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Microprocessors and Computer Interfacing

ByDavid E. Olson

Nov 25, 2022

In this article, we cover all the details about the History of Microprocessors, Intel Processors, clock cycles, executing instructions, Microprocessor-based systems, von Neumann Architecture, A machine language with instructions, Central Processing, System Bus Model  Unit, Inside a CPU, Internal Components of a Microprocessor, Registers, Arithmetic and Logic Unit (ALU), Bus Interface, Bus Interface,fetch-execute cycle and much other information about microcontrollers.


What is a computer?

  • A computer is a machine which can Input the data, Process the data and Output the results.

Then how does the computer process the input data?

  • the data was processed according to the stored program in the computer

then, if we consider about microprocessor, it is also like a computer

there is an input which may be a temperature reading from a sensor or maybe a keyboard input and then that data is processed by the processor according to stored memory instructions. finally, the final decision or final output is printed or displayed on a screen.

First of all, let’s consider computers. First-generation computers were made using vacuum tubes (1939-1954). Second-generation computers were made using transistors (1954-1959). Third-generation computers were made using IC (Integrated circuit)(1959-1971). Fourth-generation computers were made using microprocessors (1971-present). this is a simple history of microprocessors.

what is a microprocessor?

we are normally called Central Processing Unit ( CPU ). also commonly known as the heart and the brain of any computing machine. It is fabricated on a single integrated circuit (IC).

ok, .lets dive into the Microprocessors.

  • The Intel 4004 is a 4-bit microprocessor considered to be the first commercially available microprocessor
microprocessors and computer interfacing

Clock rate: 740 kHz 

                                                            Execution speed: 0.07 

                                                            MIPS Bus Width: 4 bits 

                                                            Number of Transistors: 2,300

                                                             Addressable Memory: 640 bytes

                                                             Program Memory: 4 KB

(if you want to read about clock rate, Execution speed,  Addressable Memory, Program Memory, and MIPS Bus Width then move to lower sections.)

  • then intel released other processers like 8-bit processors (8008 1972,8080 1974,8085 1976),16-bit 16-bit processors( 8086 1978,8088 1979,80186 1982,80286 1982),32-bit processors(80386 1985,80486 1989,80486 1989,Pentium II 1997,Pentium III 1999 ,Pentium IV 2000) and 64-bit processors (Intel Core 2 2006)
  • Pentium III 1999 was my first computer’s processor. (good old days🤔)
  • also, there are more stories are in the internet that how intel and other companies were clones of other companies’ pcs(like IBM).you  can find those kinds of things on the internet .but in this blog we consider technical details.

this is a table of all the processers and their details

  • Transistors -Number of transistors on the chip
  • Microns -Width of the smallest wire on the chip. ( Human hair is 100 microns thick )
  • Clock speed – Maximum rate that the chip can be clocked 
  • Data width – Data width of the ALU.(click this to read more about ALU)
  • MIPS – Millions of instructions per second. This is the rough measurement of microprocessor performance 

clock cycle

clock cycle in the microprocessor is the number of pulses per second. that means our microprocessor works according to pulses (1 and 0).there is a clock (like a cristal oscillator in Arduino  ) that generates pulses. the speed of the microprocessors mostly depends on clock speed. executing instructions

but if we consider an instruction .which is stored in memory. there are multiple clock cycles are needed to execute the instruction. according to the MIPS value of the table, the number of clock cycles to execute the instruction is varied.


Then we can calculate the speed of the instruction execution in the above case


Microprocessor-based systems

this is an electrical system and which has consist of microprocessors, memories, I/O units, and other peripherals. Microprocessors access memories, inputs, output and other units through buses.


this is a simple explanation of the Microprocessor-based system. let’s talk about inside units of Microprocessor-based systems Further.

von Neumann Architecture 

this is a computer architecture of the microprocessor-base system. there is a memory and input/output and these units are linked to the processor. the size of the memory cells is organized linearly. A machine language with instructions performing simple operations on elementary operands.

  • Sequential, centralized control of computation 
  • there is another model in computer architecture. which is called the “bus model”.

ok !! let’s go inside the CPU.

Inside a CPU

usually, a CPU has 3 sections.

Microprocessors and Computer Interfacing
  • ALU (Arithmetic and Logic Unit) – this unit calculates and does logical comparisons.
  • Registers – the numbers which are being processed are stored here. each register has a size which is called “word”. for example, if it is a 16-bit CPU, then it has 16-bit-sized registers. the external data bus has the same width as the register also external address bus has different width and it depends on the amount of memory that it may access.
  • Instruction Decoder-this controls all other parts of the CPU, sending signals in preset patterns to shift data between registers, the ALU and the main data and address buses. 

Internal Components of a Microprocessor

this is a more detailed diagram of the microprocessor

recall – ( microprocessor is like CPU and microcontroller is like a computer with the ram,rom and other elements)

Microprocessors and Computer Interfacing

there are four units inside the Microprocessors.


registers are groups of memory cells arranged as a single memory word, usually consisting of 8,16 or 32 bits. the number of registers is varied according to the types of Microprocessors. they are used for storing data temporarily in processing.

there are various types of registers in microprocessors.

Instruction Register – stores the instruction currently being executed or decoded

Accumulator (A, or ACC) – holds the result of operations performed by the arithmetic and logic unit

A, B, C, and D Registers –store data temporarily inside the Microprocessor.

Status Register-designed to contain all the appropriate 1-bit status flags, which are changed as a result of operations involving the arithmetic and logic unit. Further information can be found in the section on the ALU.

Program Counter (PC)-register in a computer processor that contains the address (location) of the instruction fetch at the current time. As each instruction gets fetched, the program counter increases its stored value by 1.

Arithmetic and Logic Unit (ALU) 

Microprocessors and Computer Interfacing

The ALU is the part of the CPU that performs arithmetic and logic operations. It consists of a number of complex logic circuits for performing operations such as addition, subtraction, and multiplication on binary numbers.

Bus Interface 

external components such as memory and I/O devices through several sets of signal lines called buses. The bus interface coordinates the activities related to transferring data into and out of the microprocessor through these buses.

Control Unit

the control unit operates the microprocessor. the normal operation of the CPU called a fetch-execute cycle.

Fetch cycle

Fetch is reading instructions from memory. mainly there are 3 busses in the microprocessor (data bus, control bus, and address bus). First CPU puts the address of the instruction on the address bus to be executed. that address information comes from the program counter(PC) and program counter maintained by the control unit. then the control bus holds the information for reading the memory location. the data bus holds the instruction from the memory which is stored in the instruction register.

the program counter is updated to point to the next instruction.

Execute cycle

this is the cycle of doing action according to recorded instructions. in this step the required data is transferred and the required logical and arithmetic operations are performed. finally, the final output is transferred to the memory or I/O devices or register.

(read the summary section to acknowledge a full fetch-execute cycle)

summary of the microprocessor mainly microcontroller has the ram, control unit, arithmetic and logical unit, registers, busses, instruction decoder and programme counter. there is a clock (pulse generator )which makes alive the 

microprocessor. when running a microprocessor, there are three cycles are included to the microprocessor.

  • First, the programme counter has the address value of the RAM .usually the program counter increase from 1 in every fetch-execute cycle. the program counter operated by the control unit.
  • then the instruction is taken from the memory through the data bus.

        (above steps are in the fetch cycle)

  • then the above instruction is decoded then do calculations 

         (this is decoding the instruction cycle)

  • finally, that decoded data is stored in the memory

        (this is executing cycle)

  •  we can manually change the program counter value after the decoding cycle. which programming counter-changing instruction should be included in the memory (registers) .then the fetch-executing cycle can vary from the above explanation.
  • the count of clock cycles to execute the instruction varies according to the instruction, for example, if we consider the assembly language, if we want to execute JUMP instruction then we need 2 clock cycles, INC (increase) instruction needs more than 2 cycles.
  • the computer always works according to the fetch-execute cycle .but in an interrupt, a reset signal and a HALT instruction, it works differently.

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