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At the core of every computer is the processor, which executes instructions. There are two main processor architectures: CISC (Complex Instruction Set Computer) and RISC (Reduced Instruction Set Computer). These architectures determine how instructions are processed and executed, impacting performance and energy efficiency.
Understanding these differences is crucial in fields like software development and hardware design. Modern devices, from smartphones to supercomputers, rely on these architectures.
By the end of this lesson, you will understand how CISC and RISC processors function and their real-world applications.
Learning Objectives
Learning Objectives
By the end of this lesson, I can:
Define CISC and RISC architectures.
Compare the advantages and disadvantages of both architectures.
Identify real-world applications of each architecture.
Key Terms
Key Terms
CISC (Complex Instruction Set Computer) 🏗️ – Processors with a large set of complex instructions designed to complete tasks in fewer lines of code.
RISC (Reduced Instruction Set Computer) ⚡ – Processors with a smaller set of simpler instructions that execute in a single cycle.
Instruction Set 📜 – The collection of machine-level commands a processor can execute.
Pipelining 🚀 – A technique where multiple instruction phases (fetch, decode, execute) overlap to improve efficiency.
Clock Cycle ⏳ – A unit of time in which a processor completes an operation.
Key Ideas
Key Ideas
Computer processors follow two main architectures: CISC (Complex Instruction Set Computing) and RISC (Reduced Instruction Set Computing). CISC processors are designed to execute complex tasks in fewer instructions, reducing the workload on software but requiring more powerful hardware. They often use multiple clock cycles per instruction and have a variable instruction length. This makes them ideal for desktop computers and legacy systems. Intel x86 processors still use a modified version of CISC.
In contrast, RISC processors focus on simpler instructions that complete in a single clock cycle, allowing faster execution and better power efficiency. This makes RISC the dominant architecture for mobile devices, embedded systems, and ARM-based processors. RISC’s uniform instruction length also enables pipelining, where multiple instructions are processed simultaneously, significantly increasing performance. Modern smartphones, tablets, and even Apple’s M-series chips use RISC-based designs for efficiency.
CISC Processors
CISC Processors
Fewer lines of assembly code needed for complex tasks.
MULT instruction can perform multiple operations (load, multiply, store) in one step.
Common in Intel x86 processors (desktop and laptops).
More complex hardware but reduces compiler workload.
Uses more transistors, increasing power consumption.
RISC Processors
RISC Processors
Each instruction is simple and executes in one clock cycle.
Requires more lines of assembly code to complete a task.
Found in ARM processors (smartphones, tablets, embedded devices).
Easier to manufacture, more energy-efficient.
Supports pipelining, improving efficiency.
Comparison of CISC and RISC
Comparison of CISC and RISC
Comparing the RISC and CISC processes.
The diagram compares RISC and CISC architectures. RISC executes machine instructions directly, making it faster and more efficient. CISC, on the other hand, uses microcode conversion, breaking instructions into microinstructions before execution, which increases flexibility but can slow down performance because it will require multiple cycles to process a single instruction.
Tips & Mistakes to Avoid
Tips & Mistakes to Avoid
✅ Tip: RISC processors are used in most modern smartphones and embedded systems because of their energy efficiency.
🚫 Mistake: Thinking CISC is always faster than RISC. While CISC reduces instruction count, RISC executes instructions more efficiently with pipelining.
Video: CISC vs RISC Processors
Video: CISC vs RISC Processors
Overview of how CISC and RISC differ in instruction execution.
Examples of real-world applications.
Case Study
Case Study
ARM Processors in Smartphones
ARM Processors in Smartphones
Smartphones require efficient processing power while maintaining low energy consumption to maximize battery life. Unlike traditional desktop processors, which prioritize raw performance, ARM-based RISC processors are optimized for efficiency and power savings. This is why nearly all modern smartphones, including iPhones and Android devices, use ARM architecture.
ARM chips use simple, low-power instructions that execute in a single clock cycle, reducing heat generation and allowing devices to run longer on a single charge. Apple’s M1 and M2 chips—found in iPads and MacBooks—leverage RISC principles to achieve high performance while maintaining excellent energy efficiency. By focusing on low power consumption and efficient instruction execution, ARM processors have become the industry standard for mobile and embedded devices.
RED TASKS
RED TASKS
Use your g-slides workbook to complete the red tasks for this lesson. Everything you need is on the lesson webpage—only do online research if specifically told to.
Title Slide
Add a new section titled "L1 CISC vs. RISC Processors"
Choose an image/icon that represents processors or computing power
Key Terms & Definitions
Create a slide defining:
Instruction Set
CISC (Complex Instruction Set Computing)
RISC (Reduced Instruction Set Computing)
Pipelining
Clock Cycle
Keep definitions clear and easy to understand but with enough detail for revision.
Comparison Table
Make a comparison table with at least six key differences between CISC and RISC.
Include points like instruction complexity, speed, efficiency, and where they are used.
Keep your table neatly organized so it’s easy to revise from.
How CISC & RISC Execute Instructions
Review the video again around 2:16 and make two diagrams:
One showing how a CISC processor executes a complex instruction in one step.
One showing how a RISC processor breaks the same task into multiple simple steps.
Add a short explanation of pipelining and why it makes RISC more efficient.
Case Study: ARM in Smartphones
Explain why ARM processors use RISC architecture.
Focus on battery life, power efficiency, and performance benefits.
Use Apple’s M1/M2 chips vs. Intel processors as an example to show why ARM is dominant in mobile computing.
✅ Before you finish:
Check that your slides are clear, well-organized, and complete.
Make sure your answers come from the lesson webpage (unless asked to research).
Exam questions must have full answers in a clear, markable format.
Your diagrams and tables should be correctly labeled and easy to understand.
- Exam-Style Questions
Answer these two questions to your slides:
Explain two advantages of RISC over CISC. (4 marks)
Compare how multiplication executes in CISC and RISC processors. (6 marks)
- Real-World Reflection
Summarize where CISC and RISC are commonly used (e.g., desktops vs. mobile devices).
Answer this question in a full, structured response: If you were designing a processor for a gaming console, would you choose CISC or RISC? Explain your choice using performance, efficiency, and compatibility.
ACTIVE LEARNING
ACTIVE LEARNING
Pair Activity: Instruction Set Breakdown
Pair Activity: Instruction Set Breakdown
Each pair is given a simple task (e.g., adding two numbers, sorting a list) and must write a set of assembly-style instructions for both CISC and RISC approaches. Compare answers with another pair and discuss how the number of instructions and efficiency differ.
Small Group Activity: Processor Debate
Small Group Activity: Processor Debate
Each group is assigned either CISC or RISC and must prepare a short argument on why their processor type is superior. Consider factors like performance, power consumption, complexity, and real-world applications. After five minutes, groups debate their case with each other.
Large Group / Whole Class Activity: Processor Card Sorting Challenge
Large Group / Whole Class Activity: Processor Card Sorting Challenge
The class is split into two teams. Each team is given a set of statements or characteristics about CISC and RISC (printed or digital). Teams race to sort them into the correct category. The team that correctly sorts all cards first wins the challenge.
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