---

The RISC philosophy of CPU design

---

• Historical development
  • Before (around) 1980:

    Computers are all Complex Instruction Set Computers (CISC)

    ---

  • CISC:

    CISC computers have complex instructions that perform a large amount of work per instruction

    ---

  • You have learned to program a CISC machine in CS255: M68000

    Example of a complex instruction:

    add.l 10(A0, D1.w), 20(A2, D3.w)

    An example of a simple instruction is:

    add,l D0, D1 // why "simple" ? Because even the CPU that // we made can execute this instruction!

  ---

  ---

  ---

  ---

• Reason for complex instructions
  • Main reason:

    Before around 1980, the computer memory were made up with magnetic rings !!! Magnetic core memory are very slow It take a long time to fetch something from memory.... What will you do in such a situation ???

    ---

    ---

  • Analogy:

    You live in Macon, Georgia and the only place you can buy food is in Atlanta It's an one-hour trip... What will you do ??? You will buy a lot of food on 1 single trip !!!

    The analogy in the computer is:

    Provide complex instructions that perform a lot of work

  ---

  ---

  ---

  ---

• Solid memory and the change to RISC
  • After around 1980:

    Solid state memory were invented Solid state memory is relatively much faster than megnetic core memory

    ---

  • With the advent of faster memory (like living in Alpharetta instead of Macon):

    There was less urgent need to fetch complex instructions

    because we could "visit" the memory in shorter time

    ---

    ---

  • Fact:

    In order to execute a complex instruction, the CPU must have more complex circuitry When a CPU do not support complex instructions, the silicon used for the complex circuitry can be used for other purpose Some CPUs use the freed silicon for on-chip cache (MIPS) Other CPUs use the freed silicon for window register (SPARC)

    ---

    ---

  • By reducing the complexity of the instructions, the instructions become more homogeneous (regular):

    Both the MIPS and the SPARC CPU use instructions that are fixed length (CISC machines use instructions of variable length)

    These computers with reduced complexity in instructions are called:

    Reduced Instruction Set Computers (RISC)

  ---

  ---

  ---

  ---

• RISC and pipelined execution
  • The designers of the RISC computers (Hennessey and Patterson) incorporarted:

    Pipelined instruction execution in the RISC computer architecture See: click here