[Computer Architecture] Computing Industry

What has made computing pervasive? What is the backbone of the computing industry?

  • Computation: Although we had computation in calculators before, it is about programmability!
  • Communication: Although we had phones to do this, we can do more (social media, games, news…) and it is about networking!
  • What has made computing pervasive is its programmability and networking capability

What makes computers programmable?

Model of computation gives us programmability.

Von-Neumann Model: Notion of computer + Execution model

  • Computer is a collection of components
    • Memory (Random Access Memory: go anywhere in the memory and grab data)
    • Central Processing Unit
      • Control Unit: in charge of giving the instructions
      • Arithmetic Logical Unit (ALU): does the arithmetic logic operations on the data
    • Input/Output System: provides data from the outside
  • Memory that stores program and data
  • Program instructions execute sequentially (sequential execution paradigm)
    • Program counter (PC): keeps track of the current instruction

Programmability vs Efficiency

  • Pipeline: Instruction Fetch, Decode, Execute, Memory, Write Back
  • We do a lot of extra work to support programmability, and this lets us use economy of scale to reduce the price. As a result, commoditize computing!

* Specialized hardware is similar to ASIC. ASIC does one application, specialized hardware does more than one.

What is the difference between the computing industry and the paper towel industry?

  • Computing industry is an industry of new possibilities.

Can we continue being an industry of new possibilities?

  • Moore’s law
    • Doubling number of transistors every two years or 18 months
    • Build higher performance general-purpose processors
    • This itself is of no use; computer architecture offers abstraction and generalization to help programmers write codes.
  • Dennard scaling: provides the physics and equations on how shrinking device sizes shrinks voltage, current, current density, thus makes power density a constant
    • Transistor: two dimensional voltage controlled switch
    • Power = Capacitance x Frequency x Voltage^2

      Power Density = Power / Area
      • 0.7x dimension
      • 0.7x capacitance
      • 1.4x frequency
      • 0.7x voltage

What is the catch?

  • End of Dennard scaling
    • Pushing down threshold voltage exponentially increases the leakage current. So voltage cannot get under 0.9 volts.
    • Lower capacitance is impossible because we cannot reduce the distance
    • We have to stop scaling the frequency.
  • Dark Silicon
    • If you reduce the dimensions of transistors but not the power and reducing frequency does not compensate for the lack of power, you cannot turn all of them on.

Single-core era to multicore era

  • Two issues with the idea
    • Can we continuously increase the number of cores?
    • How will it be programmed?
  • It is a stopgap solution

Radical departures from conventional approaches are necessary

  • Bio computing, quantum computing
  • Specialization and co-design, approximate computing

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