Component 01

Processors: Structure, Performance and Types

91 questions in the guide, across 7 subtopics. More from this topic

1. Inside the CPU: ALU, control unit, registers and buses

   What is held in the current instruction register, and what two parts does it split into?

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   The CIR holds the instruction currently being decoded and executed, copied there from the MDR at the end of the fetch. The control unit splits it into the opcode, which says what operation to perform, and the operand, which says what to perform it on: a value, a register, or a memory address.

2. Inside the CPU: ALU, control unit, registers and buses

   What travels on the control bus?

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   Control and timing signals rather than data. Typical signals are read, write, the clock pulse used to synchronise components, memory request, interrupt request, bus request and bus grant, and reset. It is bidirectional, because devices send signals back to the processor, an interrupt request being the obvious example.

3. The fetch-decode-execute cycle

   Which register tells you what the processor is doing right now, and which tells you what it is about to do?

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   The current instruction register holds the instruction being decoded or executed at this moment. The program counter holds the address of the instruction that will be fetched next.

Component 01

Input, Output and Storage

46 questions in the guide, across 4 subtopics. More from this topic

Component 01

Systems Software

97 questions in the guide, across 7 subtopics. More from this topic

1. Operating systems: need, function and purpose

   How does an operating system provide a 'platform' for application software?

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   It exposes a consistent set of services and interfaces (often called an application programming interface) that applications call instead of controlling hardware. A word processor asks the operating system to save a file; it does not need to know whether the storage is a hard disk drive, a solid state drive or a network share.

2. Memory management: paging, segmentation and virtual memory

   What is virtual memory?

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   A technique in which a section of secondary storage is used as though it were RAM. Pages that are not currently needed are written out to this area, freeing real RAM for the pages that are needed, so programs larger than physical memory can run and more programs can be open at once than would otherwise fit.

3. Memory management: paging, segmentation and virtual memory

   A machine has 8 KiB of RAM available to processes and a page size of 1 KiB. A process needs 5.2 KiB. How many frames does it occupy and how much space is wasted?

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   5.2 KiB requires 6 pages, because a partly used page still needs a whole frame, so it occupies 6 frames of 1 KiB. The wasted space is 6 - 5.2 = 0.8 KiB, sitting unused inside the last page. This waste inside an allocated page is internal fragmentation.

Component 01

Applications Generation

64 questions in the guide, across 5 subtopics. More from this topic

1. Applications and utilities

   What is application software?

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   Software that carries out a task the end user actually wants done, rather than running or maintaining the computer itself - for example a word processor, a spreadsheet, a web browser, a database front end, a photo editor or a game. It relies on system software to reach the hardware.

2. Open source and closed source

   What is open source software?

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   Software released with its source code, under a licence that allows anyone to read it, modify it and redistribute it. It is usually, though not necessarily, free of charge; the defining feature is the freedom granted by the licence, not the price.

3. Translators: interpreters, compilers and assemblers

   What is a compiler?

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   A translator that takes the whole of a high-level source program and translates it in one operation into object or machine code, producing an executable file. Once compiled, the program can be run repeatedly without the compiler or the source code being present.

Component 01

Software Development

41 questions in the guide, across 3 subtopics. More from this topic

1. The five development methodologies

   State the stages of the waterfall lifecycle in order.

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   Feasibility study; requirements analysis; design; implementation (coding); testing; installation, then ongoing evaluation and maintenance. Each stage is completed and approved before the next begins.

2. Writing and following algorithms

   What is a trace table, and why is it used?

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   A table with a column for each variable, in which each row records the values after one step or one pass of the algorithm, together with any output produced. It is used to dry run an algorithm by hand, so that its behaviour can be checked without a computer and the exact point at which the values go wrong can be located.

Component 01

Types of Programming Language

64 questions in the guide, across 5 subtopics. More from this topic

1. Programming paradigms and procedural languages

   What is a programming paradigm?

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   A style or approach to programming - a whole way of thinking about how a program should be organised and what a program fundamentally is. It determines how the problem is broken down, how data and instructions are represented and how the flow of the program is controlled, rather than merely which keywords are typed.

2. Assembly language and the Little Man Computer

   What does the DAT instruction do in the Little Man Computer, and why is it not really an instruction?

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   It reserves a mailbox to hold data rather than a program instruction, and optionally sets its initial value. It is an assembler directive: it generates no operation for the processor to carry out, it simply tells the assembler to set that memory location aside, usually with a label so the program can refer to it by name.

3. Modes of addressing memory

   In immediate addressing, what does the operand actually mean?

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   The operand is the data itself. No memory access is needed to fetch it, because the value to be used is held in the instruction, so the processor simply uses it. If the instruction is LDA with immediate addressing and the operand is 7, the accumulator is set to 7.

Component 01

Exchanging Data

122 questions in the guide, across 9 subtopics. More from this topic

1. Compression

   Lossy compression generally produces much smaller files than lossless. Why, then, is lossless ever used?

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   Because for many kinds of data any loss at all destroys the file's meaning. Lossless is used whenever the exact original bits are needed, and the price of that guarantee is a smaller reduction in size. Lossy is only acceptable where an approximation is good enough for the human viewer or listener.

2. Encryption

   Describe how symmetric encryption works.

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   In symmetric encryption a single shared key is used both to encrypt the plaintext and to decrypt the ciphertext. The sender and the recipient must both hold that same secret key. The sender encrypts with it, transmits the ciphertext, and the recipient applies the same key to recover the plaintext.

3. Hashing

   What is a collision?

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   A collision occurs when two different inputs produce the same hash value. Collisions are unavoidable in principle because there are more possible inputs than there are possible fixed-size outputs, so a good hash function aims to make them rare and hard to engineer rather than impossible.

Component 01

Networks

92 questions in the guide, across 8 subtopics. More from this topic

1. Characteristics of networks, LANs and WANs, protocols and standards

   Is the internet a LAN or a WAN?

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   The internet is the largest example of a wide area network - in fact a global collection of interconnected wide area networks. It spans the whole planet, no single organisation owns its infrastructure, and it exists to join together enormous numbers of separate networks including individual LANs.

2. Protocol layering and the TCP/IP stack

   Which protocols operate at each layer of the TCP/IP stack?

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   Application layer: HTTP, HTTPS, FTP, SMTP, POP3, IMAP, and DNS queries. Transport layer: TCP and UDP. Network layer: IP, which carries the addressing and routing. Link layer: the technologies that move frames over the physical medium, such as Ethernet on cable and Wi-Fi over radio.

Component 01

Web Technologies

71 questions in the guide, across 6 subtopics. More from this topic

1. Structure, presentation and behaviour, and HTML

   What is the link element used for in the head of a page?

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   It attaches an external resource to the document, and its main use is to link the page to an external CSS stylesheet. Putting one link element in each page means every page shares one stylesheet, so the whole site is styled consistently from a single file and the browser can cache that file and reuse it across pages.

2. CSS

   Write and explain a CSS rule that makes all main headings blue.

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   h1 { color: blue; } The selector h1 chooses every first-level heading in any document that uses the stylesheet. The declaration inside the braces sets the property color to the value blue. Every such heading is then drawn in blue, and changing that one word changes them all.

3. JavaScript

   What does this produce? var total = 0; for (var i = 1; i <= 5; i++) { total = total + i; } alert(total);

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   A pop-up alert box showing 15. total starts at 0 and the loop runs with i taking the values 1, 2, 3, 4 and 5, adding each to total in turn: 0+1=1, then 3, then 6, then 10, then 15. When i becomes 6 the condition i <= 5 is false and the loop ends, so the alert displays the final value 15.

Component 01

Data Types and Number Representation

152 questions in the guide, across 9 subtopics. More from this topic

1. Primitive data types

   Why is it dangerous to test two real numbers for exact equality?

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   Because rounding errors mean two calculations that should give the same value may differ in the last bits. A total built by adding 0.1 ten times may not equal 1.0 exactly. The safe test is whether the difference between the two values is smaller than a small tolerance, rather than whether they are identical.

2. Primitive data types

   In a language where + works on both, what is the difference between 5 + 3 and "5" + "3"?

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   5 + 3 is integer addition and gives 8. "5" + "3" is string concatenation and gives "53", because the operands are strings of characters, not numbers. The operator symbol is the same but the type of the operands decides which operation is performed.

3. Binary, hexadecimal and denary conversion

   Why does hexadecimal need the letters A to F?

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   Hexadecimal is base 16, so it needs sixteen distinct digit symbols. The denary digits only supply ten of them, so the remaining six values, ten to fifteen, are written A, B, C, D, E and F. A is 10, B is 11, C is 12, D is 13, E is 14 and F is 15.

Component 01

Data Structures

124 questions in the guide, across 9 subtopics. More from this topic

1. Arrays and records

   How many elements does an array declared as 2 by 3 by 4 hold?

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   24. The sizes of the dimensions multiply: 2 x 3 x 4 = 24 elements.

2. Arrays and records

   An array of 4-byte integers begins at address 2000 and is indexed from 0. What is the address of element 6?

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   2024. Address = 2000 + (6 x 4) = 2000 + 24 = 2024. Note that if the array were indexed from 1 instead, you would use (6 - 1) x 4, giving 2020 - always check the base index.

3. Lists and tuples, and creating, traversing, adding and removing items

   What is a list?

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   An ordered, dynamic collection of items where each item has a position. Unlike an array, a list can grow and shrink at run time, items can be inserted and removed at any position, and in many languages the items do not all have to be the same data type. Underneath it may be implemented with a dynamic array or a linked list.

Component 01

Boolean Algebra

85 questions in the guide, across 5 subtopics. More from this topic

1. Logic Gates and Truth Tables

   Describe the NAND gate and give its truth table.

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   The symbol is the AND gate's D shape with a small circle on its output, the circle indicating inversion. It is NOT AND, so its output is the exact opposite of AND: 1 in every case except when all inputs are 1. Truth table: 0,0 gives 1; 0,1 gives 1; 1,0 gives 1; 1,1 gives 0.

2. Logic Gates and Truth Tables

   What happens if you connect both inputs of a NAND gate to the same signal? Work it out from the truth table.

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   Only two rows are then reachable. With both inputs 0, NAND gives 1; with both inputs 1, NAND gives 0. That is exactly the behaviour of a NOT gate, so a NAND with its inputs tied together inverts. The same trick works with a NOR gate, whose 0,0 row gives 1 and 1,1 row gives 0.

3. Defining Problems Using Boolean Logic

   Write an expression, in terms of A and B only, that is true when exactly one of the two inputs is true, without using the XOR symbol.

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   Q = A.B' + A'.B - that is, A true and B false, or A false and B true. This is the sum-of-products form of exclusive OR, and it is exactly what an XOR gate does. Writing it out this way is useful when you need to build the function from AND, OR and NOT gates only.

Component 01

Legal, Moral, Cultural and Ethical Issues

73 questions in the guide, across 7 subtopics. More from this topic

Component 02

Elements of Computational Thinking

78 questions in the guide, across 5 subtopics. More from this topic

Component 02

Problem Solving and Programming

119 questions in the guide, across 10 subtopics. More from this topic

1. Programming techniques: Programming constructs

   Why must a file be closed after a program has finished reading or writing it?

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   Closing releases the file for other programs and makes sure any data still held in a buffer is actually written to storage.

2. Programming techniques: Programming constructs

   What do the MOD and DIV operators return?

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   MOD returns the remainder of an integer division and DIV returns the whole-number quotient, so 12 MOD 5 is 2 and 12 DIV 5 is 2.

Component 02

Algorithms

230 questions in the guide, across 17 subtopics. More from this topic

1. Designing and analysing an algorithm

   Explain how decomposition helps in the design of an algorithm.

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   Decomposition breaks a large problem into smaller sub-problems, each of which can be solved, tested and understood separately. It makes the design manageable, allows different sub-problems to be worked on independently or reused elsewhere, and means a fault can be located in one small part rather than across the whole solution.

2. Comparing algorithms and choosing one for a task

   Besides the number of operations, what else should you weigh when choosing an algorithm for a task?

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   How much memory it needs and how much is available, the size of the data set now and in the future, whether the data is already partly sorted, whether the data arrives all at once or item by item, whether equal items must keep their original order, and how complicated the algorithm is to implement correctly and maintain.

3. Queues

   Describe the algorithm for removing an item from a queue.

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   Check whether the queue is empty; if it is, report the error and stop. Otherwise read the item at the position given by the front pointer, move the front pointer on by one, decrease the count of items held, and return the item read.