8085 microprocessor questions

  1. What are the various registers in 8085? - Accumulator register, Temporary register, Instruction register, Stack Pointer, Program Counter are the various registers in 8085 .
  2. In 8085 name the 16 bit registers? - Stack pointer and Program counter all have 16 bits.
  3. What are the various flags used in 8085? - Sign flag, Zero flag, Auxillary flag, Parity flag, Carry flag.
  4. What is Stack Pointer? - Stack pointer is a special purpose 16-bit register in the Microprocessor, which holds the address of the top of the stack.
  5. What is Program counter? - Program counter holds the address of either the first byte of the next instruction to be fetched for execution or the address of the next byte of a multi byte instruction, which has not been completely fetched. In both the cases it gets incremented automatically one by one as the instruction bytes get fetched. Also Program register keeps the address of the next instruction.
  6. Which Stack is used in 8085? - LIFO (Last In First Out) stack is used in 8085.In this type of Stack the last stored information can be retrieved first.
  7. What happens when HLT instruction is executed in processor? - The Micro Processor enters into Halt-State and the buses are tri-stated.
  8. What is meant by a bus? - A bus is a group of conducting lines that carriers data, address, & control signals.
  9. What is Tri-state logic? - Three Logic Levels are used and they are High, Low, High impedance state. The high and low are normal logic levels & high impedance state is electrical open circuit conditions. Tri-state logic has a third line called enable line.
  10. Give an example of one address microprocessor? - 8085 is a one address microprocessor.
  11. In what way interrupts are classified in 8085? - In 8085 the interrupts are classified as Hardware and Software interrupts.
  12. What are Hardware interrupts? - TRAP, RST7.5, RST6.5, RST5.5, INTR.
  13. What are Software interrupts? - RST0, RST1, RST2, RST3, RST4, RST5, RST6, RST7.
  14. Which interrupt has the highest priority? - TRAP has the highest priority.
  15. Name 5 different addressing modes? - Immediate, Direct, Register, Register indirect, Implied addressing modes.
  16. How many interrupts are there in 8085? - There are 12 interrupts in 8085.
  17. What is clock frequency for 8085? - 3 MHz is the maximum clock frequency for 8085.
  18. What is the RST for the TRAP? - RST 4.5 is called as TRAP.
  19. In 8085 which is called as High order / Low order Register? - Flag is called as Low order register & Accumulator is called as High order Register.
  20. What are input & output devices? - Keyboards, Floppy disk are the examples of input devices. Printer, LED / LCD display, CRT Monitor are the examples of output devices.
  21. Can an RC circuit be used as clock source for 8085? - Yes, it can be used, if an accurate clock frequency is not required. Also, the component cost is low compared to LC or Crystal.
  22. Why crystal is a preferred clock source? - Because of high stability, large Q (Quality Factor) & the frequency that doesn’t drift with aging. Crystal is used as a clock source most of the times.
  23. Which interrupt is not level-sensitive in 8085? - RST 7.5 is a raising edge-triggering interrupt.
  24. What does Quality factor mean? - The Quality factor is also defined, as Q. So it is a number, which reflects the lossness of a circuit. Higher the Q, the lower are the losses.
  25. What are level-triggering interrupt? - RST 6.5 & RST 5.5 are level-triggering interrupts.
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114 Comments on 8085 microprocessor questions

  1. Kunal
    Posted 10/17/2008 at 1:09 pm | Permalink

    RST 5.5, 6.5, 7.5 as well as RST 0 to 7 give the vector locations of the memory they point to.
    For eg: RST 4.5(TRAP)corresponds to 4.5*8=0036(D)=0024(H) location of memory.
    Thus, PC jumps to this location.

  2. Kunal
    Posted 10/17/2008 at 1:11 pm | Permalink

    I forgot to mention one thing.
    RST 0 to RST 7 are mnemonics whereas interrupts don’t hav mnemonics. So, all RST’s are like 1 BYTE call/jump instruction.

  3. subha
    Posted 11/9/2008 at 2:07 am | Permalink

    what is the timing diagram of CALL instruction?

  4. harsha
    Posted 11/9/2008 at 11:43 pm | Permalink

    Call has 18 T states.

    opcode fetch(6) + memory read(3) + memory read(3) + memory write(3) + memory write(3)

  5. subha
    Posted 11/17/2008 at 7:05 am | Permalink

    what is difference between microprocessor & processor?

  6. KuNaL
    Posted 11/18/2008 at 10:21 am | Permalink

    A processor which executes a task in micro seconds is a micro processor i.e. a processor having clock frequency in MHz.

  7. malli
    Posted 12/4/2008 at 2:27 am | Permalink

    1.What is the function of the microprocessor?
    2.How many address location can the microprocessor point for 32-bit wide
    program counter?
    3.What is the function of registers in microprocessor? Explain in detail the
    general-purpose registers and special-purpose registers.
    4.Which is the most frequently used register of the microprocessor? Explain
    5.Explain how Procomm and Primer Trainer communicate.

  8. Posted 1/14/2009 at 12:03 am | Permalink

    what are sp & pc

  9. harshahossur
    Posted 1/15/2009 at 2:22 am | Permalink


    stack is a part of RAM. SP is the stack pointer which always points to the top of the stack(TOS).

    PC is the program counter which always points to the next instruction to be fetched.

  10. anuj katiyar
    Posted 1/17/2009 at 10:42 am | Permalink

    very good questions please send me answers of some questions
    1.why 8085 microprocessor and 8086 microprocessor are called as 8085 and 8086.
    2.why are the program counter & the stack pointer 16-bit registers?

  11. harshahossur
    Posted 1/21/2009 at 11:59 pm | Permalink


    2.why are the program counter & the stack pointer 16-bit registers?

    memory accessing capability of 8085 is 64kbits. each memory location will hav a unique address. Hence it requires 16 bits to address each memory location uniquely. Since program counter n the stack pointer points to the memory(they contain memory address), they are 16 bits. Hope u got this

  12. abhijith
    Posted 1/23/2009 at 1:59 pm | Permalink

    Whats the difference b/w software and hardware interrupts?

  13. harshahossur
    Posted 1/29/2009 at 1:35 am | Permalink


    SI:software interrupt
    HI: hardware interrupt

    > HI needs a signal , where as SI needs an instruction

    >HI can use priorities but SI cannot as they are instructions which are excecuted in sequence

    > HI may be maskable or non maskable but its not applicable for SI

    > there are 5 HIs(trap, rst 7.5, rst 6.5, rst 5.5, intr) in 8085 but 8 SIs(rst0 to rst 7).

  14. Posted 2/7/2009 at 11:09 am | Permalink

    The arithematic operations that do not affect
    flags are INX and DCX. The logic operation that do not affect flag is compliment(CMA).
    8085 has 6500 transistors.8085 architecture follows Von Neumann architecture.RST7.5 is edge triggered and RST5.5,RST6.5 are level triggered.

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