COSC 4349 and 5349 Computer Architecture

1. COSC 4349 and 5349Computer Architecture Fundamentals of Computer Design (Part 2) Dr. Lappoon R. Tang

2. Overview • How improvements in technology can affect possible design choices • Practical limitations brought about by improvements in technology and how they also affect design choices

3. Readings • Section 1.4 to Section 1.5 • Skim over the Math in Section 1.5: important thing is to understand the essence or the concepts, not so much the details

4. Changes in technology affects design New Technology New Processor Functionalities Design Choices of Processor Architecture

5. Four critical implementation technologies • Integrated circuit logic technology • Increase in transistor density allows an increase transistor count on a chip, which improves CPU speed and enables new functionalities • Semiconductor DRAM • RAM capacity continues to increase by about 40% per year • Magnetic disk technology • Cost per bit stored in the disk continues to drop while access time keeps improving – it can affect design of storage system • Network technology

6. MOS technology quantum leap: A case study on how technology can affect design decisions • MOS technology in the early 80s reached a point that allowed 25k to 50k transistors to be fit on a single chip • That amount of transistors finally made it possible for one to build a 32 bit CPU on a single chip • Effect: Dramatic speed up in CPU due to elimination of need to pass information across multiple chips • New design possible: A CPU with onboard cache (perhaps even several levels of CPU cache)

7. Performance trends: Bandwidth over Latency • Bandwidth – total amount of work done in a given time. • Latency – time between start and completion of an event. • Observation: Bandwidth improves much more rapidly than latency. • Conclusion: That improvement in bandwidth completely outpaced latency can • affect design choices

8. Scaling of transistor performance • Feature size – minimum size of a transistor in either the x or y dimension; the smaller the feature size, the better is the performance of a transistor • Transistor performance improves linearly with decreasing feature size • However, transistor count per chip improves quadratically with a linear improvement in transistor performance • For a linear amount of downsizing in transistor size, one can actually pack more than a linear increase in the amount of transistors per chip! • Implications: Density improvement of transistor per chip allows introduction of 64 bit architecture and innovations in pipelining and cache design

9. Trends in power in integrated circuit • The general rule of thumb is that power consumption by a chip is proportional to • Square of voltage • and some other factors related to transistors • One drops power consumption mainly by reducing voltage of a chip

10. Trends in power in integrated circuit • A 3.2 GHz Pentium 4 consumes 135W • The amount of heat generated by a P4 at this clock rate has basically stretched the limit of heat dissipation naturally allowed • Solution and challenge: • Distributing power across the chip • Turning of the clock of inactive units (e.g. floating point unit) • Removing heat from the chip

11. Trends in power in integrated circuit • Even when a transistor is off, leakage current still flows • Increasing the number of transistors increases power even if they are turned off and leakage current increases in processors with smaller transistor sizes • In 2006, the goal for leakage is 25% of the total power consumption

12. Trends in power in integrated circuit • Limitation in cooling of processor has led to use of multiple processors on a chip running at lower voltage and clock rates