by assembler?

An assembler is a program which translates an assembly language program (ALP) into machine language program.

4. Give the sequence of operation that takes place during the execution of CALL instruction.
  1. The address of the next instruction of the main program is saved in the stack.
  2. The content of the SP is decremented by 2 indicate the new stack top.
  3. Then the program jumps to subroutine staring address specified the label.
5. What is program counter?

Program counter is a 16-bit special purpose register. It is used to hold the memory address of the next instruction to be executed. It keeps the track of memory address of the instructions in a program while they are being executed.

6. Define indirect addressing mode and give and example.

In the indirect addressing mode, the instruction specifies the name of the register in which the address of the data is available.

Example : MOV A, M
[A] ← [[HL]]

7. What are the types of instruction for 8085 microprocessor?
  1. Data transfer instruction
  2. Arithmetic instruction
  3. Logical and rotate instruction
  4. Branch instruction
  5. Stack instruction
  6. I/O and machine control instruction
8. What are the addressing modes for 8085 microprocessor?
  1. Direct addressing mode
  2. Register addressing mode
  3. Register Indirect addressing mode
  4. Immediate addressing mode
9. Write the operation carried out when 8085 executes DAD H instruction.
[H] ← [HL] + [HL]

The content of HL pair is added with the same content and the result is stored in hl pair itself.

10. What is an interrupt?

Interrupt is an signal send by an external device to the processor so as to request the processor to perform a particular task or work.

11. What is the difference between Opcode and Operand?

Opcode is the part of an instruction that identifies a specific operation.
Operand is a part of an instruction that represents a value on which the instruction acts.

12. When the READY signal of 8085 processor is sampled by the processor?

The 8085 microprocessor samples (checks) the READY signal at the Second T-state of every machine cycle.

13. What is the function of SID and SOD pins in 8085?

SID (Serial Input Data) line can be input to the most significant bit of the accumulator.

SOD (Serial Output Data) line can be used to output the most significant bit of the accumulator.

14. What is the operation carried out when RST 1 instruction is executed by 8085?

RST 1 is one of the software interrupts. Its vector address is 0008 H.

When RST 1 instruction is executed by 8085, the microprocessor pushes the content of PC to stack. Then loads the vector address (0008 H) in PC and executing an ISR stored in this address.

15. What is stack and subroutine?

Stack : It is set of memory locations in memory.
Subroutine : It is a group of instructions written separately from the main program to perform a function that occurs repeatedly in the main program.

16. Why data bus is bidirectional?

The microprocessor has to fetch (read) the data from memory or input device for processing and after processing, it has to store (Write) the data to memory or output device. Hence the data has is bidirectional.

17. What is T-state?

The T-state is the time period of the internal clock signal of the microprocessor. The time taken by the microprocessor to execute the machine cycle is expressed in T-state.

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Santhakumar Raja


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