Click a paper to view its questions. The specification for the course is shown on the right. For each question, its specification references are listed and can be clicked to scroll the relevant part of the specification into view.
Only A-Level questions relating to the AS-Level specification are shown.
Shade one lozenge to indicate which of the unsigned numbers listed in Table 1 has the largest value.
| Number base | Number | Largest value (shade one) |
|---|---|---|
| Binary | 101101001 | |
| Hexadecimal | 30A | |
| Decimal | 396 |
- A:Binary 101101001
- B:Hexadecimal 30A
- C:Decimal 396
30A;
R. if more than one lozenge shaded.
Calculate the minimum amount of storage space that is required to store the bitmap image in Figure 2 excluding metadata. Express your answer in bytes. You must show your working.
50 x 50 x 2 / 8 = 625 (bytes)
2 marks for the correct answer with some working shown
OR
1 mark for one of:
- multiplying 50 by 50 in the working // 2500 in the working
- multiplying by 2 in the working
- giving the correct solution of 625 (bytes) with no working shown
Figure 3 shows an enlarged view of part of one row of pixels from the image in Figure 2.
Describe how a row of pixels, such as that shown in Figure 3, could be represented in compressed form by using run length encoding.
A run is a sequence/series of pixels of the same colour // the number of consecutive pixels of the same colour would need to be counted;
(Pairs of values would be stored), which would consist of a run length and the colour of the pixels in the run;
Example of how the specific row of pixels would be compressed eg 7 Yellow, 4 Blue, 9 Yellow; A. assignment of numeric values to colours
A. "row" for "run" as BOD
Max 2
The image in Figure 2 is compressed using run length encoding. The compressed file is 80% smaller than the original file. The image in Figure 4 is compressed using the same technique and the compressed file is approximately the same size as the original file.
Explain why the run length encoding method was not able to compress the image in Figure 4 as much as it could compress the image in Figure 2.
Runs will be of shorter length // the image (in the second figure) contains a lot more different colours; A. colour depth is higher in the second image
(For short runs) the additional run length data may (largely) cancel out (or even outweigh) the reduction in storage of pixel colour data;
A. responses given in reverse ie why first figure was compressed more effectively
Figure 5 shows a logic circuit.
Complete the part of the truth table for the circuit in Figure 5 that is shown below.
| Inputs | Outputs | ||||||
|---|---|---|---|---|---|---|---|
| A | B | C | D | E | F | G | H |
| 0 | 0 | 0 | |||||
| 0 | 0 | 1 | |||||
| 0 | 1 | 1 | |||||
| 1 | 1 | 1 | |||||
| Inputs | Outputs | ||||
|---|---|---|---|---|---|
| A B C | D | E | F | G | H |
| 0 0 0 | 0 | 0 | 0 | 0 | 0 |
| 0 0 1 | 1 | 0 | 0 | 0 | 1 |
| 0 1 1 | 0 | 1 | 0 | 1 | 0 |
| 1 1 1 | 0 | 1 | 0 | 1 | 1 |
1 mark: Column D correct or column E or column F correct
1 mark: Column G correct
1 mark: Column H correct
Max 2 if any incorrect values in table
Using Figure 5, write a Boolean expression to show how the output G is calculated from the inputs A, B and C.
B⋅C+A⋅(B⊕C)
1 mark: B⋅C or B⊕C somewhere in expression
1 mark: A is ANDed with B⊕C
1 mark: Fully correct expression
A. award second mark even if brackets around B⊕C are missing
A. use of AND, OR, XOR instead of symbols
A. (B̅∙C)+(B∙C̅) for (B⊕C)
If a student has written an expression but then tried to simplify it and made an error then mark the original expression that the student has written down and ignore the simplification.
Describe the principles of operation of an optical disk drive that is used to read data from an optical disk such as a CD-ROM or DVD-ROM.
Level 3 (5-6 marks): A detailed, coherent, description that covers both the reading mechanism and how data is represented, demonstrating a very good level of understanding.
Level 2 (3-4 marks): An adequate description, including at least three points from the list below. The description may cover one or both of the reading mechanism and how data is represented. The description is logically organised so that it makes sense when read as a whole and therefore demonstrates a reasonable understanding of the principles of operation of an optical disk drive.
Level 1 (1-2 marks): A small number of relevant points have been recalled (in this case award one mark per point, up to a maximum of two from lists below). The structure of the response, or lack of it, means that only a very limited understanding of the principles of operation is demonstrated.
Indicative Content
Reading mechanism
- (Low power beam of) laser / light is shone at disk NE. implication because it is reflected
- Light is focussed on spot on track
- (Some) light is reflected back from disk
- Amount of light reflected back is measured // light sensor detects reflection
- Disc spins at constant linear velocity // zoned constant linear velocity // variable (angular) velocity A. variable speed R. constant speed
How data is represented
- Data is stored on one/spiral track A. data is read in a spiral
- Continuation of land/pit reflects light whereas transition between land and pit scatters light A. land reflects light whereas pits scatter light/do not reflect light or vice-versa A. "bump" for "land"
- Transition between land and pit indicates a 1 and continuation of land/pit represents 0 A. land represents 1, pit 0 or vice-versa A. reflection represents 1, no reflection 0 or vice-versa A. "bump" for "land" or other wording which clearly reflects nature of pits and lands
USB Flash Drives (a type of SSD) are a more popular choice for transferring files such as images and word processed documents from one computer to another than CD-Rs (a type of optical disk).
Explain why this is the case.
- Flash drives can have a higher (storage) capacity; R. references that could be to physical size eg "Flash drives are bigger"
- Flash drives have faster access/read/write times;
- No drive is required to use a flash drive // flash drive and medium are integrated;
- Flash drives can be reused;
- Flash drives are more compact;
- Flash drives not damaged by scratches; NE. more robust without a reason why
R. points about cost unless they are supported by a reason, such as no separate drive being required
NE. more portable unless this is supported with a valid reason that would not also apply to a CD
Explain some of the challenges that face legislators in the digital age.
One mark per challenge that is explained.
- Information can be combined/processed/transferred in ways that were not previously possible; A. an example of this NE. there is a lot more data
- Technology evolves quickly (so difficult for law to keep up with changes) // new types of crime become possible // some crimes are easier // future problems may not be understood;
- Global nature of Internet means crimes may be committed in one country from outside its direct jurisdiction // laws are often national/local whilst the Internet is global // digital crime can be committed from a great distance // different countries have different laws;
- Some crimes may be committed by states rather than individuals;
- Different countries/cultures may have different attitudes to principles important to computer science (such as copyright, intellectual property, privacy); Note: this point relates to attitudes not legislation
- Methods such as encryption make it harder to monitor criminal activity // electronic evidence may be harder to gather than physical evidence // can be harder to identify culprits online (eg by use of proxies, VPN) // peer-to-peer systems make it harder to identify criminal; NE. hard to catch criminals
- Individuals may have access to large amounts of sensitive information that may be of public interest // conflicts between freedom of speech/information and privacy/state secrets;
- Technology companies (can use their wealth) to lobby for their own interests // concern over influence of companies on legislators;
- Resources required to enforce legislation may not be available;
NE. Copyright, Data Protection, Misuse, Hacking
Refer responses containing other relevant points to team leaders.
Max 3
Three numbers are listed in the first column of Table 2. For each row in Table 2, shade one or more lozenges, in the appropriate column(s), to indicate which set(s) of numbers contain(s) the number on the row. As an example, the first row has been completed for you, to indicate that π is a member of the set of irrational numbers and the set of real numbers, but is not a member of the sets of natural, integer or rational numbers.
| Natural | Integer | Rational | Irrational | Real | |
|---|---|---|---|---|---|
| π | ✓ | ✓ | |||
| 15/23 | |||||
| 108 |
15/23: Rational, Real;
108: Natural, Integer, Rational, Real;
R. answers in which additional lozenges are shaded
Figure 10 shows a list of eight numbers, stored in an array.
Figure 10
| Index | [0] | [1] | [2] | [3] | [4] | [5] | [6] | [7] |
|---|---|---|---|---|---|---|---|---|
| Contents | 48 | 9 | 201 | 62 | 82 | 92 | 30 | 72 |
Describe what an ordinal number is and what an ordinal number would be used for in the context of this array.
What is – 1 mark:
Shows order / position / rank / place;
Use in array – 1 mark:
The ordinal numbers would represent the position / index / location of the values in the array;
Using the laws of Boolean algebra, show that:
(A + B) ∙ (B + C ∙ (D + D')) = A ∙ C + B
You must show your working.
Award up to four marks for the working shown, but Max 3 if the response does not show that (A + B) ∙ (B + C ∙ (D + D')) = A ∙ C + B
1 mark for each application of an identity or theorem that produces an expression that is logically equivalent to the original expression but uses fewer logical operators.
1 mark for a successful application of the distribution law – only one mark, regardless of how many times this has been applied
Continue marking until an incorrect step is encountered. If a student misses out some steps but does not make an error then continue marking.
Example Solution 1
(A + B) ∙ (B + C ∙ (D + D̅ ))
= (A + B) ∙ (B + C ∙ 1) By identity X + X̅ = 1
= (A + B) ∙ (B + C) By identity X ∙ 1 = X
= A ∙ B + A ∙ C + B ∙ B + B ∙ C Using distribution law
= A ∙ B + A ∙ C + B + B ∙ C By identity X ∙ X = X
= A ∙ B + A ∙ C + B By redundancy theorem X + X ∙ Y = X
= A ∙ C + B By redundancy theorem X + X ∙ Y = X
Example Solution 2
(A + B) ∙ (B + C ∙ (D + D̅ ))
= (A + B) ∙ (B + C ∙ 1) By identity X + X̅ = 1
= (A + B) ∙ (B + C) By identity X ∙ 1 = X
= A ∙ C + B Using distribution law (this jump is worth 2 marks)
Figure 11 shows an analogue signal represented as a waveform. The analogue signal is being converted to a digital signal by an analogue to digital convertor (ADC). Points A and B in Figure 11 indicate the amplitude of the waveform (A), at a point in time, and the value that was recorded for this measurement when the waveform was sampled (B).
The waveform's amplitude is measured and recorded using a scale with 16 divisions, which are shown on the Y axis in Figure 11. The recorded digital data indicates which division on the Y axis each measurement is closest to. For example, the closest division to measurement A is 13.
What sample resolution has been used?
4 bits // nibble / half a byte;
NE. 4, 0.5
The graph covers a time period of 0.01 seconds. During this time period, 10 samples have been recorded at the times indicated by the divisions on the X axis in Figure 11.
What sample rate has been used?
1000 // 1 x 103 // 103 (Hz / samples per second) // 1kHz;
A. 10 ÷ 0.01
Explain the impact of the difference between A and B and how this difference could be reduced by redesigning the sampling system.
Significance: It will not be possible to reproduce the original signal (completely) accurately // the recording is not (completely) accurate;
NE. "error" without explaining that this affects the quality of the recording/reproduction
NE. lower
How reduced: Increase the sample resolution // increase the number of bits used to record each sample;
TO. references to changing the sample rate (even if changing sample resolution also mentioned)
A different analogue signal is being sampled. The highest frequency present in the signal's waveform is 1200 Hz.
What is the minimum sample rate that must be used during sampling in order to preserve all of the frequencies in the waveform?
2400 (Hz) // 2.4kHz;
A. 1200 x 2
The pseudo-code in Figure 12 shows one method for carrying out encryption of a single character using the Caesar Cipher. If the character to be encrypted is a capital letter, then the encrypted character will be shifted along the alphabet by the number of positions specified by the key. If the character is not a capital letter, then the encrypted character is set to be equal to the original character.
Figure 13 shows an incomplete assembly language program that has been written to implement the pseudo-code algorithm shown in Figure 12.
By analysing the assembly language program in Figure 13, explain the purpose for which the registers R1, R2 and R3 have been used.
One mark for correct purposes given for one or two registers OR two marks for correct purposes given for all three registers.
| Register | Purpose |
|---|---|
| R1 | The plaintext letter // the letter before it has been encrypted // the original letter // the characterCode |
| R2 | The key // the number of positions to shift letters by // the value to add to the letter // the keyValue |
| R3 | The ciphertext letter // the encrypted letter // the encryptedCode |
A. "letter" for "character" and vice-versa
On Figure 13, write the assembly language instruction that is missing from position 1.
MOV R3, R1;
I. missing comma
A. another command which would achieve the same affect eg ORR R3, R1, #0
On Figure 13, write the assembly language instructions that are missing from position 2.
- R3 compared to 90 or 91as first command;
- 26 subtracted from R3 and result stored back into R3 after comparison (whether comparison is correct or not);
- Fully working code;
DPT omission of # for immediate addressing values
DPT use of register number other than R3
I. missing commas
Example Solution 1
CMP R3, #91 BLT finished SUB R3, R3, #26 B finished
Example Solution 2
CMP R3, #90 BGT moveBack B finished moveBack: SUB R3, R3, #26 B finished
Example Solution 3
CMP R3, #90 BLT finished BEQ finished SUB R3, R3, #26 B finished
A family uses a wireless computer network at home.
Describe two security measures that the family should put in place to ensure that the wireless access point is secure and explain how these security measures will make wireless connections to the access point more secure.
Award one mark for each pair of measure and how this is effective.
Do not award marks for both rows 1 and 2.
| Measure | How Makes Effective |
|---|---|
| Use WPA (WiFi Protected Access)/WPA2 (A. WEP) (which is strong) NE. use of password | To encrypt transmission // so that if intercepted transmissions cannot be understood/read by someone who does not have the key / by an unauthorised person |
| Encrypt transmissions | So that if intercepted cannot be understood/read by someone who does not have the key / by an unauthorised person |
| Disable broadcast of SSID (Service Set Identifier) | So that the network is harder to discover // so that you must know the SSID to connect |
| Use a MAC address white list A. "Hardware" for "MAC" NE. whitelist without reference to MAC addresses | So that only devices with a known address // address on the list can connect |
Max 2
The network uses the CSMA/CA access method with Request to Send/Clear to Send (RTS/CTS). A computer on the network has data to send to another computer. Explain how the CSMA/CA access method with RTS/CTS will be used during this transmission.
Level 3 (5-6 marks): A detailed, coherent, description that includes the use of RTS/CTS and that conveys good understanding of how the access method works. Whilst there may be some omissions from the description it contains no misunderstandings.
Level 2 (3-4 marks): An adequate description, including at least three points from the list below. The description is logically organised so that it makes sense when read as a whole and therefore demonstrates a reasonable understanding of how the system works. The description may or may not include the use of RTS/CTS.
Level 1 (1-2 marks): A small number of relevant points have been recalled (in this case award one mark per point, up to a maximum of two from lists below). However, the structure of the response, or lack of it, demonstrated only a very limited understanding, if any, of the access method used.
Indicative Content
- computer with data to send monitors/listens for (data signal)
- if (data) signal present/another transmission in progress then continue to wait
- when no (data) signal present computer sends a Request to Send / RTS A. if no valid points made about RTS/CTS in response then accept that when no data signal is present computer starts to transmit data, but with no marks awarded for RTS/CTS then response is limited to max Level 2
- two computers could start transmitting simultaneously if they both detect there is no data signal
- receiver/WAP responds (to RTS) with a Clear to Send / CTS signal A. router
- RTS/CTS signal blocks any other transmissions from nodes in range
- if/when CTS received then start to transmit A. by implication as BOD if the student states that the computer will begin to transmit after the receiver sends the CTS
- if CTS not received continue to wait (until transmission ends)
- receiver sends acknowledgement / ack after (all) data received
- after transmitting (the transmitter) waits to receive acknowledgement packet (to confirm data received and not corrupted)
- if no acknowledgement/ack received (within reasonable time period) then: wait a time period then listen again / retransmit.
- the acknowledgement/ack also notifies other computers that they can transmit again
- waiting periods are (often) random A. an example waiting period that is random
- collisions cannot be detected by transmitter
Paper 2 covers sections 5-9 of the AQA AS Computer Science specification.
5 Fundamentals of data representation
5.1 Number systems
5.1.1 Natural numbers
| Reference | Content | Additional information |
|---|---|---|
| 5.1.1.1 View | Be familiar with the concept of a natural number and the set ℕ of natural numbers (including zero). | ℕ = {0, 1, 2, 3, …} |
5.1.2 Integer numbers
| Reference | Content | Additional information |
|---|---|---|
| 5.1.2.1 View | Be familiar with the concept of an integer and the set ℤ of integers. | ℤ = { …, -3, -2, -1, 0, 1, 2, 3, … } |
5.1.3 Rational numbers
| Reference | Content | Additional information |
|---|---|---|
| 5.1.3.1 View | 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. |
5.1.4 Irrational numbers
| Reference | Content | Additional information |
|---|---|---|
| 5.1.4.1 View | Be familiar with the concept of an irrational number. | An irrational number is one that cannot be written as a fraction, for example √2. |
5.1.5 Real numbers
| Reference | Content | Additional information |
|---|---|---|
| 5.1.5.1 View | 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'. |
5.1.6 Ordinal numbers
| Reference | Content | Additional information |
|---|---|---|
| 5.1.6.1 View | 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 1st object, 'b' the 2nd, and so on. |
5.1.7 Counting and measurement
| Reference | Content | Additional information |
|---|---|---|
| 5.1.7.1 View | Be familiar with the use of natural numbers for counting. | |
| 5.1.7.2 View | Be familiar with the use of real numbers for measurement. |
5.2 Number bases
5.2.1 Number base
| Reference | Content | Additional information |
|---|---|---|
| 5.2.1.1 View | Be familiar with the concept of a number base, in particular:
| Students should be familiar with expressing a number's base using a subscript as follows:
|
| 5.2.1.2 View | Convert between decimal, binary and hexadecimal number bases. | |
| 5.2.1.3 View | Be familiar with, and able to use, hexadecimal as a shorthand for binary and to understand why it is used in this way. |
5.3 Units of information
5.3.1 Bits and bytes
| Reference | Content | Additional information |
|---|---|---|
| 5.3.1.1 View | Know that the bit is the fundamental unit of information. | A bit is either 0 or 1. |
| 5.3.1.2 View | Know that a byte is a group of 8 bits. | |
| 5.3.1.3 View | Know that 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. |
5.3.2 Units
| Reference | Content | Additional information |
|---|---|---|
| 5.3.2.1 View | Know the names, symbols and corresponding powers of 10 for the decimal prefixes:
| |
| 5.3.2.2 View | Know the names, symbols and corresponding powers of 2 for the binary prefixes:
| |
| 5.3.2.3 View | 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. | Historically the terms kilobyte, megabyte, etc have often been used when kibibyte, mebibyte, etc are meant. |
5.4 Binary number system
5.4.1 Unsigned binary
| Reference | Content | Additional information |
|---|---|---|
| 5.4.1.1 View | 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. |
| 5.4.1.2 View | Know that in unsigned binary the minimum and maximum values for a given number of bits, n, are 0 and 2n - 1 respectively. |
5.4.2 Unsigned binary arithmetic
| Reference | Content | Additional information |
|---|---|---|
| 5.4.2.1 View | Be able to add two unsigned binary integers. | |
| 5.4.2.2 View | Be able to multiply two unsigned binary integers. |
5.4.3 Signed binary using two's complement
| Reference | Content | Additional information |
|---|---|---|
| 5.4.3.1 View | 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. |
| 5.4.3.2 View | Know how to represent negative and positive integers in two's complement. | |
| 5.4.3.3 View | Know how to perform subtraction using two's complement. | |
| 5.4.3.4 View | Know how to calculate the range of a given number of bits, n. |
5.4.4 Numbers with a fractional part
| Reference | Content | Additional information |
|---|---|---|
| 5.4.4.1 View | Know how numbers with a fractional part can be represented in fixed point form in binary in a given number of bits. | |
| 5.4.4.2 View | Be able to convert decimal to fixed point binary of a given number of bits. | |
| 5.4.4.3 View | Be able to convert fixed point binary to decimal of a given number of bits. |
5.5 Information coding systems
5.5.1 Character form of a decimal digit
| Reference | Content | Additional information |
|---|---|---|
| 5.5.1.1 View | Differentiate between the character code representation of a decimal digit and its pure binary representation. |
5.5.2 ASCII and Unicode
| Reference | Content | Additional information |
|---|---|---|
| 5.5.2.1 View | Describe ASCII and Unicode coding systems for coding character data. | |
| 5.5.2.2 View | Explain why Unicode was introduced. |
5.5.3 Error checking and correction
| Reference | Content | Additional information |
|---|---|---|
| 5.5.3.1 View | Describe and explain the use of parity bits. | |
| 5.5.3.2 View | Describe and explain the use of majority voting. | |
| 5.5.3.3 View | Describe and explain the use of check digits. | |
| 5.5.3.4 View | Evaluate the use of parity bits, majority voting and check digits |
5.6 Representing images, sound and other data
5.6.1 Bit patterns, images, sound and other data
| Reference | Content | Additional information |
|---|---|---|
| 5.6.1.1 View | Describe how bit patterns may represent other forms of data, including graphics and sound. |
5.6.2 Analogue and digital
| Reference | Content | Additional information |
|---|---|---|
| 5.6.2.1 View | Understand the difference between analogue and digital:
|
5.6.3 Analogue/digital conversion
| Reference | Content | Additional information |
|---|---|---|
| 5.6.3.1 View | Describe the principles of operation of an analogue to digital converter (ADC). | |
| 5.6.3.2 View | Describe the principles of operation of a digital to analogue converter (DAC). |
5.6.4 Bitmapped graphics
| Reference | Content | Additional information |
|---|---|---|
| 5.6.4.1 View | Explain how bitmaps are represented. | |
| 5.6.4.2 View | Explain resolution. | Resolution is expressed as number of dots per inch where a dot is a pixel. |
| 5.6.4.3 View | Know that colour depth is the number of bits stored for each pixel. | |
| 5.6.4.4 View | Know that the size of an image in pixels is width of image in pixels × height of image in pixels. | The size of an image is also alternatively sometimes described as the resolution of an image. |
| 5.6.4.5 View | 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. |
| 5.6.4.6 View | Be familiar with typical metadata. | eg width, height, colour depth. |
5.6.5 Digital representation of sound
| Reference | Content | Additional information |
|---|---|---|
| 5.6.5.1 View | Describe the digital representation of sound. | |
| 5.6.5.2 View | Understand sample resolution and its effect on the quality of audio recordings. | |
| 5.6.5.3 View | Understand sampling rate and its effect on the quality of audio recordings. | |
| 5.6.5.4 View | Know Nyquist's theorem. | |
| 5.6.5.5 View | Calculate sound sample sizes in bytes. |
5.6.6 Musical Instrument Digital Interface (MIDI)
| Reference | Content | Additional information |
|---|---|---|
| 5.6.6.1 View | Describe the purpose of MIDI and the use of event messages in MIDI. | |
| 5.6.6.2 View | Describe the advantages of using MIDI files for representing music. |
5.6.7 Data compression
| Reference | Content | Additional information |
|---|---|---|
| 5.6.7.1 View | Know why images and sound files are often compressed and that other files, such as text files, can also be compressed. | |
| 5.6.7.2 View | Understand the difference between lossless and lossy compression and explain the advantages and disadvantages of each. | |
| 5.6.7.3 View | Explain the principles behind run length encoding (RLE) for lossless compression. | |
| 5.6.7.4 View | Explain the principles behind dictionary-based methods for lossless compression. |
5.6.8 Encryption
| Reference | Content | Additional information |
|---|---|---|
| 5.6.8.1 View | Understand what is meant by encryption and be able to define it. | 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. |
| 5.6.8.2 View | Be familiar with the term cipher. | |
| 5.6.8.3 View | Be familiar with the term plaintext. | |
| 5.6.8.4 View | Be familiar with the term ciphertext. | |
| 5.6.8.5 View | Be familiar with Caesar cipher and be able to apply it to encrypt a plaintext message and decrypt a ciphertext. | |
| 5.6.8.6 View | Be able to explain why Caesar cipher is easily cracked. | |
| 5.6.8.7 View | Be familiar with Vernam cipher or one-time pad and be able to apply it to encrypt a plaintext message and decrypt a ciphertext. | 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. |
| 5.6.8.8 View | Explain why Vernam cipher is considered as a cypher with perfect security. | |
| 5.6.8.9 View | Compare Vernam cipher with ciphers that depend on computational security. | Vernam cipher is the only one to have been mathematically proved to be completely secure. The worth of all other ciphers ever devised is based on computational security. In theory, every cryptographic algorithm except for Vernam cipher can be broken, given enough ciphertext and time. |
6 Fundamentals of computer systems
6.1 Hardware and software
6.1.1 Relationship between hardware and software
| Reference | Content | Additional information |
|---|---|---|
| 6.1.1.1 View | Define the term hardware. | |
| 6.1.1.2 View | Define the term software. | |
| 6.1.1.3 View | Understand the relationship between hardware and software. |
6.1.2 Classification of software
| Reference | Content | Additional information |
|---|---|---|
| 6.1.2.1 View | Explain what is meant by system software. | |
| 6.1.2.2 View | Explain what is meant by application software. | |
| 6.1.2.3 View | Understand the need for, and attributes of, different types of software. |
6.1.3 System software
| Reference | Content | Additional information |
|---|---|---|
| 6.1.3.1 View | Know that system software includes operating systems (OSs), utility programs, libraries and translators (compiler, assembler, interpreter). | |
| 6.1.3.2 View | Understand the need for, and functions of operating systems (OSs). | |
| 6.1.3.3 View | Understand the need for, and functions of utility programs. | |
| 6.1.3.4 View | Understand the need for, and functions of libraries. | |
| 6.1.3.5 View | Understand the need for, and functions of translators (compiler, assembler, interpreter). |
6.1.4 Role of an operating system (OS)
| Reference | Content | Additional information |
|---|---|---|
| 6.1.4.1 View | Understand that a role of the operating system is to hide the complexities of the hardware. | |
| 6.1.4.2 View | Know that the OS handles resource management, managing hardware to allocate processors, memories and I/O devices among competing processes. |
6.2 Classification of programming languages
6.2.1 Classification of programming languages
| Reference | Content | Additional information |
|---|---|---|
| 6.2.1.1 View | Show awareness of the development of types of programming languages and their classification into low- and high-level languages. | |
| 6.2.1.2 View | Know that low-level languages are considered to be:
| |
| 6.2.1.3 View | Know that high-level languages include imperative high-level language. | |
| 6.2.1.4 View | Describe machine-code language and assembly language. | |
| 6.2.1.5 View | Understand the advantages and disadvantages of machine-code and assembly language programming compared with high-level language programming. | |
| 6.2.1.6 View | Explain the term 'imperative high-level language' and its relationship to low-level languages. |
6.3 Types of program translator
6.3.1 Types of program translator
| Reference | Content | Additional information |
|---|---|---|
| 6.3.1.1 View | Understand the role of assemblers. | |
| 6.3.1.2 View | Understand the role of compilers. | |
| 6.3.1.3 View | Understand the role of interpreters. | |
| 6.3.1.4 View | Explain the differences between compilation and interpretation. Describe situations in which each would be appropriate. | |
| 6.3.1.5 View | Explain why an intermediate language such as bytecode is produced as the final output by some compilers and how it is subsequently used. | |
| 6.3.1.6 View | Understand the difference between source and object (executable) code. |
6.4 Logic gates
6.4.1 Logic gates
| Reference | Content | Additional information |
|---|---|---|
| 6.4.1.1 View | Construct truth tables for the NOT logic gate. | Students should know and be able to use ANSI/IEEE standard 91-1984 Distinctive shape logic gate symbols for these logic gates. |
| 6.4.1.2 View | Construct truth tables for the AND logic gate. | |
| 6.4.1.3 View | Construct truth tables for the OR logic gate. | |
| 6.4.1.4 View | Construct truth tables for the XOR logic gate. | |
| 6.4.1.5 View | Construct truth tables for the NAND logic gate. | |
| 6.4.1.6 View | Construct truth tables for the NOR logic gate. | |
| 6.4.1.7 View | Be familiar with drawing and interpreting logic gate circuit diagrams involving one or more of the above gates. | |
| 6.4.1.8 View | Complete a truth table for a given logic gate circuit. | |
| 6.4.1.9 View | Write a Boolean expression for a given logic gate circuit. | |
| 6.4.1.10 View | Draw an equivalent logic gate circuit for a given Boolean expression. |
6.5 Boolean algebra
6.5.1 Using Boolean algebra
| Reference | Content | Additional information |
|---|---|---|
| 6.5.1.1 View | Be familiar with the use of Boolean identities and De Morgan's laws to manipulate and simplify Boolean expressions. |
7 Fundamentals of computer organisation and architecture
7.1 Internal hardware components of a computer
7.1.1 Internal hardware components of a computer
| Reference | Content | Additional information |
|---|---|---|
| 7.1.1.1 View | Have an understanding and knowledge of the basic internal components of a computer system. | Although exam questions about specific machines will not be asked, it might be useful to base this section on the machines used at the centre. |
| 7.1.1.2 View | Understand the role of the processor. | |
| 7.1.1.3 View | Understand the role of main memory. | |
| 7.1.1.4 View | Understand the role of the address bus. | |
| 7.1.1.5 View | Understand the role of the data bus. | |
| 7.1.1.6 View | Understand the role of the control bus. | |
| 7.1.1.7 View | Understand the role of I/O controllers. | |
| 7.1.1.8 View | Be able to explain the difference between von Neumann and Harvard architectures and describe where each is typically used. | Embedded systems such as digital signal processing (DSP) systems use Harvard architecture processors extensively. Von Neumann architecture is used extensively in general purpose computing systems. |
| 7.1.1.9 View | Understand the concept of addressable memory. |
7.2 The stored program concept
7.2.1 The meaning of the stored program concept
| Reference | Content | Additional information |
|---|---|---|
| 7.2.1.1 View | Be able to describe the stored program concept: machine code instructions stored in main memory are fetched and executed serially by a processor that performs arithmetic and logical operations. |
7.3 Structure and role of the processor and its components
7.3.1 The processor and its components
| Reference | Content | Additional information |
|---|---|---|
| 7.3.1.1 View | Explain the role and operation of the arithmetic logic unit. | |
| 7.3.1.2 View | Explain the role and operation of the control unit. | |
| 7.3.1.3 View | Explain the role and operation of the clock. | |
| 7.3.1.4 View | Explain the role and operation of general-purpose registers. | |
| 7.3.1.5 View | Explain the role and operation of the program counter. | |
| 7.3.1.6 View | Explain the role and operation of the current instruction register. | |
| 7.3.1.7 View | Explain the role and operation of the memory address register. | |
| 7.3.1.8 View | Explain the role and operation of the memory buffer register. | |
| 7.3.1.9 View | Explain the role and operation of the status register. |
7.3.2 The Fetch-Execute cycle and the role of registers within it
| Reference | Content | Additional information |
|---|---|---|
| 7.3.2.1 View | Explain how the Fetch-Execute cycle is used to execute machine code programs, including the stages in the cycle (fetch, decode, execute) and details of registers used. |
7.3.3 The processor instruction set
| Reference | Content | Additional information | |||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 7.3.3.1 View | Understand the term 'processor instruction set' and know that an instruction set is processor specific. | ||||||||||||||||||
| 7.3.3.2 View | Know that instructions consist of an opcode and one or more operands (value, memory address or register). | A simple model will be used in which the addressing mode will be incorporated into the bits allocated to the opcode so the latter defines both the basic machine operation and the addressing mode. Students will not be expected to define opcode, only interpret opcodes in the given context of a question. For example, 4 bits have been allocated to the opcode (3 bits for basic machine operation, eg ADD, and 1 bit for the addressing mode). 4 bits have been allocated to the operand, making the instruction, opcode + operand, 8 bits in length. In this example, 16 different opcodes are possible (24 = 16).
| |||||||||||||||||
7.3.4 Addressing modes
| Reference | Content | Additional information |
|---|---|---|
| 7.3.4.1 View | Understand and apply immediate addressing. | Immediate addressing: the operand is the datum. |
| 7.3.4.2 View | Understand and apply direct addressing. | Direct addressing: the operand is the address of the datum. Address to be interpreted as meaning either main memory or register. |
7.3.5 Machine-code/assembly language operations
| Reference | Content | Additional information |
|---|---|---|
| 7.3.5.1 View | Understand and apply the basic machine-code operations of:
Use the basic machine-code operations above when machine-code instructions are expressed in mnemonic form—assembly language, using immediate and direct addressing. |
7.3.6 Factors affecting processor performance
| Reference | Content | Additional information |
|---|---|---|
| 7.3.6.1 View | Explain the effect on processor performance of multiple cores. | |
| 7.3.6.2 View | Explain the effect on processor performance of cache memory. | |
| 7.3.6.3 View | Explain the effect on processor performance of clock speed. | |
| 7.3.6.4 View | Explain the effect on processor performance of word length. | |
| 7.3.6.5 View | Explain the effect on processor performance of address bus width. | |
| 7.3.6.6 View | Explain the effect on processor performance of data bus width. |
7.4 External hardware devices
7.4.1 Input and output devices
| Reference | Content | Additional information |
|---|---|---|
| 7.4.1.1 View | Know the main characteristics, purpose and suitability of barcode readers and understand their principles of operation. | |
| 7.4.1.2 View | Know the main characteristics, purpose and suitability of digital cameras and understand their principles of operation. | |
| 7.4.1.3 View | Know the main characteristics, purpose and suitability of laser printers and understand their principles of operation. | |
| 7.4.1.4 View | Know the main characteristics, purpose and suitability of RFID and understand their principles of operation. |
7.4.2 Secondary storage devices
| Reference | Content | Additional information |
|---|---|---|
| 7.4.2.1 View | Explain the need for secondary storage within a computer system. | |
| 7.4.2.2 View | Know the main characteristics, purposes, suitability and understand the principles of operation of the hard disk. | |
| 7.4.2.3 View | Know the main characteristics, purposes, suitability and understand the principles of operation of the optical disk. | |
| 7.4.2.4 View | Know the main characteristics, purposes, suitability and understand the principles of operation of the solid-state disk (SSD). | SSD = NAND flash memory + a controller that manages pages, and blocks and complexities of writing. Based on floating gate transistors that trap and store charge. A block, made up of many pages, cannot overwrite pages; a page has to be erased before it can be written to but technology requires the whole block to be erased. Lower latency and faster transfer speeds than a magnetic disk drive. |
| 7.4.2.5 View | Compare the capacity and speed of access of various media and make a judgement about their suitability for different applications. |
8 Consequences of uses of computing
8.1 Individual (moral), social (ethical), legal and cultural issues and opportunities
| Reference | Content | Additional information |
|---|---|---|
| 8.1.1 View | Show awareness of current individual (moral), social (ethical), legal and cultural opportunities and risks of computing. Understand that:
Be able to discuss the challenges facing legislators in the digital age. | Teachers may wish to employ two very powerful techniques, hypotheticals and case studies, to engage students in the issues. Hypotheticals allow students to isolate quickly important ethical principles in an artificially simplified context. For example, a teacher might ask students to explain and defend how, as a Google project manager, they would evaluate a proposal to bring Google's Street View technology to a remote African village. What questions should be asked? Who should be consulted? What benefits, risks and safeguards considered? What are the trade-offs? Case studies allow students to confront the tricky interplay between the sometimes competing ethical values and principles relevant in real world settings. For example, the Google Street View case might be used to tease out the ethical conflicts between individual and cultural expectations, the principle of informed consent, Street View's value as a service, its potential impact on human perceptions and behaviours, and its commercial value to Google and its shareholders. There are many resources available on the Internet to support teaching of this topic. |
9 Fundamentals of communication and networking
9.1 Communication
9.1.1 Communication methods
| Reference | Content | Additional information |
|---|---|---|
| 9.1.1.1 View | Define serial transmission methods. | |
| 9.1.1.2 View | Define parallel transmission methods. | |
| 9.1.1.3 View | Discuss the advantages of serial over parallel transmission. | |
| 9.1.1.4 View | Define and compare synchronous and asynchronous data transmission. | |
| 9.1.1.5 View | Describe the purpose of start and stop bits in asynchronous data transmission. |
9.1.2 Communication basics
| Reference | Content | Additional information |
|---|---|---|
| 9.1.2.1 View | Define baud rate. | |
| 9.1.2.2 View | Define bit rate. | |
| 9.1.2.3 View | Define bandwidth. | |
| 9.1.2.4 View | Define latency. | |
| 9.1.2.5 View | Define protocol. | |
| 9.1.2.6 View | Differentiate between baud rate and bit rate. | Bit rate can be higher than baud rate if more than one bit is encoded in each signal change. |
| 9.1.2.7 View | Understand the relationship between bit rate and bandwidth. | Bit rate is directly proportionate to bandwidth. |
9.2 Networking
9.2.1 Network topology
| Reference | Content | Additional information |
|---|---|---|
| 9.2.1.1 View | Understand and explain the operation of a physical star topology. | |
| 9.2.1.2 View | Understand and explain the operation of a logical bus network topology. | A network physically wired in star topology can behave logically as a bus network by using a bus protocol and appropriate physical switching. |
| 9.2.1.3 View | Differentiate between the physical star topology and the logical bus network topology. |
9.2.2 Types of networking between hosts
| Reference | Content | Additional information |
|---|---|---|
| 9.2.2.1 View | Explain peer-to-peer networking and describe situations where it might be used. | In a peer-to-peer network, each computer has equal status. |
| 9.2.2.2 View | Explain client-server networking and describe situations where it might be used. | In a client-server network, most computers are nominated as clients and one or more as servers. The clients request services from the servers, which provide these services, for example file server, email server. |
9.2.3 Wireless networking
| Reference | Content | Additional information |
|---|---|---|
| 9.2.3.1 View | Explain the purpose of WiFi. | A wireless local area network that is based on international standards. Used to enable devices to connect to a network wirelessly. |
| 9.2.3.2 View | Be familiar with the components required for wireless networking. |
|
| 9.2.3.3 View | Be familiar with the purpose of Service Set Identifier (SSID). | |
| 9.2.3.4 View | Be familiar with how wireless networks are secured using WPA (Wifi Protected Access)/WPA2. | |
| 9.2.3.5 View | Be familiar with how wireless networks are secured by disabling SSID (Service Set Identifier) broadcasting. | |
| 9.2.3.6 View | Be familiar with how wireless networks are secured using a MAC (Media Access Control) address allow list. | |
| 9.2.3.7 View | Explain the wireless protocol Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA) with and without Request to Send/Clear to Send (RTS/CTS). |