Understanding Computers

1 Explain why computers use binary to store data. 2

One mark for each of the following, to a maximum of 2.

• Electronic components have two states, on and off
• These two states can represent 1 and 0 reliably

Model answer. Computers are built from electronic components that have exactly two reliable states: a circuit is either carrying current or it is not. These two states can be used to represent the digits 1 and 0, and because there are only two of them they can be told apart reliably even if the voltage varies a little. Trying to use ten different levels for ordinary denary digits would be far harder to build and far more likely to produce errors.

2 Convert the denary number 89 into 8 bit binary. Show your working. 3

One mark for each of the following, to a maximum of 3.

• Uses place values starting from 128
• Shows subtraction working
• Correct answer 01011001

Model answer. Working from the largest place value downwards: 128 does not fit into 89, so write 0. 64 fits, leaving 25, so write 1. 32 does not fit into 25, so write 0. 16 fits into 25, leaving 9, so write 1. 8 fits into 9, leaving 1, so write 1. 4 does not fit, write 0. 2 does not fit, write 0. 1 fits, leaving 0, so write 1. The answer is 01011001, and checking gives 64 + 16 + 8 + 1 = 89.

3 Describe the difference between RAM and a hard drive. 3

One mark for each of the following, to a maximum of 3.

• RAM holds the data and programs currently in use
• RAM is wiped when the power is switched off
• A hard drive stores data permanently and is much larger but slower

Model answer. RAM holds the programs and data that the computer is using at that moment, and the CPU reads directly from it because it is very fast. However, RAM is temporary: its contents are wiped as soon as the power is switched off, which is why unsaved work is lost in a power cut. A hard drive stores data permanently, so files are still there when the computer is switched back on. It has a much larger capacity than RAM but is far slower to access, which is why programs are copied from the hard drive into RAM when they are opened.

4 Explain how an image is stored in a computer. 4

One mark for each of the following, to a maximum of 4.

• The image is divided into a grid of pixels
• Each pixel is given a colour stored as a binary number
• More pixels means more detail
• More bits per pixel means more possible colours, and both increase file size

Model answer. An image is stored as a grid of tiny squares called pixels. Each pixel holds a single colour, and that colour is represented by a binary number, so the whole image becomes a long list of binary values. The number of pixels is called the resolution, and using more pixels means the picture captures finer detail and looks sharper. The number of bits used for each pixel decides how many different colours are available, so 8 bits gives 256 colours while 24 bits gives over 16 million. Increasing either the number of pixels or the number of bits per pixel improves the quality of the image but also increases the amount of data that has to be stored, so the file becomes larger.

5 State what the CPU does and explain what clock speed measures. 3

One mark for each of the following, to a maximum of 3.

• The CPU processes instructions
• It fetches, decodes and executes instructions repeatedly
• Clock speed measures how many cycles the CPU performs each second

Model answer. The CPU is the part of the computer that processes all the instructions, and it does this by repeatedly fetching the next instruction from memory, decoding it to work out what it means, and then executing it. Clock speed measures how many of these cycles the CPU carries out each second, and it is given in hertz. A processor running at 3 gigahertz completes three billion cycles every second, so a higher clock speed generally means more instructions are processed and the computer feels faster.