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Knowledge/A Level/H446/01 Computer systems/Types of processor/The differences between and uses of CISC…

The differences between and uses of CISC and RISC processors

1.1.2 Types of processor

Practise exam questions

Types of processor — Note 1 of 3

Candidates need to understand the differences between the CISC and RISC processors and the key features and benefits of each. They should be aware of the relative benefits of each architecture

What you need to know

CISC (Complex Instruction Set Computer) and RISC (Reduced Instruction Set Computer) are two approaches to designing a processor's instruction set. They trade off the complexity of the hardware against the complexity of the compiled code.

What to know

Key terms

Exam tipCommon mistake

CISC

A processor with a large set of complex instructions, many of which take several clock cycles. A single instruction can carry out a multi-step operation.

  • shorter programs (fewer instructions), so the compiler's job is simpler and less RAM is needed for code.
  • more complex circuitry, so it is more expensive, uses more power and generates more heat.
  • Uses: CISC is common in desktop and laptop processors (e.g. x86). RISC is common in mobile phones, tablets and embedded systems (e.g. ARM) where battery life and heat matter.
RISC

A processor with a small set of simple instructions, most of which complete in one clock cycle.

  • simpler hardware, so cheaper, lower power and less heat; instructions of a fixed length and similar duration make pipelining easier.
  • more instructions are needed for complex tasks, so programs are longer and the compiler does more work.
  • Exam tip: When recommending one for a scenario, link your choice to the scenario, e.g. 'a smartwatch should use RISC because lower power consumption extends battery life'.
  • Common mistake: Saying RISC is always faster. Each instruction is faster, but more instructions may be needed for the same task.

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Practise 1.1.2 questions
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Back to Types of processor

Types of processor

Note 1 of 3

  1. 1The differences between and uses of CISC and RISC processors
  2. 2GPUs and their uses (including those not related to graphics)
  3. 3Multicore and parallel systems
Topic overview