- Generally we are interested in questions involving computer speed.
- What are the units of speed in a computer?

- We will look at different ways you could possibly build a computer.
- We will want to be able to discuss performance of different options so we can compare.
- To do this, we must define what performance is.
- We will discuss performance in some detail later, but for now we will define it to be time spent on a program.

- The basic components of time in a program.
- The clock
- The number of instructions in the program.
- The number of cycles per instruction (CPI)

- The clock
- Discussed on page 33 of the book.
- Discussed in appendix B.7
- This is the driver of the CPU
- It is an electronic signal that changes between a low signal and a high signal.
- This could be a sin/cos wave, or probably a shaped.

- A clock cycle
- Is one iteration of the clock.
- It generally has a rising edge, where most logic takes place.
- But we can shape the clock if we wish, more on that later.
- (From alanclements.org)

- The
**clock frequency**is the amount of cycles run in a given time period.- Normally this is cycles per second.
- The unit for this is Hertz (Hz).
- A 1 Hz Clock has a frequency of one cycle per second.
- $\frac{\text{cycles}}{\text{second}}$
- What are typical clock rates?

- Clock cycle time
- This is $\frac{\text{seconds}}{\text{cycle}}$
- My windows machine claims to run at 2.6GHz, how long is each cycle?
- $2.6GHz = \frac{2.6\times10^{9}\text{cycles}}{1\text{second}}$.
- That is the same as $\frac{1\text{second}}{2.6\times10^{9}\text{cycles}}$
- Or $\frac{1}{2.6} \times 10^{-9} \frac{\text{seconds}}{\text{cycle}}$
- or $0.38 \times 10^{-9} \frac{\text{seconds}}{\text{cycle}}$
- So a cycle is $0.38 \times 10^{-9} \text{seconds}$.
- So a cycle is $3.8 \times 10^{-10} \text{seconds}$.
- This doesn't seem like a good answer, let's change units.
- What unit do I want? (nano seconds or pico seconds?) (pico)
- $ 1 \text{second} = 1 \times 10^{12} \text{pico seconds}$
- $3.8 \times 10^{-10} \text{seconds} \times \frac{1 \times 10^{12} \text{pico seconds}}{1 \text{seconds}}$
- $3.8 \times 10^{\cancel{-10} 1} \cancel{\text{seconds}} \times \frac{1 \times 10^{\cancel{12} 2} \text{pico seconds}}{1 \cancel{\text{seconds}}}$
*That 12 should be canceled* - Or $3.8 \times 10^2$ps
- Or 380 ps/cycle.

- Why is this important?
- this is, in theory, how long it will take to execute a single instruction!

- The number of instructions per program.
- These are machine/assembly instructions.
- We will be able to tell the difference later, but for now you can think of them as semi-equivalent.
- Right now we will consider all instructions of equal weight.
- We will discover that this is not the case later.
- We will discuss this in more detail later.

- The final part of the mix is CPI or IPC .
- Cycles per instruction.
- Ideally we would like to be able to execute an instruction every cycle.
- But we might not be able to do this.