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Complete Summary of the AQA AS Computer Science Syllabus $80.59
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Complete Summary of the AQA AS Computer Science Syllabus
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The complete summary of the AQA AS level Computer Science specification Written by a Cambridge offer-holder and A*A*A* predicted student. Hand-drawn diagrams and detailing included. Programming, theory and ethics all within this document.

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Document information

  • July 30, 2022
  • 53
  • 2021/2022
  • Summary

Subjects

  • computer science
  • data science
  • machine learning
  • ethics
  • a level
  • a level cs
  • a level computer science
  • aqa
  • as computer science
  • programming
  • coding
  • as coding
  • year 12
  • year 13
  • uni
  • university
  • cs
  • ai

Written for

  • A/AS Level
  • AQA
  • Computer Science
  • AS Level
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Computer Science
Encryption
 Caesar cipher = shift all letters by certain number of places in alphabet
 Vernam cipher = create a key of random 0s and 1s; if plaintext and key digits are the same
then ciphertext is a 0, if they are different then ciphertext is a 1
 Substitution cipher = a method of encrypting by which units of plaintext are replaced with
ciphertext according to a fixed system.
 Transposition cipher = rearranging letters of plaintext
 Frequency analysis considers how often a letter occurs in normal text and correlates this to
the ciphertext units to decipher substitution cipher
 Key = a piece of information used in combination with an algorithm (cipher) to form
ciphertext from plaintext and vice versa

Number Systems and General Number Bases
 Natural numbers = positive integers
 Integers = numbers that can be written without a fractional counterpart
 Rational numbers = numbers that can be represented as a/b where a and b are integers and
b≠0
 Real numbers = numbers of continuous quantities that can be represented on a number line
 Irrational numbers = real numbers that cannot be represented in the form a/b where a and b
are integers
 Complex numbers = numbers that can be expressed in the form a+bi where a and b are real
numbers and i represents the imaginary unit

ASCII and Unicode
 ASCII uses 7 bit encoding to code 128 different characters; includes English alphabet and
punctuation and digits but not emojis and other languages
 Extended ASCII code is an 8-bit character set that represents 256 different characters,
making it possible to use characters such as é or ©. Extended ASCII is useful for European
languages
 Unicode uses between 8 and 32 bits per character, so it can represent far more characters;
takes up more storage
 A character code is a unique binary code representing a character
 Ord(char) gives a character’s ASCII number
 Chr(n) gives a number’s letter symbol in ASCII

(Unsigned) Binary Numbers
 Base 10 to binary:
o Repeated subtraction of powers of 2
o Repeated division by 2 and reverse the remainders
 An overflow error is when the answer cannot be represented in the given number of bits. It
does not matter if the addition gives too many bits as long as the number Can be
represented; just take the correct number of bits from the right.
 BCD= binary coded decimal; parts of binary encoded as denary

2s Complement Binary Numbers

,  2s complement binary = place value of each column is the same as unsigned binary but left-
most column has negative value
 To convert a negative number in base 10 to 2s complement binary:
o Find the positive number in binary
o Change 0s to 1s and vice versa
o Add 1 to the rightmost column

Hexadecimal
 Higher information density and readability vs binary
 Used in colour references, assembly language programs and error messages [memory locs]
 Hexadecimal to binary
o Take each hexadecimal digit
o Interpret each one in base 10 eg. 2 is 2, F is 15
o Convert each to 4 bit binary
 Binary to hexadecimal
o If the number of bits is not a multiple of 4, add 0s at the front of the data stream
o Take groups of 4 bits and convert each block of 4 bits to denary
o Interpret each of these denary numbers as a hexadecimal digit (10 is A, 11 is B, etc.)

Units of Data
 1 bit = 1 binary digit
 1 byte = 8 bits
 1 kilobyte = 103bytes
1 megabyte = 106 bytes
1 gigabyte = 109 bytes
1 terabyte = 1012 bytes
 1 kibibyte = 210 bytes
1 mebibyte = 220 bytes
1 gibibyte = 230 bytes
1 tebibyte = 240 bytes
 Bits may represent an instruction, opcode & operand or an integer
 Word = group of bits which can be manipulated as a single unit

Values, Datatypes and Expressions
 The scope of a variable means where the variable is accessible – global vs local
 Global variables are defined in the beginning of a program and are used to store values that
need to be used throughout the program
 Local variables are declared inside a subroutine and used within the body of the subroutine
 Local variables aid modularisation
 Local variables better because easier to debug, easier to read, same name for diff variables
 Datatype = an attribute given to all variables/constants that dictates how the data is to be
stored and handled
 Integer = whole number
 String = string of characters
 Float = decimal number
 Boolean = True or False

,  Explicit type conversion can force a value to take on a specific datatype and as such be
manipulated with special operations for that data type
 Expression = a statement that can be “evaluated” to result in a value which has a certain
datatype
 Condition = an expression that has a Boolean value attached to it, which then results in
selection and some event taking place
 Variable = a value that can change during the running of a program
 Constant = a value that cannot change again once you have set it
 Declaration = reserving a block of memory and giving it a named identifier and data type
 Assignment = changing the value at a particular memory location; changing the value of the
variable




Boolean Algebra

, Simplification Laws:

 A . A= A
 A+ A= A
 A . A=0
 A+ A=1
 A .1= A
 A+0= A
 A .0=0
 A+1=1
 A . B=B . A
 A+ B=B+ A
 A . ( B . C )=( A . B ) . C
 A+ ( B+C )=( A+ B ) +C
 A . ( B+C )= A . B+ A . C
 A . ( A + B )= A
 A+ ( A . B )= A
 A+ ( B .C )= ( A +B ) . ( A+C )
XOR Laws:

 A ⊕B=B ⊕ A
 A ⊕ A=1
 A ⊕ 0=A
 A ⊕ 1= A
 A ⊕B=B ⊕ A
 ( A ⊕ B ) ⊕C= A ⊕ ( B⊕ C )= A ⊕ B ⊕ C
 A ( B⊕ C )= AB ⊕ AC
De Morgan’s Laws

 ( A . B )= A+ B
 ( A+ B)= A . B

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