Structure and function of the processor (1.1.1)
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a) The Arithmetic and Logic Unit; ALU, Control Unit and Registers (ProgramCounter; PC, Accumulator; ACC, MemoryAddress Register; MAR, Memory Data Register; MDR, Current Instruction Register; CIR). Busses: data, address and control: How this relates to assembly language programs
b) The Fetch-Decode-Execute Cycle, including its effect on registers
c) The factors affecting the performance of the CPU: clock speed, number of cores
e) The use of pipelining in a processor to improve efficiency
f) Von Neumann, Harvard and contemporary processor architecture.
Approaches to teaching the content
This topic starts by exploring in detail the CPU/Processor at a working level. Students will learn about the various parts of the CPU and how they interact and function to process instructions and ‘compute’.
There are a selection of interactive sites which explain and provide animations for the various component parts of a processor.
This video demonstrates how the CPU works.
The Fetch-Decode-Execute Cycle is a model to demonstrate what the parts of the Processor are doing and the effect that this has on the internal memory registers. There are many videos showing this cycle in action which will support students’ understanding of the process: see Video 1 and Video 2.
Students need to understand the Fetch-Decode-Execute Cycle at the registry and component level. Little Man Computer is free software which models a simple von Neumann architecture computer; it has all of the basic features of a modern computer. Little Man Computer can be programmed in machine or assembly code, which will enable you to run your programs. This does require software to be installed first, although students may be familiar with the interface from GCSE: York University - Little Man Computer.
The topic covers the factors that affect CPU performance; students are required to identify what they are as well as explain the impact that each will have on performance. This can be achieved by comparing several PROCESSOR specifications and professional reviews of the performance. Benchmark testing is a useful tool to compare CPU performance across a range of brands and technologies. Students can then discuss and analyse, in relation to the factors, why one processor outperforms another. This resource allows students to compare and contrast a range of CPU benchmarks or look for a specific model.
Depending on what access rights students have to computers, they could predict the performance of a computer, based on the factors, and install the free software to benchmark test it.
A further technique that improves the efficiency of the processor is pipelining. This is a commonly used structure that improves the performance of a processor by using the output from one execution as the input for another. This approach is common in manufacturing and assembly lines. This site gives a good overview.
Pipelining is a technique used to speed up the fetch-decode-execute cycle; this is important when comparing the differences between and benefits of CISC and RISC in 1.1.2. Types of Processor.
Finally, this topic traces back to the original concepts of processor architecture developed by Von Neumann and Harvard. These contemporary processor architectures are also historical and many resources provide a detailed guide to how the architectures were developed and how they differ from each other: see Wikipedia - Harvard architecture, Princeton - Harvard architecture and E E Herald.
Students will gain an insight into how these architectures have developed into today’s modern-day processors, supporting discussions around future advancements.
Common misconceptions or difficulties students may have
Students who have studied GCSE Computing will have an understanding of the CPU and its function. Students may consider a CPU/Processor as three elements, the ALU, CU and Cache. Within these are more elements which function together to ‘process’ a set of instructions.
Conceptual links to other areas of the specification – useful ways to approach this topic to set students up for topics later in the course
This unit content will support elements 1.1.2 Types of Processor, where students explore types of processors, and 1.2.1 Systems Software, where students study Interrupts, the role of Interrupts and Interrupt Service Routines (ISR) and their role within the fetch-decode-execute cycle.
- Cut out the cards in Learner Resource 1 and place them in a box or bag.
- Students take it in turns to pull one out. Depending on resources, the students could use plasticine to model the word or a concept that represents the word.
- Students could draw the word instead or use actions to explain the word without talking.
- To add an element of competition, students could be split into groups and play against each other, for example two students have the same word and have to use the plasticine to create something that represents the word which enables their team to guess correctly.
- After each correct guess, the students should be asked how they guessed it; this can be an opportunity to ask further questions or clarify misunderstanding.
- Using the cards in Learner Resource 1, cut them up, give one to each student or give them all to a group of students. The task is to put them into the correct order of the FETCH actions and the EXECUTE actions. Students should justify why they have ordered the actions and explain what is happening.
- Make an animation of the Fetch-Decode-Execute Cycle. Learner Resource 2 shows a basic explanation.
- Use the sites shown below to support your knowledge and check that your animation is correct:
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