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introduction
Introduction
Introduction

🔗Lecture on Udacity (39 min)

Computer Architecture

What is it?

Architecture - design a building that is well-suited for its purpose
Computer Architecture - design a computer that is well-suited for its purpose.

Why do we need it?

  1. Improve Performance (e.g. speed, battery life, size, weight, energy effficiency, ...)
  2. Improve Abilities (3D graphics, debugging support, security, ...)

Takeaway: Computer Architecture takes available hardware (fabrication, circuit designs) to create faster, lighter, cheaper, etc. computers.

Technology Trends

If you design given current technology/parts, you get an obsolete computer by the time the design is complete. Must take into account technological trends and anticipate future technology

Moore's Law

Used to predict technology trends. Every 18-24 months, you get 2x transistors for same chip area:
\( \Rightarrow \) Processor speed doubles
\( \Rightarrow \) Energy/Operation cut in half
\( \Rightarrow \) Memory capacity doubles

Memory Wall

Consequence of Moore's Law. IPS and Capacity double every 2 years. If Latency only improves 1.1x every two years, there is a gap between latency and speed/capacity, called the Memory Wall. Caches are used to fill in this gap.

Memory Wall Graph

Processor Speed, Cost, Power

Speed Cost Power
2x 1x 1x
OR 1.1x .5x .5x

Improvements may differ, it's not always speed. Which one of the above do you really want? It depends.

Power Consumption

Two kinds of power a processor consumes.

  1. Dynamic Power - consumed by activity in a circuit
  2. Static Power - consumed when powered on, but idle

Active Power

Can be computed by

$$P = \tfrac 12 *C*V^2*f*\alpha$$ Where $$\\C = \text{capacitance}\\P = \text{power supply voltage}\\f = \text{frequency} \\ \alpha = \text{activity factor (some percentage of transistors active each clock cycle)}$$

For example, if we cut size/capacitance in half but put two cores on a chip, the active power is relatively unchanged. To get power improvements, you really need to lower the voltage.

Static Power

The power it takes to maintain the circuits when not in use. Static Power Decreasing the voltage means the reduced electrical "pressure" on a transistor results in more static power usage. However, increasing the voltage means you consume more active power. There is a minimum at which total power (static + dynamic) occurs.

Fabrication Cost and Yield

Circuits are printed onto a wafer in silicon, divided up into small chips, and packaged. The packaged chip is tested and either discarded or shipped/sold. Larger chips could have more fallout due to probabilities of using larger areas of the wafer, and thus cost more.

$$ \text{fabrication yield} = \tfrac{\text{working chips}}{\text{chips on wafer}} $$

Fabrication Yield

Fabrication cost example: If a wafer costs $5000, and has approximately 10 defects, how much does each chip cost in the following example? Remember \( \text{chip cost} = \tfrac{\text{wafer cost}}{\text{fabrication yield} } \)

Size Chips/Wafer Yield Cost
Small 400 390 $12.80
Large 96 86 $58.20
Huge 20 11 $454.55

Benefit from Moore's Law in two ways:

  • Smaller \( \Rightarrow \) much lower cost
  • Same Area \( \Rightarrow \) faster for same cost

*[IPS]: Instructions per Second