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Fixed length (RISC) vs variable length (CISC) instructions

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• Fixed length (RISC) instructions
  • Some computer CPUs use a fixed length instruction encoding format.

    That means:

    Every computer instruction is represented using the same number of bytes

    For example: ARM instructions are always 4 bytes long

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• Consequences of fixed length (RISC) instructions
  • Because each instruction is the same length, we know how many bytes we will need to increase to get the next instruction

  • But fixed sized instruction is very inflexible:

    The more complex an operation is, the more information we need to encode in a computer instruction Because the instruction length is fixed, the complexity of a (single) computer instruction will be limited

    So we often find that:

    Computer that uses fixed length computer instruction do not have complex computer instruction Such a computer is called Reduced Instruction Set Computer (RISC) because the complexity of its instruction set is reduced.

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  • Example of RISC computers that has fixed length instructions are:

    ARM processor --- each instruction is 4 bytes long SPARC processor --- each instruction is 4 bytes long

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• Variable length (CISC) instructions
  • Some computer CPUs use a "variable" length instruction encoding format.

    That means:

    Different computer instructions is represented using different number of bytes         There is basically no constraint on how many bytes is used in encoding the computer instruction (i.e.: use as many bytes you need)

    For example: INTEL (the CPU in your laptop) instructions are can have different number of bytes

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• Consequences of variable length (RISC) instructions
  • Because each instruction can have different length, we can encode simpler instruction using fewer number of bytes

  • Furthermore:

    Because there is no restriction on the number of bytes that we can use, we can encode very complex instructions

    So we often find that:

    Computer that uses variable length computer instruction usually have complex computer instruction Such a computer is called Complex Instruction Set Computer (CISC)

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  • Example of CISC computer that has variable length instructions are:

    The very successful INTEL processor (the CPU used in your laptop) The very elegantly designed but unsuccesful M68000 processor (we used to teach CS255 in M68000 assembler code)

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• Power (speed) of a processor and the complexity of its instruction set
  • You may think that CISC computer has an advantage over RISC because they can execute very complex instructions

    The power of a computer is not measured by the complexity of its instructions.

    The power (= how fast) a computer rather depends on the circuitry used to execute the instructions.

    Specifically: how many steps of the execution of different instructions can you overlap with each other.

    If you can overlap the execution of multiple instructions (i.e.: multi-tasking), then you can do things faster

    Multi-asking (or pipelining) is a topic in CS355 - so if you want to learn more, you will need to take that course....