8085 Microprocessor Interview Questions and answers

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

• Accumulator register
• Temporary register
• Instruction registers
• Stack Pointer
• Program Counter

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.

Stack pointer is a special purpose 16-bit register in the Microprocessor, which holds the address of the top of the stack.

LIFO (Last In First Out) stack is used in 8085.In this type of Stack the last stored information can be retrieved first.

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.

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

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.

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.

TRAP, RST7.5, RST6.5, RST5.5, INTR

RST0, RST1, RST2, RST3, RST4, RST5, RST6, RST7.

TRAP has the highest priority.

• Immediate
• Direct
• Register
• Register indirect
• Implied addressing modes.

There are 12 interrupts in 8085.

3 MHz is the maximum clock frequency for 8085.

RST 4.5 is called as TRAP.

Keyboards, Floppy disk are the examples of input devices.

Printer, LED / LCD display, CRT Monitor are the examples of output devices.

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.

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.

RST 6.5 & RST 5.5 are level-triggering interrupts

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

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.

• 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.