Cambridge IGCSE Computer Science Past Paper PDF

Head to www.savemyexams.com for more awesome resources Cambridge (CIE) IGCSE Your notes Computer Science Computer Architecture Contents The CPU & Microprocessors Von Neumann Architecture The Fetch-Decode-Execute C...

Head to www.savemyexams.com for more awesome resources Cambridge (CIE) IGCSE Your notes Computer Science Computer Architecture Contents The CPU & Microprocessors Von Neumann Architecture The Fetch-Decode-Execute Cycle Characteristics of the CPU CPU Instruction Sets Embedded Systems Page 1 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The CPU & Microprocessors Your notes The CPU & Microprocessors What is the purpose of the CPU? The purpose of the Central Processing Unit (CPU) is to fetch, decode and execute instructions The CPU is the brain of the computer and its job is to take an input, process data and produce an output It is central to all devices including: Laptops Desktops Games-Consoles Mobile Devices The words CPU and Microprocessor are interchangeable and can be used to mean the same thing A microprocessor is a type of integrated circuit on a single chip Data and commands are inputted by the user using an input device, the central processing unit (CPU) processes data by executing instructions and the results are outputted to an output device Below is an example of data being inputted, processed and the results being outputted Step Example Input A keyboard is used to input a number Process If the instruction being executed is ADD, the inputted value is added to an existing value Output The result of the calculation is outputted to the user via the monitor Page 2 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources A diagram showing the input, process, output sequence followed by computer systems Your notes Page 3 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Von Neumann Architecture Your notes Von Neumann Architecture What is the Von Neumann architecture? The Von Neumann Architecture is a design of the CPU which was proposed by Mathematician John Von Neumann in the 1940s, which most general-purpose computers are built upon The Von Neumann Architecture outlines how the computer memory, input/output devices and processor all work together The Von-Neumann architecture The Von Neumann architecture consists of: Control unit (CU) Arithmetic logic unit (ALU) Registers Buses Page 4 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources What is the function of each component? Arithmetic logic unit (ALU) Your notes Performs arithmetic operations Performs logical decisions IF X > 5 THEN DO ………. Control unit (CU) Coordinates how data moves around the CPU by sending a signal to control the movement of the data Decodes the instructions fetched from memory Registers Extremely small, extremely fast memory located in the CPU Hold small amounts of data needed as part of the fetch-execute cycle Each register has its own specific purpose It consists of 5 main registers The Program Counter (PC) The Memory Address Register (MAR) The Memory Data Register (MDR) The Accumulator (ACC) Current Instruction Register (CIR) For each of the registers you must know The name of the register Its acronym The purpose of the register Name Acronym Purpose Program Counter PC Holds the memory address of the next instructions to be executed Page 5 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Increments by 1 as the fetch-decode-execute cycle runs Your notes Memory Address MAR Holds the memory address of where data or instructions are Register to be fetched from memory Memory Data Register MDR Stores the data or instruction which has been fetched from memory Current Instruction CIR Stores the instruction the CPU is currently decoding or Register executing Accumulator ACC Stores the results of any calculations that have taken place in the Arithmetic Logic Unit (ALU) Buses Components within the CPU and wider computer system are connected by buses A bus is a set of parallel wires through which data/signals are transmitted from one component to another There are 3 types of bus: Address - unidirectional, carries location data (addresses), data is written to/read from Data - bidirectional, carries data or instructions Control - bidirectional, carries commands and control signals to tell components when they should be receiving reads or writes etc.. Page 6 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Examiner Tips and Tricks If asked to describe the purpose of the PC, MAR or the MDR make sure you explain how the data is being fetched or written to memory Worked Example Describe the role of the control unit, the control bus, the data bus and the address bus when fetching an instruction from memory Answer The address of memory (holding instruction) is placed on the address bus (1) The control unit sends a signal (1) on the control bus (to start a read operation) (1) The instruction is/the contents of the memory are placed on the data bus (1) Page 7 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The Fetch-Decode-Execute Cycle Your notes Fetch-Decode-Execute Cycle (FDE) What is the purpose of the CPU? The purpose of the Central Processing Unit (CPU) is to fetch, decode and execute instructions The CPU is the brain of the computer and its job is to take an input, process data and produce an output What is the Fetch-Decode-Execute cycle? The Fetch-Decode-Execute Cycle is the cycle that the central processing unit (CPU) runs through billions of times per second to make a computer work A computer takes an input, processes the input and then delivers an output for the user Input: Clicking a button on the gamepad Process: The CPU inside the console follows a set of instructions to carry out the task Output: The player moving on screen Page 8 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The Fetch-Decode-Execute cycle stages Fetch stage Your notes During the fetch stage of the cycle, the program counter holds the address of the next instruction to be fetched from memory The address of the next instruction or data to be fetched is copied into the memory address register (MAR) The address of the instruction or data is then sent along the address bus and awaits a signal from the control bus The signal sent along the control bus is sent from the control unit (CU) to the main memory The data or instructions received from main memory is fetched to the memory data register (MDR) via the data bus A copy of the instruction or data is stored in the current instruction register (CIR) The program counter (PC) increments by 1 so it is pointing to the next instruction to be executed Decode stage During the decode stage of the cycle, the CPU needs to work out what is required from the instruction This is done as the instruction is split into two parts: Opcode - what the instruction is Operand - what to do it to This could be either data or an address where the data is stored Execute stage During the execute stage of the cycle, the CPU will carry out the instruction that was fetched Some examples that would take place at this stage are Performing a calculation Storing a result or data back in main memory (RAM) Going to main memory to fetch data from a different location The important things to remember are: An instruction or data is fetched from memory Page 9 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources The instruction is decoded The instruction is executed Your notes The cycle repeats billions of times per second Examiner Tips and Tricks Make sure you read the question carefully and look at the number of marks allocated to judge the level of detail required. Often questions on the fetch-decode-execute cycle only require you to describe the steps rather than explain how the registers and buses are used during each step Worked Example Explain how an instruction is fetched using Von Neumann architecture Answer The Program Counter (PC) holds the address/location of the next instruction to be fetched The address held in the PC is sent to the Memory Address Register (MAR) The memory address is sent using the address bus The Program Counter is incremented The instruction is sent from the address in memory to the Memory Data Register (MDR) The instruction is transferred using the data bus The instruction is sent to the Current Instruction Register (CIR) Page 10 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Characteristics of the CPU Your notes Characteristics of the CPU What are the common characteristics of the CPU? There are 3 common characteristics Clock Speed Cache Size Number of Cores Each of these characteristics has a significant impact on the performance of the CPU How do the characteristics of the CPU affect performance? Clock speed The clock speed is measured in Hertz (Hz) The clock speed measures the number of fetch-decode-execute cycles that can take place in 1 second The faster the clock speed, the more instructions can be fetched and executed per second Page 11 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Modern computers have a clock speed in Gigahertz (GHz), meaning billion A clock speed of 3.5GHz can perform up to 3.5 billion instructions per second Your notes Cache size Cache is very small, very fast memory on or close to the CPU Cache is used as temporary storage to provide quick access to a copy of frequently used instructions and data The larger the cache size, the more frequently used instructions or data can be stored This results in the CPU having to complete fewer fetch cycles from memory (RAM), speeding up the performance Cache also has a significantly faster read/write speed than RAM, making it much quicker to retrieve instructions from there instead of from memory (RAM) Number of cores A core works like it is its own CPU Multiple core processors mean they have multiple separate processing units that can fetch, decode and execute instructions at the same time For example, a dual-core processor would have 2 processing units, each with their own Control Unit (CU) Arithmetic Logic Unit (ALU) Accumulator (ACC) Registers Multi-core processors can run more powerful programs with greater ease Multiple cores increase the performance of the CPU by working with the clock speed Example: A quad-core CPU (4 cores), running at a clock speed of 3Ghz 4 cores x 3GHz 4 x 3 billion instructions 12 billion instructions per second Page 12 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes Worked Example One computer has a single core processor and the other has a dual core processor. Explain why having a dual core processor might improve the performance of the computer Answer Any 2 from: The computer with the dual core processor has two cores/double the amount of cores Parallel processing can take place Each core can execute a separate instruction at the same time Each core can process instructions independently of each other Page 13 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources CPU Instruction Sets Your notes CPU Instruction Sets What is an Instruction set? An instruction set is a list of all the commands that can be processed by a CPU Each command has a binary code which is called machine code The binary code is made up of an operation code (opcode) and an operand The opcode is the operation to be performed The operand is the location where the operation is to be performed in The table below shows an example instruction set Each instruction has a mnemonic that indicates what the instruction does alongside an example binary code After an instruction is decoded into an opcode and an operand, the CPU finds the opcode in the processor’s instruction set It then knows what operation to perform when executing the instruction Instruction Mnemonic Binary code Command Add ADD 10100001 Adds a value to the value currently stored in the accumulator (ACC) Subtract SUB 00100010 Subtract a value from the values stored in the accumulator Load LDA 10111111 Load the value stored in a memory location into the accumulator Store STA 01100000 Store the value in the accumulator in a specific location in memory Stop HLT 00000000 Stop the program Instruction lists are machine-specific Page 14 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources A program created using one computer’s instruction set would not run on a computer containing a processor made by a different manufacturer Your notes For example, a computer program created using Intel’s instruction set would not run on a device containing an ARM processor Worked Example Using the instruction set in the table above what would be the operation if the instruction was 00100010 00000010? Answer Either of: The operation would be SUB If the operand was raw data the complete instruction would be to subtract 2 from the value in the accumulator Page 15 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Embedded Systems Your notes Embedded Systems What is an embedded system? An embedded system is a computer system which is used to perform a dedicated function, inside a larger mechanical unit Examples of embedded systems include Heating thermostats Hospital equipment Washing machines Dishwashers Coffee machines Satellite navigation systems Factory equipment Security systems Traffic lights Page 16 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Your notes What are the properties of an embedded system? They are small in size They use less power than a general-purpose computer They have a lower cost Examiner Tips and Tricks Always use key examples from the list above and don’t try to use different examples such as a fridge or kettle as these will not appear on mark schemes because although they have a single purpose, most fridges and kettles do not have a CPU. Worked Example 1) Tick two boxes below to show which are an example of an embedded system Page 17 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers  Head to www.savemyexams.com for more awesome resources Is it an example of an embedded system Laptop Your notes Washing Machine Mobile Phone Car Engine Management System 2) Justify your choice to question 1 Answers 1) Tick two boxes below to show which are an example of an embedded system. Is it an example of an embedded system Laptop Washing Machine ✓ Mobile Phone Car Engine Management System ✓ 2) Justify your choice to question 1 Any two of: A washing machine and car engine management system are not general-purpose computers A washing machine and car engine management system have a single purpose and are both housed inside a larger mechanical unit A washing machine and car engine management system have a microprocessor Page 18 of 18 © 2015-2025 Save My Exams, Ltd. · Revision Notes, Topic Questions, Past Papers