A-level Computer Science₇₅₁₇

Be familiar with the concept of a natural number and the set ℕ of natural numbers (including zero).

ℕ = {0, 1, 2, 3, … }

Be familiar with the concept of an integer and the set ℤ of integers.

ℤ = { …, -3, -2, -1, 0, 1, 2, 3, … }

Be familiar with the concept of a rational number and the set ℚ of rational numbers, and that this set includes the integers.

ℚ is the set of numbers that can be written as fractions (ratios of integers). Since a number such as 7 can be written as 7/1, all integers are rational numbers.

Be familiar with the concept of an irrational number.

An irrational number is one that cannot be written as a fraction, for example √2.

Be familiar with the concept of a real number and the set ℝ of real numbers, which includes the natural numbers, the rational numbers and the irrational numbers.

ℝ is the set of all 'possible real world quantities'.

Be familiar with the concept of ordinal numbers and their use to describe the numerical positions of objects.

When objects are placed in order, ordinal numbers are used to tell their position. For example, if we have a well-ordered set S = {‘a’, ‘b’, ‘c’, ‘d’}, then ‘a’ is the 1 st object, ‘b’ the 2nd, and so on.

Be familiar with the use of:

• natural numbers for counting
• real numbers for measurement.

Be familiar with the concept of a number base, in particular:

• decimal (base 10)
• binary (base 2)
• hexadecimal (base 16).

Students should be familiar with expressing a number’s base using a subscript as follows:

Base 10: Number₁₀ , eg 67₁₀

Base 2: Number₂ , eg 10011011₂

Base 16: Number₁₆ , eg AE₁₆

Convert between decimal, binary and hexadecimal number bases.

Be familiar with, and able to use, hexadecimal as a shorthand for binary and to understand why it is used in this way.

Know that:

• the bit is the fundamental unit of information
• a byte is a group of 8 bits.

A bit is either 0 or 1.

Know that the 2ⁿ different values can be represented with n bits.

For example, 3 bits can be configured in 2³ = 8 different ways.

000, 001, 010, 011, 100, 101, 110, 111

Know that quantities of bytes can be described using binary prefixes representing powers of 2 or using decimal prefixes representing powers of 10, eg one kibibyte is written as 1KiB = 2¹⁰ B and one kilobyte is written as 1 kB = 10³ B.

• kibi, Ki - 2¹⁰
• mebi, Mi - 2²⁰
• gibi, Gi - 2³⁰
• tebi, Ti - 2⁴⁰

• kilo, k - 10³
• mega, M - 10⁶
• giga, G - 10⁹
• tera, T - 10¹²

Historically the terms kilobyte, megabyte, etc have often been used when kibibyte, mebibyte, etc are meant.

Know the difference between unsigned binary and signed binary.

Students are expected to be able to convert between unsigned binary and decimal and vice versa.

Know that in unsigned binary the minimum and maximum values for a given number of bits, n , are 0 and 2ⁿ -1 respectively.

Be able to:

• add two unsigned binary integers
• multiply two unsigned binary integers.

Know that signed binary can be used to represent negative integers and that one possible coding scheme is two’s complement.

This is the only representation of negative integers that will be examined. Students are expected to be able to convert between signed binary and decimal and vice versa.

Know how to:

• represent negative and positive integers in two’s complement
• perform subtraction using two’s complement
• calculate the range of a given number of bits, n .

Know how numbers with a fractional part can be represented in:

• fixed point form in binary in a given number of bits
• floating point form in binary in a given number of bits.

Students are not required to know the Institute of Electrical and Electronic Engineers (IEEE) standard, only to know, understand and be able to use a simplified floating representation consisting of mantissa + exponent.

Be able to convert for each representation from:

• decimal to binary of a given number of bits
• binary to decimal of a given number of bits.

Exam questions on floating point numbers will use a format in which both the mantissa and exponent are represented using two's complement.

Know and be able to explain why both fixed point and floating point representation of decimal numbers may be inaccurate.

Use binary fractions. For a real number to be represented exactly by the binary number system, it must be capable of being represented by a binary fraction in the given number of bits. Some values cannot ever be represented exactly, for example 0.1₁₀ .

Be able to calculate the absolute error of numerical data stored and processed in computer systems.

Be able to calculate the relative error of numerical data stored and processed in computer systems.

Compare absolute and relative errors for large and small magnitude numbers, and numbers close to one.

Compare the advantages and disadvantages of fixed point and floating point forms in terms of range, precision and speed of calculation.

Know why floating point numbers are normalised and be able to normalise un-normalised floating point numbers with positive or negative mantissas.

Explain underflow and overflow and describe the circumstances in which they occur.

Differentiate between the character code representation of a decimal digit and its pure binary representation.

Describe ASCII and Unicode coding systems for coding character data and explain why Unicode was introduced.

Describe and explain the use of:

• parity bits
• majority voting
• checksums
• check digits.

Describe how bit patterns may represent other forms of data, including graphics and sound.

Understand the difference between analogue and digital:

• data
• signals.

Describe the principles of operation of:

• an analogue to digital converter (ADC)
• a digital to analogue converter (DAC).

Know that ADCs are used with analogue sensors.

Know that the most common use for a DAC is to convert a digital audio signal to an analogue signal.

Explain how bitmaps are represented.

Explain the following for bitmaps:

• resolution
• colour depth
• size in pixels.

The size of an image is also alternatively sometimes described as the resolution of an image.

Size of an image in pixels is width of image in pixels x height of image in pixels.

Resolution is expressed as number of dots per inch where a dot is a pixel.

Colour depth = number of bits stored for each pixel.

Calculate storage requirements for bitmapped images and be aware that bitmap image files may also contain metadata.

Ignoring metadata,

storage requirements = size in pixels x colour depth

where size in pixels is width in pixels x height in pixels .

Be familiar with typical metadata.

eg width, height, colour depth.

Explain how vector graphics represents images using lists of objects.

The properties of each geometric object/shape in the vector graphic image are stored as a list.

Give examples of typical properties of objects.

Use vector graphic primitives to create a simple vector graphic.

Compare the vector graphics approach with the bitmapped graphics approach and understand the advantages and disadvantages of each.

Be aware of appropriate uses of each approach.

Describe the digital representation of sound in terms of:

• sample resolution
• sampling rate and the Nyquist theorem.

Calculate sound sample sizes in bytes.

Describe the purpose of MIDI and the use of event messages in MIDI.

Describe the advantages of using MIDI files for representing music.

Know why images and sound files are often compressed and that other files, such as text files, can also be compressed.

Understand the difference between lossless and lossy compression and explain the advantages and disadvantages of each.

Explain the principles behind the following techniques for lossless compression:

• run length encoding (RLE)
• dictionary-based methods.

Understand what is meant by encryption and be able to define it.

Students should be familiar with the terms cipher, plaintext and ciphertext.

Caesar and Vernam ciphers are at opposite extremes. One offers perfect security, the other doesn’t. Between these two types are ciphers that are computationally secure – see below. Students will be assessed on the two types. Ciphers other than Caesar may be used to assess students' understanding of the principles involved. These will be explained and be similar in terms of computational complexity.

Be familiar with Caesar cipher and be able to apply it to encrypt a plaintext message and decrypt a ciphertext.

Be able to explain why it is easily cracked.

Be familiar with Vernam cipher or one-time pad and be able to apply it to encrypt a plaintext message and decrypt a ciphertext.

Explain why Vernam cipher is considered as a cypher with perfect security.

Since the key k is chosen uniformly at random, the ciphertext c is also distributed uniformly. The key k must be used once only. The key k is known as a one-time pad.

Compare Vernam cipher with ciphers that depend on computational security.