RISC & CISC

Processors come in a number of different types which can be confusing. However, dividing them into two main categories of RISC (Reduced Instruction Set Computer) and CISC (Complex Instruction Set Computer) can help us to understand how different machines are able to process instructions.

Before reading into RISC & CISC processor types, it’s useful to familiarise yourself with Computer Architectures first.

CPU Processor

An instruction set allows the processor to decode the binary instructions sent into the computer. Because different CPUs read instructions in different ways, the instruction set acts like a look up table matching each binary bit pattern with an instruction.

RISC – Reduced Instruction Set Computers

RISC processors use a set of instructions that are more simplistic that CISC, and because of this it can take several cycles to complete a task. Whilst both sets will complete the same tasks, a RISC architecture will reduce the tasks into smaller, more simple instructions (much like decomposition in programming!)

CISC – Complex Instruction Set Computers

CISC archtitectures allow for more complex instructions, like loading data & adding at the same time, whereas RISC would process this as two separate instructions.

Comparing RISC & CISC

RISCCISC
Simple InstructionsComplex Instructions
Small number of instructionsMany instructions
Instructions take 1 cycleInstructions can take multiple cycle
Software handles processing loadHardware circuits handle processing load
Heavy use of RAMEfficient use of RAM

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