Internal workings

In general, a basic electronic calculator consists of the following components:

Power source (mains electricity, battery and/or solar cell)
An office calculating machine with a paper printer.
To perform the calculation 25 + 9, one presses keys in the following sequence on most calculators: 2 5 + 9 =.
When 2 5 is entered, it is picked up by the scanning unit; the number 25 is encoded and sent to the X register;

Keypad (input device) – consists of keys used to input numbers and function commands (addition, multiplication, square-root, etc.)

Display panel (output device) – displays input numbers, commands and results. Liquid-crystal displays (LCDs), vacuum fluorescent displays (VFDs), and light-emitting diode (LED) displays use seven segments to represent each digit in a basic calculator. Advanced calculators may use dot matrix displays.

A printing calculator, in addition to a display panel, has a printing unit that prints results in ink onto a roll of paper, using a printing mechanism.

Processor chip (microprocessor or central processing unit).

Processor chip's contents

Unit Function

Scanning (Polling) unit When a calculator is powered on, it scans the keypad waiting to pick up an electrical signal when a key is pressed.

Encoder unit Converts the numbers and functions into binary code.

X register and Y register They are number stores where numbers are stored temporarily while doing calculations. All numbers go into the X register first; the number in the X register is shown on the display.

Flag register The function for the calculation is stored here until the calculator needs it.

Permanent memory (ROM) The instructions for in-built functions (arithmetic operations, square roots, percentages, trigonometry, etc.) are stored here in binary form. These instructions are programs, stored permanently, and cannot be erased.

User memory (RAM) The store where numbers can be stored by the user. User memory contents can be changed or erased by the user.

Arithmetic logic unit (ALU) The ALU executes all arithmetic and logic instructions, and provides the results in binary coded form.

Binary decoder unit Converts binary code into decimal numbers which can be displayed on the display unit.

Clock rate of a processor chip refers to the frequency at which the central processing unit (CPU) is running. It is used as an indicator of the processor's speed, and is measured in clock cycles per second or the SI unit hertz (Hz). For basic calculators, the speed can vary from a few hundred hertz to the kilohertz range.

Example

A basic explanation as to how calculations are performed in a simple 4-function calculator:

Next, when the + key is pressed, the "addition" instruction is also encoded and sent to the flag or status register;

The second number 9 is encoded and sent to the X register. This "pushes" (shifts) the first number out into the Y register;

When the = key is pressed, a "message" (signal) from the flag or status register tells the permanent or non-volatile memory that the operation to be done is "addition";

The numbers in the X and Y registers are then loaded into the ALU and the calculation is carried out following instructions from the permanent or non-volatile memory;

The answer, 34 is sent (shifted) back to the X register. From there, it is converted by the binary decoder unit into a decimal number (usually binary-coded decimal), and then shown on the display panel.

Other functions are usually performed using repeated additions or subtractions. Where calculators have added functions (such as square root, or trigonometric functions), software algorithms are required to produce high precision results. Sometimes significant design effort is needed to fit all the desired functions in the limited memory space available in the calculator chip, with acceptable calculation time.