8085 Microprocessor Interview Questions - These are some FAQs on microprocessors which is solely prepared by myself keeping in the interviews of various IT as well as Core companies.

What is Microprocessor?
Microprocessor is a program-controlled device, which fetches the instructions from memory, decodes and executes the instructions. Most Micro Processor are single- chip devices.

What are the various registers in 8085?
  • Accumulator register
  • Temporary register
  • Instruction register
  • Stack Pointer
  • Program Counter

What are the different types of flags in 8085 microprocessor?
There are 5 different flags in 8085 microprocessor. Though the flag register is of 8 bit but 3 bits are not in use. Only 5 bits are used for the different flags. They are:-
  • Sign flag(s) – This is designated by the letter ‘S’. If sign bit is 1 then the sign flag is set to 1 and if the sign bit is zero then sign flag is reset to zero.
  • Zero flag(z) – This is designated by the letter ‘z’. If the result of any arithmetic or logical operation is zero i.e. all the bits are zero then zero flag is set to 1 else it is set to zero.
  • Auxiliary carry (AC) - This flag is set to 1 only when any intermediate carry is produced. Else it is reset to 0.
  • Parity flag (P) – when the result of any operation has odd number of ones then parity flag is set to 1 else if it has even number of ones then it is reset to 0.
  • Carry flag(C) – this flag is set to 1 only when a carry is produced in the result i.e. the carry bit is 1 else if the carry bit is zero then the flag is reset to zero.

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.

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.

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.
What is meant by a bus?
A bus is a group of conducting lines that carriers data, address, & control signals.

What are the types of buses?
Three types of buses are there:-
Address bus: It carries the Address to the memory to fetch either instruction.
Data bus: It carries data from the memory.
Control bus: It carries the control signals like RD/WR, Select etc.

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.

What are Hardware interrupts?
TRAP, RST7.5, RST6.5, RST5.5, INTR

What are Software interrupts?

Which interrupt has the highest priority?
TRAP has the highest priority.

Name 5 different addressing modes?
  • Immediate
  • Direct
  • Register
  • Register indirect
  • Implied addressing modes.

How many interrupts are there in 8085?
There are 12 interrupts in 8085.

What is clock frequency for 8085?
3 MHz is the maximum clock frequency for 8085.

What is the RST for the TRAP?
RST 4.5 is called as TRAP.

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.

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.

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.

What are level-triggering interrupt?
RST 6.5 & RST 5.5 are level-triggering interrupts

How can signals be classified for the 8085 Microprocessor?
The signals of the 8085 Microprocessor based on their functions can be classified into 7 categories namely:
  • Frequency and power signals
  • Address and data buses
  • The control bus
  • Interrupt Signals
  • Serial Input / Output signals
  • DMA signals
  • Reset Signals

What are the various functional blocks of the 8085 Microprocessor?
The various functional blocks of the 8085 Microprocessor are:
  • Registers
  • Arithmetic logic unit
  • Address buffer
  • Increment / decrement address latch
  • Interrupt control
  • Serial I/O control
  • Timing and control circuitry
  • Instructions decoder and machine cycle encoder.

What are the different types of data transfers possible in the 8085?
  • Data transfer is possible between two registers.
  • It is also possible between a memory location and a register.
  • Also it can occur between an input/output device and an accumulator.
  • In reality data is never transferred it can only be copied from one location to another.

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