1.1 System Architecture

This unit examines the most important part of a computer system, the CPU. It examines in more detail how modern CPUs actually follow instructions and how the performance of the CPU can be improved by tweaking the internal design of the actual CPU.

It also looks at how the CPU is internally organised and how different instructions move data around the CPU in different ways.

Embedded systems are an important use of CPUs that we often recognise as "smart" but not necessarily a computer system. This unit includes studying some of those embedded systems.

The CPU (Central Processing Unit) is the part of a computer system that is commonly referred to as the "brains" of a computer. The CPU is also known as the processor or microprocessor.

The CPU is responsible for executing a sequence of stored instructions called a program. This program will take inputs from an input device, process the input in some way and output the results to an output device.

CPUs aren’t only found in desktop or laptop computers, many electronic devices now rely on them for their operation. Mobile phones, DVD players and washing machines are examples of equipment that have a CPU.

The Function of the CPU

The function of the Central Processing Unit is to fetch, decode, and execute instructions.

The program code instructions are stored in the systems RAM. They are fetched, one at a time, and retrieved along the data bus on the computer. The Control Unit decodes the instruction and sends data and signals around the CPU that carry out (or execute) the instruction.

This cycle repeats constantly. Even when the computer system seems to be doing nothing, it is actually running through this cycle, checking the keyboard, refreshing the display etc.

1. A CPU running at a faster clock rate will work more quickly than a slower one. CPU chips are designed to work at a certain rate and if they run too quickly they can be damaged, but most CPUs are capable of some overclocking.

2. If a CPU is swapped for a similar one with more cores, then the system will usually work much more quickly. Doubling the number of cores could potentially double the speed of the computer, but the software must have been specially written to take advantage of multi-core processors.

3. CPUs with a large amount of cache will run the same code considerably more quickly than CPUs with no or little cache. This is because they can pre-load the code and data into the faster cache memory and re-use it rather than loading it from the slower RAM.

You will need to know these three factors for your exam, and you should also be able to explain the reason why improve the speed of a computer system.

The Clock

The computer system will continue to run through the FDE cycle all the time that the power is turned on. The CPU is very complex and there are often several things happening at once. To coordinate all of these different actions, an electronic clock makes everything run at the same time and in sync. The clock speed is measure in hertz. One hertz is one cycle per second - but the computer runs at gigahertz speeds - this is one billion cycles per second. It is difficult to image such fast speeds, in one billionth of a second, light only travels about 30 cm.

It is possible to change the clock speed, but this can only be done by a small amount. Increasing the clock speed is known as overclocking and most CPUs allow you to increase the clock speed by 10-20%. If you try to overclock the processor any more than that it may not work reliably, may run at an extreme temperature and will almost certainly void the manufacturers warranty. Temperature is the enemy of performance computers and they usually have a large and expensive cooling system to remove the heat as quickly as possible. The CPU will get very hot and may melt if the heatsink is removed.

To improve the performance of the CPU, the clock rate could be increased. This would make the whole fetch, decode, execute cycle faster. It is usually possible to use the same CPU when the clock rate is increased slightly, but the processor will generate more heat and may be unreliable if the clock rate is increased too much,

Number of Cores

Because computer systems are working at the very limits of what we understand to be the physical properties of computers, we need to look at other ways of making improvements in computer speed. The circuits inside the CPU are arranged on a grid that is often just 14 nanometres across. Human fingernails grow about one nanometre a second, so 14 nanometres is tiny. Electrical signals behave in unusual ways at such a small scale, they cannot go around sharp corners and jump across the gaps which makes their behavior erratic and unreliable.

The answer seems obvious! Instead of trying to make one CPU run through the fetch, decode, execute cycle at a faster and faster rate, we could just use two processors. This has recently led to some rapid development and a big boost in CPU performance. Reasonably priced CPU chips now have four or eight separate processors inside them, and are known as quad cored (4 CPUs) and octo-core (8 CPUs). High end servers take this even further and massively powerful super computers may have many hundreds of thousands of processors.

In 2018, the top ranked mobile phones use 8 core CPUs and some also have a graphics processor which may also be octo-core.

It is important to realise that using a CPU with more cores does not automatically make a computer run faster. The operating system and application software will need to be specially written to take advantage of the multi-core processor and not all tasks can be broken down into parallel tasks.

To improve the performance of the computer system, the CPU would be swapped for one that has more cores. This would mean that the computer system can carry out instructions at the same time, but only if the program had been written to make use of the extra cores. Modern operating systems can use many cores at once because they are written to make the maximum use of the CPUs features.

We often think of computer systems as the PCs, laptops, tablet computers and mobile phones which are all around us. There are actually many more computers than that! Embedded computer systems are built into larger appliances or devices such as washing machines, dishwashers and microwaves. These devices are not primarily intended to process data but perform another task instead.

The purpose of the embedded computer system is to monitor and control various components in the host device, for example, a washing machine is primarily intended to wash clothes, the embedded computer is there to help it keep track of washing programs, timings, water temperature and so on.

Many household devices are examples of embedded systems, but this does not mean that they all are. If it can be programmed or controlled with buttons that have a display then there is a good chance that a computer will be behind it and the device is an embedded system.