How is the amount of data measured, and why is binary used?
Why data must be represented in binary, the units of information (bit, nibble, byte, kB, MB, GB, TB, PB) and how to convert between them.
An OCR J277 1.2.3 answer on why computers use binary, the units of information from bit and nibble up to petabyte, and how to convert between units of data capacity.
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What this dot point is asking
OCR wants you to explain why data inside a computer is stored in binary, to know the units of information in order (bit, nibble, byte, then the larger units), and to convert between units, including in file-size and capacity problems. The conversions appear in calculation questions, so the method must be automatic.
Why binary is used
The units of information
The order to memorise, smallest to largest, is: bit, nibble, byte, kilobyte, megabyte, gigabyte, terabyte, petabyte. Note that bits make byte, and nibbles make byte.
Converting between units
Larger conversions
The same rule scales to any unit. To turn 2 GB into megabytes, multiply by 1000 (going down a unit): MB. To turn 6000 MB into gigabytes, divide by 1000 (going up a unit): GB. Capacity questions often combine a conversion with a division, for example "how many 5 MB photos fit on a 2 GB card": convert the card to MB ( MB) then divide by the file size ( photos). Show each step so a marker can follow your working even if one arithmetic slip occurs.
Try this
Q1. State how many bits are in a byte and how many in a nibble. [2 marks]
- Cue. A byte is 8 bits; a nibble is 4 bits.
Q2. Convert 8000 bytes to kilobytes (use 1 kB = 1000 bytes). [1 mark]
- Cue. kB.
Q3. How many 4 MB songs could be stored on a 2 GB memory card (use 1 GB = 1000 MB)? [2 marks]
- Cue. MB, then songs.
Exam-style practice questions
Practice questions written in the style of OCR exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
OCR 20203 marksState how many bits are in a nibble and how many bits are in a byte, and explain why data is represented in binary inside a computer.Show worked answer →
A nibble is 4 bits. A byte is 8 bits. (One mark each.)
Data is represented in binary because the components inside a computer (such as transistors and switches) have two reliable states, on and off, which can represent the two binary digits 1 and 0. Using only two states makes the hardware simpler and more reliable than trying to detect many different voltage levels, so all data and instructions are stored and processed as patterns of 1s and 0s. (One mark.)
Markers reward "two states / on and off" linked to the two binary digits. "Because computers like binary" with no reason does not earn the mark.
OCR 20224 marksA file is 5000 kilobytes in size. Convert this size to megabytes, and then state how many such files could be stored on a 4 gigabyte memory card (using 1 GB = 1000 MB). Show your working.Show worked answer →
Convert kilobytes to megabytes by dividing by 1000: MB. (One mark for the method, one for 5 MB.)
The card holds 4 GB. Using , that is MB. (One mark.)
Number of files: files. (One mark.)
Markers accept either the decimal convention () or the binary convention () as long as it is used consistently and the working is shown. OCR commonly uses 1000 for these conversions at GCSE.
Related dot points
- The need for primary storage, the purpose and characteristics of RAM and ROM, the differences between them, and the need for virtual memory.
An OCR J277 1.2.1 answer on the need for primary storage, the purpose and characteristics of RAM and ROM, the differences between them, and why virtual memory is needed.
- The need for secondary storage, the common types (optical, magnetic, solid state) and how to choose a suitable device using capacity, speed, portability, durability, reliability and cost.
An OCR J277 1.2.2 answer on the need for secondary storage, the three common types (optical, magnetic, solid state), and how to choose a suitable device by capacity, speed, portability, durability, reliability and cost.
- Converting between denary and binary (up to and including 8 bits), binary addition and the detection of overflow, and binary shifts and their effect.
An OCR J277 1.2.4 answer on converting between denary and binary up to 8 bits, adding binary numbers and detecting overflow, and binary shifts and their effect on a value.
- Representing characters with ASCII and Unicode; representing images with pixels, colour depth, resolution and metadata; representing sound with sample rate, sample resolution and bit rate; and the effect on file size and quality.
An OCR J277 1.2.4 answer on representing characters (ASCII, Unicode), images (pixels, colour depth, resolution, metadata) and sound (sample rate, sample resolution, bit rate), and the effect of each on file size and quality.
- The need for compression and the difference between lossy and lossless compression, with their typical uses.
An OCR J277 1.2.5 answer on the need for data compression and the difference between lossy and lossless compression, including how each works and their typical uses.
Sources & how we know this
- OCR GCSE (9-1) Computer Science (J277) specification — OCR (2020)