12.3.2.6 explain how the clock speed, word length, and bus width affect the performance  

Computer performance

There are several things we can try to improve the performance of your computer:

• Increasing the clock speed
• Adjusting word length
• Increasing bus widths

Clock speed (Clock rate)

Clock speed - the number of cycles that are performed by the CPU per second.

The most obvious way to increase the speed of your computer is to increase the speed of your computer clock. At a higher clock speed, the CPU will be forced to execute more instructions per second.

For example, if we compare two identical processor models, which differ only in clock speed. One operates at 3.4 GHz and the other operates at 2.6 GHz. In this case, the 3.4 GHz processor will run 30% faster when both are running at maximum speed.

Word length (Word size)

Word length - the number of bits of information that the CPU can process at a time.

Another way to improve computer performance is to increase the word size. This means an increase in the number of bits that the computer can handle at one time.

The longer the word, the more accurate calculations can be done by computers and more complex things. Modern computers are mostly in word sizes: 32 bits or 64 bits.

For example, on a 32-bit machine, pointers can address ~ 4 GB of RAM, whereas, on a 64-bit machine, they can address over 16.7 million TB. If more than 4 GB is to be addressed on a 32-bit system, there will be a lot of extra clock cycles to fetch and parse these wider numbers, whereas these operations are much faster on a processor that can handle it all in one word.

Bus width (A bus size)

Bus width - the number of bits of information a bus can carry at one time.

For example, imagine that you have a processor capable of understanding 32-bit words in one go. But what happens if the bus sending words from memory to the processor is only 8 bits wide? We will have the neck of the bottle. This will require sending four blocks of data over the data bus before we get a word for the processor to execute. In other words, to increase performance, we also need to increase the bus size to avoid bottlenecks.

Additionally

Also, computer performance is affected by

• number of cores
• cache size

Cores

A CPU consists of multiple processing units. Each processing unit is known as the core. A core consists of a processor (ALU and CU) and registers. Computers may be dual (two), quad (four), or even more cores. 
As the number of cores increases, the computer has more power to execute multiple programs at a particular instant of time. 

When the number of cores is increased, the communication channels between the cores also use up some of the speed. Hence, increasing the number of cores four times, will not increase the speed of the computer four times.

Cache

A cache is a small memory part located closer to the CPU when compared to RAM. 
Cache temporarily holds the data and instructions that the CPU is likely to use more frequently. 
To reduce the access time, the control unit checks the cache first before requesting any instruction from RAM or main memory. 

Accessing cache memory
Due to the presence of cache, the speed of the CPU increases. 
As the cache is near to CPU, it is faster to use a cache when compared to RAM. 

Questions:

1. Explain terms "clock speed", "word length" and "bus width".
2. Why does increasing the number of cores does not increase the speed of the CPU?
3. How does clock speed affect the performance of the CPU?
4. What is cache? How does it reduce the access time for data requests?
5. What are the different levels of cache? Specify the location, size, and access time for each of them.
    

Exercises:

Ex. 1 Fill in the blanks

Ex. 2 Crossword

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Ex. 3 Compare two computer systems
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