Microprocessor Questions and Answers

Microprocessor Questions and Answers

Q.What is Microprocessor? Give the power supply & clock frequency of 8085

A microprocessor is a multipurpose, programmable logic device that reads
binary instructions from a storage device called memory accepts binary data as
input and processes data according to those instructions and provides result as
output. The power supply of 8085 is +5V and clock frequency in 3MHz.

Q. List few applications of microprocessor-based system.

It is used:

i. For measurements, display and control of current, voltage, temperature, pressure, etc.
ii. For traffic control and industrial tool control.
iii. For speed control of machines.

Q. What are the functions of an accumulator?

The accumulator is the register associated with the ALU operations and sometimes I/O operations. It is an integral part of ALU. It holds one of data to be processed by ALU. It also temporarily stores the result of the operation performed by the ALU.

Q. List the 16 – bit registers of 8085 microprocessor.

Stack pointer (SP) and Program counter (PC).

Q. List the allowed register pairs of 8085.

• B-C register pair
• D-E register pair
• H-L register pair

Q. Mention the purpose of SID and SOD lines

SID (Serial input data line):
It is an input line through which the microprocessor accepts serial data.

SOD (Serial output data line):
It is an output line through which the microprocessor sends output serial data.

Q. What is an Opcode?

The part of the instruction that specifies the operation to be performed is called the Operation code or opcode.

Q. What is the function of IO/M signal in the 8085?

It is a status signal. It is used to differentiate between memory locations and I/O operations. When this signal is low (IO/M = 0) it denotes the memory related operations. When this signal is high (IO/M = 1) it denotes an I/O operation.

Q. What is an Operand?

The data on which the operation is to be performed is called as an Operand.

Q. How many operations are there in the instruction set of 8085 microprocessor?

There are 74 operations in the 8085 microprocessor.

Q. List out the five categories of the 8085 instructions. Give examples of the instructions for each group.

• Data transfer group – MOV, MVI, LXI.
• Arithmetic group – ADD, SUB, INR.
• Logical group –ANA, XRA, CMP.
• Branch group – JMP, JNZ, CALL.
• Stack I/O and Machine control group – PUSH, POP, IN, HLT.

Q. Explain the difference between a JMP instruction and CALL instruction .

A JMP instruction permanently changes the program counter. A CALL instruction leaves information on the stack so that the original program execution sequence can be resumed.

Q. Explain the purpose of the I/O instructions IN and OUT.

The IN instruction is used to move data from an I/O port into the accumulator.

The OUT instruction is used to move data from the accumulator to an I/O port.

The IN & OUT instructions are used only on microprocessor, which use a separate address space for interfacing.

Q. What is the difference between the shift and rotate instructions?

A rotate instruction is a closed loop instruction. That is, the data moved out at one end is put back in at the other end. The shift instruction loses the data that is moved out of the last bit locations.

Q. How many address lines in a 4096 x 8 EPROM CHIP?

12 address lines.

Q. Control signals used for DMA operation are ____________


Q. What is meant by Wait State?

This state is used by slow peripheral devices. The peripheral devices can
transfer the data to or from the microprocessor by using READY input line. The
microprocessor remains in wait state as long as READY line is low. During the
wait state, the contents of the address, address/data and control buses are held

Q. List the four instructions which control the interrupt structure of the 8085

  • DI ( Disable Interrupts )
  • EI ( Enable Interrupts )
  • RIM ( Read Interrupt Masks )
  • SIM ( Set Interrupt Masks )

Q. What is meant by polling?

Polling or device polling is a process which identifies the device that has interrupted the microprocessor.

Q. What is meant by interrupt?

Interrupt is an external signal that causes a microprocessor to jump to a specific subroutine.

Q. Explain priority interrupts of 8085.

The 8085 microprocessor has five interrupt inputs. They are TRAP, RST 7.5, RST 6.5, RST 5.5, and INTR. These interrupts have a fixed priority of interrupt service.

If two or more interrupts go high at the same time, the 8085 will service them on priority basis. The TRAP has the highest priority followed bye RST 7.5, RST 6.5, RST 5.5. The priority of interrupts in 8085 is shown in the table.

Interrupts Priority

  1. TRAP
  2. RST 7.5
  3. RST 6.5
  4. RST 5.5
  5. INTR

Q. What is a microcomputer?

A computer that is designed using a microprocessor as its CPU is called microcomputer.

Q. What is the signal classification of 8085

All the signals of 8085 can be classified into 6 groups

  • Address bus
  • Data bus
  • Control and status signals
  • Power supply and frequency signals
  • Externally initiated signals
  • Serial I/O ports

Q. What are operations performed on data in 8085

The various operations performed are

  • Store 8-bit data
  • Perform arithmetic and logical operations
  • Test for conditions
  • Sequence the execution of instructions
  • Store data temporarily during execution in the defined R/W memory locations called the stack

Q. Steps involved to fetch a byte in 8085

i. The PC places the 16-bit memory address on the address bus
ii. The control unit sends the control signal RD to enable the memory chip
iii. The byte from the memory location is placed on the data bus
iv. The byte is placed in the instruction decoder of the microprocessor and
the task is carried out according to the instruction

Q. How many interrupts does 8085 have, mention them

The 8085 has 5 interrupt signals; they are INTR, RST7.5, RST6.5, RST5.5
and TRAP

Q. Basic concepts in memory interfacing

The primary function of memory interfacing is that the microprocessor
should be able to read from and write into a given register of a memory chip. To
perform these operations the microprocessor should

• Be able to select the chip
• Identify the register
• Enable the appropriate buffer

Q. Define instruction cycle, machine cycle and T-state

Instruction cycle is defined, as the time required completing theexecution of an instruction. Machine cycle is defined as the time required completing one operation of accessing memory, I/O or acknowledging an external request. Tcycle is defined as one subdivision of the operation performed in one clock period

Q. What is an instruction?

An instruction is a binary pattern entered through an input device to command the microprocessor to perform that specific function

Q. What is the use of ALE

The ALE is used to latch the lower order address so that it can be available in T2 and T3 and used for identifying the memory address. During T1 the ALE goes high, the latch is transparent ie, the output changes according to the input data, so the output of the latch is the lower order address. When ALE goes low the lower order address is latched until the next ALE.

Q. How many machine cycles does 8085 have, mention them

The 8085 have seven machine cycles.

They are

• Opcode fetch
• Memory read
• Memory write
• I/O read
• I/O write
• Interrupt acknowledge
• Bus idle

Q. Explain the signals HOLD, READY and SID

HOLD indicates that a peripheral such as DMA controller is requesting
the use of address bus, data bus and control bus.

READY is used to delay the microprocessor read or write cycles until a slow responding peripheral is ready to send or accept data. SID is used to accept serial data bit by bit

Q. Mention the categories of instruction and give two examples for each

The instructions of 8085 can be categorized into the following five

• Data transfer MOV Rd,Rs STA 16-bit
• Arithmetic ADD R DCR M
• Logical XRI 8-bit RAR
• Branching JNZ CALL 16-bit
• Machine control HLT NOP

Q. Explain LDA, STA and DAA instructions

LDA copies the data byte into accumulator from the memory location
specified by the 16-bit address. STA copies the data byte from theaccumulator in
the memory location specified by 16-bit address. DAA changes the contents of
the accumulator from binary to 4-bit BCD digits.

Q. Explain the different instruction formats with examples

The instruction set is grouped into the following formats

• One byte instruction MOV C,A
• Two byte instruction MVI A,39H
• Three byte instruction JMP 2345H

Q. What is the use of addressing modes, mention the different types

The various formats of specifying the operands are called addressing modes, it is used to
access the operands or data.

The different types are as follows

• Immediate addressing
• Register addressing
• Direct addressing
• Indirect addressing
• Implicit addressing

Q. What is the use of bi-directional buffers?

It is used to increase the driving capacity of the data bus. The data bus of a
microcomputer system is bi-directional, so it requires a buffer that allows the data
to flow in both directions.

Q. Define stack and explain stack related instructions

The stack is a group of memory locations in the R/W memory thatis used
for the temporary storage of binary information during the execution of the
program. The stack related instructions are PUSH & POP

Q. Why do we use XRA A instruction
The XRA A instruction is used to clear the contents of the Accumulator and store the value 00H.

Q. What is Microcontroller and Microcomputer

Microcontroller is a device that includes microprocessor; memory and I/O
signal lines on a single chip, fabricated using VLSI technology. Microcomputer is
a computer that is designed using microprocessor as its CPU. It includes
microprocessor, memory and I/O.

Q. Define Flags

The flags are used to reflect the data conditions in the accumulator. The 8085
flags are S-Sign flag, Z-Zero flag, AC-Auxiliary carry flag, P-Parity flag, CYCarry flag
D7 D6 D5 D4 D3 D2 D1 D0

Q. How does the microprocessor differentiate between data and instruction

When the first m/c code of an instruction is fetched and decoded in the
instruction register, the microprocessor recognizes the number of bytes required
to fetch the entire instruction.

For example MVI A, Data, the second byte is
always considered as data. If the data byte is omitted by mistake whatever is in
that memory location will be considered as data & the byte after the “data”will be
treated as the next instruction.

Q. What is assembler

The assembler translates the assembly language program text which is given as input
to the assembler to their binary equivalents known as object code. The time required to
translate the assembly code to object code is called access time. The assembler checks for
syntax errors & displays them before giving the object code.

Q. What is loader

The loader copies the program into the computer’s main memory at load time and begins the program execution at execution time.

Q. What is linker

A linker is a program used to join together several object files into one large object

For large programs it is more efficient to divide the large program modules into
smaller modules. Each module is individually written, tested & debugged. When all the
modules work they are linked together to form a large functioning program.


The ALIGN directive forces the assembler to align the nextsegment at an address
divisible by specified divisor. The format is ALIGN number where number can be 2, 4, 8
or 16. Example ALIGN 8.

The ASSUME directive assigns a logical segment to a physical segment at any given
time. It tells the assembler what address will be in the segment registers at execution
time. Example ASSUME CS: code, DS: data, SS: stack

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