Read the following passage and mark the letter A, B, C or D on your answer sheet to indicate the correct answer to each of the following questions.

In the history of technology, computers and calculators were innovative developments. They are essentially different from all other machines because they have a memory. This memory stores instructions and information. In a calculator, the instructions are the various functions of arithmetic, which are permanently remembered by the machine and cannot be altered or added to. The information consists of the numbers which are keyed in.

An electronic pocket calculator can perform almost instant arithmetic. A calculator requires an input unit to feed in numbers, a processing unit to make the calculation, a memory unit, and an output unit to display the result. The calculator is powered by a small battery or by a panel of solar cells. Inside is a microchip that contains the memory and processing units and also controls the input unit, which is the keyboard, and the output unit, which is the display.

The input unit has keys for numbers and operations. Beneath the key is a printed circuit board containing a set of contacts for each key. Pressing a key closes the contacts and sends a signal along a pair of lines in the circuit board to the processing unit, in which the binary code for that key is stored in the memory. The processing unit also sends the code to the display. Each key is connected by a different pair of lines to the processing unit, which repeatedly checks the lines to find out when a pair is linked by a key.

The memory unit stores the arithmetic instructions for the processing unit and holds the temporary results that occur during calculation. Storage cells in the memory unit hold the binary codes for the keys that have been pressed. The number codes, together with the operation code for the plus key, are held in temporary cells until the processing unit requires them.

When the equals key is pressed, it sends a signal to the processing unit. This takes the operation code - for example, addition - and the two numbers being held in the memory unit and performs the operation on the two numbers. After the addition is done, the result goes to the decoder in the calculator's microchip. This code is then sent to the liquid crystal display unit, which shows the result, or output, of the calculation.

What is the main purpose of the passage?