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Computer **performance **is a *traffic problem*, moving data and instructions from memory and around inside the chip. Most people think of "traffic" in terms of cars and highways. However, there is a more relevant * traffic analogy* that everyone experienced before they learned to drive.

## The Analogy

Students have been sitting in class for a long time. Finally the bell rings throughout the school signaling the end of the current period. Everyone gets up and moves through the hall to their next classroom. After a few minutes the bell rings again to signal the start of the next period. The bell has to ring everywhere in the school at the same time to coordinate movement. Without the bell, some classes would be released early and others would be released late.

The various parts of a computer hold instructions and data. Periodically they send this data along wires to the next processing station. To coordinate this activity, the computer provides a clock pulse. The clock is a regular pattern of alternating high and low voltages on a wire. To compare this with a clock in the hall, lets say the high voltage signal is a "tick" and the low voltage signal is a "tock". The clock speed is measured in millions per second (Megahertz) or billions per second (Gigahertz). A 100Mh PC mainboard has a clock which "ticks" and "tocks" 100 million times each second. Each tick-tock sequence is called a cycle. The clock pulse tells some circuits when to start sending data on the wires, while it tells other circuits when the data from the previous pulse should have already arrived.

A small point of notation: The standard clock speeds are some multiple of 33.3333... MHz. Three times this speed is 100 MHz. By convention, the speeds are rounded down to 33 and 66 MHz, but the fraction explains why three times a 33 MHz clock is 100 and not 99.

There are five ways to increase the processing power of a CPU or the teaching power of a High School.

Raise the clock speed - In the analogy, this corresponds to reducing the time available for each class period. If the teacher can talk faster, and if the students behave and listen more closely, this can work up to a point. Each student gets done with the school day earlier.

**Build a Pipeline** - A more complicated solution shortens the class period, but then breaks each subject into a sequence of steps. If it takes 45 minutes to cover *Algebra*, and that time cannot be reduced, then the subject could be covered in three consecutive 15 minute periods. A simpler subject might be covered in just one period. After all, there is no reason other than the convenience of scheduling why every every class for every subject lasts the same period of time. Students get done quicker, but only if some of the subjects are light weight.

**Parallelism **- Add more classrooms and more students. No one student learns anything faster, but at the end of the day the school has taught more people in the same amount of time. Of course, this only works if you have more students in the school district to teach.

**Class Size** - double the number of students in each classroom. High Schools don't like to do this. Computers, however, can easily switch from 32 to 64 bit operations. This will not effect most programs, but the particular applications that need processing power (games, multimedia) can be distributed in a 64 bit form to get more work done per operation.

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Tell me what is Hertz (Clock Speed) http://heelpbook.altervista.org/2013/tell-me-what-is-hertz-clock-speed/ – 12:30 – 19/05/2013 – #heelpbook @heelpbook