1 The characteristics of contemporary processors, input, output and storage devices Now test yourself 4 Give an example of when the contents of the ACC might be changed. 5 State the name of the bus that carries the contents of memory to the MDR. 6 Explain why the PC is incremented by 1 in each cycle. Answers on p. 209 The factors affecting the performance of the CPU ✚ clock speed ✚ number of cores ✚ amount of cache memory ✚ use of pipelining.

Clock speed

Processors work at incredible speeds, dictated by the clock signal. The speed of this signal, known as the clock speed, is measured in hertz (Hz). Unit 1 Hertz (Hz) 1 Kilohertz (kHz) 1 Megahertz (MHz) 1 Gigahertz (GHz) Pulses per second 1 1 000 1 000 000 1 000 000 000 Modern desktop processors tend run in the order of Gigahertz. The processor on the computer being used to write this chapter runs at a speed of 2.8 GHz. That means its processor has a clock producing 2.8 billion pulses per second.

Number of cores

A core is a processing unit within the CPU. Modern CPUs have multiple cores that can operate independently.

✚ Each core is a distinct processing unit on the CPU that can run independently. ✚ When multitasking, different cores can run different applications. ✚ It is also possible for multiple cores to work on the same problem.

✚ The more cores you have, the easier it is to run more things simultaneously. More cores also mean that tasks that can have their work shared will run more quickly. Cache Random access memory’s (RAM’s) access speed is significantly slower than the speed at which the CPU operates. To compensate for this, processors have a small amount of fast memory called cache memory: ✚ Cache memory is a temporary store.

✚ Cache memory is built into the processor itself, reducing the distance data has to travel to it. Data and instructions that are likely to be regularly accessed are kept in cache memory, ready for fast access.

✚ By allowing the processor to access cache, the overall speed at which it operates is less likely to be limited by RAM’s access speed.

✚ Because cache memory is faster when it is smaller, different levels of cache are used.

✚ Level one cache is the smallest and built directly into the CPU. Often each core will have its own level one cache. 10

✚ Each subsequent cache level is larger and further away from the heart of the CPU and is therefore slower to access.

✚ Modern CPUs tend to have three or four levels of cache. Core A core is a processing unit within the CPU.

Cache memory

A fast memory that is built on or close to the CPU and is designed to reduce the need to access RAM, which has slower access speeds. Making links Having a cache is a common concept in computing, and isn’t just used in the CPU. Web browsers cache files on your computer to avoid them having to be repeatedly downloaded.