WTF Are My Huskers Doing?
- Thread starter HuskerHackFraud
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- Fixed length (RISC) instructions
- Some computer CPUs use a fixed lengthinstruction encoding format.
That means:
- Every computer instruction is represented using the same number of bytes
- For example: ARM instructions are always 4 bytes long
- Some computer CPUs use a fixed lengthinstruction encoding format.
- 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 encodein a computer instruction
- Because the instruction length is fixed, the complexity of a (single) computer instruction will be limited
- The more complex an operation is, the more information we need to encodein a computer instruction
- 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.
- Computer that uses fixed length computer instruction do not have complex computer instruction
- Example of RISC computers that has fixed lengthinstructions are:
- ARM processor --- each instruction is 4 bytes long
- SPARC processor --- each instruction is 4 bytes long
- Because each instruction is the same length, we know how many bytes we will need to increase to get the next instruction
- Variable length (CISC) instructions
- Some computer CPUs use a "variable" lengthinstruction 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)
- Different computer instructions is represented using different number of bytes
- For example: INTEL (the CPU in your laptop) instructions are can have different number of bytes
- Some computer CPUs use a "variable" lengthinstruction encoding format.
- Consequences of fixed 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)
- Computer that uses variable length computer instruction usually have complex computer instruction
- Example of CISC computer that has variable lengthinstructions 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)
- Because each instruction can have different length, we can encode simpler instruction using fewer number of bytes
- 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....
- You may think that CISC computer has an advantage over RISC because they can execute very complex instructions