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Complex Instruction Set Computer (CISC)

Computer Software


8051 Clock and Instruction Cycle

Internal Memory

Special Function Registers (SFRs)

Internal ROM

Connecting External Memory

Timers / Counters

Serial Interface

Serial Data Transmission Modes




A computer is a multipurpose programmable machine that reads binary
instructions from its memory , accepts binary data as input ,processes the data according to those
instructions and provides results as output. It is a programmable device made up of both
hardware and software. The various components of the computer are called hardware. A set of
instructions written for the computer to solve a specific task is called program and collection of
programs is called software .

The computer hardware consists of four main components. The central processing unit which
acts as computer’s brain. Input unit through which program and data can be entered to computer,
output unit on which the results of the computations can be displayed. Memory in which data
and program are stored.

A computer that is designed using a microprocessor as its CPU , is known as a microcomputer.
Microprocessor or ‘Computer on Chip’ first became a commercial reality in 1971 with the
introduction of the 4 bit 4004 by Intel. A byproduct of Microprocessor development was
Microcontroller. The same fabrication technology and programming concept that make the
general purpose microprocessor also yielded the Microcontroller.


A microprocessor is a general purpose digital computer central processing unit (CPU). Although
known as a ‘Computer on Chip’ the Microprocessor in no sense a complete digital computer.
Block diagram of a Microprocessor CPU which contains ALU; Program counter (PC), a stack

pointer (SP) ,some working registers , a clock timing circuit and interrupt circuit s is shown in
the following figure

To make a computer microcomputer one must add memory usually RAM and ROM,
memory decoders , an oscillator and a number of Input ,Output devices such as serial and parallel
ports. In addition special purpose devices such as interrupt handler and counters may be added to
relieve the CPU from time consuming counting or timing cores. When the Microcomputer is
equipped with mass storage devices , I/O peripherals such as a key board and a display CRT it
yields a small computer that can be applied to a range of general purpose applications.

The hardware design of a microprocessor is arranged such that a very small or very large
system can be configured around the CPU as the application demands as shown in Fig1. The
prime use of the Microprocessor is to read data , perform extensive calculations on that data, and
store those calculations in a mass storage device or display the results for human use. The
programs used by microprocessor are stored in the mass storage device and loaded into RAM as
user directs. A few microprocessor program are stored in ROM . The ROM based programs are
primarily small fixed programs that operate peripherals and other fixed devices that are
connected to the system.


A Microcontroller is a programmable digital processor with necessary
peripherals. Both microcontrollers and microprocessors are complex sequential digital circuits
meant to carry out job according to the program / instructions. Sometimes analog input/output

interface makes a part of microcontroller circuit as mixed mode(both analog and digital) in

A microcontroller can be compared to a Swiss knife with multiple functions incorporated in the
same Integrated Circuits. Block diagram of a typical Microcontroller which is a true computer on
a chip is shown below. The design incorporates all the features found in microprocessor CPU :
ALU,PC, SP and registers. It also has other features needed to make a complete computer: ROM,
RAM, Parallel I/O, serial I/O, Counters and clock circuits. Like the microprocessor , a
microcontroller is a general purpose device, but one that is meant to read data, perform limited
calculations on that data and control its environment based on those calculations. The prime use
of microcontroller is to control the operation of a machine using a fixed program that is stored in
ROM and that does not change over the lifetime of the system.