Interview Question Answers based on 8051 Microcontroller

In this post, Lets discuss list of interview question answers based on 8051 Microcontroller. I hope you will find this post useful for entry level position/academic interview. 8051 or MCS-51 (Intel refers) is one of popular microcontroller architecture among 8-bit microcontrollers.
Introduction of 8051 Microcontroller Architecture:

Block Diagram of 8051 Microcontroller Architecture

In 1981, Intel Corporation introduced an 8-bit microcontroller called the 8051. The 8051 became widely popular after Intel allowed other manufacturers to make and market any flavors of the 8051. They please with the condition that they remain code-compatible with the 8051. This has led to many, versions of the 8051 with different speeds and amounts of on-chip ROM marketed by more than half a dozen manufacturers. It is important to note that although there are different flavors of the 8051 in terms of speed and amount of on-chip ROM, they are all compatible with the original 8051 as far as the instructions are concerned. This means that if you write your program for one, it will run on any of them regardless of the manufacturer.

Intel 8051 follows which architecture?

Intel 8051 is Harvard Architecture.

What is the difference between Harvard Architecture and von Neumann Architecture?

The name Harvard Architecture comes from the Harvard Mark. The most obvious characteristic of the Harvard Architecture is that it has physically separate signals and storage for code and data memory. It is possible to access program memory and data memory simultaneously. Typically, code (or program) memory is read-only and data memory is read-write. Therefore, it is impossible for program contents to be modified by the program itself.

The von Neumann Architecture is named after the mathematician and early computer scientist John von Neumann. Von Neumann machines have shared signals and memory for code and data. Thus, the program can be easily modified by itself since it is stored in read-write memory.

8051 was developed using which technology?

Intel’s original MCS-51 family was developed using NMOS technology, but later versions, identified by a letter C in their name (e.g., 80C51) used CMOS technology and consume less power than their NMOS predecessors. This made them more suitable for battery-powered devices.

Why 8051 is called 8 bit microcontroller?

The Intel 8051 is an 8-bit microcontroller which means that most available operations are limited to 8 bits.

What is the width of data bus?

8-bit data bus

What is the width of address bus?

16-bit address bus

List out the features of 8051 micro controller?

  • 40 Pin IC.
  • 128 bytes of RAM.
  • 4K ROM.
  • 2 Timers (Timer 0 and Timer 1).
  • 32 Input/ Output pins.
  • 1 serial port.
  • 6 Interrupts (Including Reset).

On-chip RAM is also called _____ memory?

Direct memory

What location code memory space and data memory space begins?

At location 0x00 for internal or external memory

How Much on chip RAM is available?

128 bytes of RAM (from 0x00 to 0x7F) and can be used to store data.

With 12 MHz clock frequency how many instructions (of 1 machine cycle and 2 machine cycle) it can execute per second?

A cycle is, in reality, 12 pulses of the crystal. That is to say, if an instruction takes one machine cycle to execute, it will take 12 pulses of the crystal to execute. Since we know the crystal is pulsing 11,059,000 times per second and that one machine cycle is 12 pulses, we can calculate how many instruction cycles the 8051 can execute per second:

11,059,000 / 12 = 921,583

This means that the 8051 can execute 921,583 single-cycle instructions per second. Since a large number of 8051 instructions are single-cycle instructions it is often considered that the 8051 can execute roughly 1 million instructions per second, although in reality it is less–and, depending on the instructions being used, an estimate of about 600,000 instructions per second is more realistic.

NOTE: It is again important to emphasize that not all instructions execute in the same amount of time. The fastest instructions require one machine cycle (12 crystal pulses), many others require two machine cycles (24 crystal pulses), and the two very slow math operations require four machine cycles (48 crystal pulses).

About Umesh Lokhande

Umesh Lokhande holds a Master degree in Scientific Instrumentation from University of Applied Sciences Jena, Germany. and has previously worked at Orbotech, Alere Technologies etc. Umesh is also a founder and first author of BINARYUPDATES.COM


  1. QUESTION : What is the difference between synchronous, asynchronous and Interrupt driven data transfer?

    ANSWER : Let us take an example for each.

    Synchronous Mode : Take a case of boiling milk. In this mode the processor or controller continuously checks the status whether the milk is boiled or not ? in simple whether the task is completed or not ? so whats disadvantage here.

    All the processors efficient time wasted in only checking the status. it cannot concentrate on other tasks.

    Asynchronous Mode : Here the status is checked but not continuously. so some delay is made and then status is checked.

    Here comes Interrupt driven Mode: the most efficient one. why do i say this?

    Take an example: You are boiling some food in a cooker. when the rice is boiled the cooker blows whistle and tells us the rice is boiled. similarly when the task is completed interrupts informs micro controller. So one can concentrate on other works without looking the status of task.
    CPU efficient time is utilised.

  2. very good questions and answers for easy understanding for beginners.

  3. vidya sagar verma

    How to access carry bit

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