1.7 - Instruction Sets


An instruction set is a list of all the instructions that a CPU can process as part of the FDE cycle.

CPUs can have different sets of instructions that they can perform based on their function. The two most common instruction sets are the simpler RISC (Reduced Instruction Set Computer) and more complicated CISC (Complex Instruction Set Computer).




Clock Speed


RISC has fewer instructions than CISC and is, therefore, slower for carrying out complex commands but quick for basic tasks.


RISC is generally cheaper to mass produce because less circuitry is required for the smaller instruction set. 

RISC CPUs are designed to use less power and run without dedicated cooling systems (like fans) so that they can be used in devices like smartphones.

RISC CPUs run at lower clock speeds than CISC CPUs. They can perform more straightforward tasks more quickly than CISC but are generally not used to carry out complicated instructions.


CISC has more complex instructions available and can, therefore, perform complicated tasks.

CISC CPUs are generally more expensive because they require more circuitry to operate.


Because CISC CPUs require more circuitry, this means that they generate more heat and may need a fan. CISC CPUs, therefore, are commonly used in desktop computers.


CISC CPUs run at higher clock speeds
than RISC CPUs. They can perform
complex tasks more quickly than RISC.

Questo's Corner


a. What do RISC and CISC stand for?

b. Describe the major differences between RISC & CISC for each of the following features:​ Complexity, Cost, Power, Clock Speed.

c. Suggest why devices like a Kindle use RISC instead of CISC.

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