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