Embedded Files
Students should have studied the following:
d) The function of the CPU as fetch and execute instructions stored in memory
e) How common characteristics of CPU's affect their performance:
The Function of the 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.
The Fetch Decode Execute Cycle Simplified.
The Fetch Decode Execute Cycle Simplified.
The Fetch Cycle
The Fetch Cycle
- The PC value is copied to the MAR
- PC is incremented
- The Instruction at the MAR address is read from RAM
The Decode Cycle
The Decode Cycle
- The instruction in the MAR is decoded by the CU
The Execute Cycle
The Execute Cycle
- The data at the MAR address is read from RAM
- The ALU performs any calculations required on the ACC
- The value in the MDR is written to the address in the MAR.
The Clock
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.
Number of Cores
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.
Cache Memory
Cache Memory
Another physical restriction on computer speed is that the RAM chips can only work at a certain level of performance. This means that the fetch, decode, execute cycle is slowed down by the fetch part. Adding more cores will actually lead to an even bigger drop in performance, because all of the cores will be trying to fetch instructions from the RAM at the same time.
This can be improved by using a smaller area of super high performance memory which is actually inside the CPU chip. This is known as cache memory. Once instructions and data are loaded into the cache, they can be used by the CPU at a far higher speed, which has a huge impact on the CPU performance.
This is a greatly magnified view of the inside of a CPU. The Cache memory can be seen on the right hand side, and looks quite similar to the RAM memory used as primary storage.
The Factors Affecting the Performance of a CPU
The Factors Affecting the Performance of a CPU
- 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.
- 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.
- 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.
Embedded Systems
Embedded Systems
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.
Embedded systems have software that controls the hardware connected to the system. This is a special kind of software known as firmware. The firmware can be updated, but this does not happen very often. Firmware is usually very basic but also very reliable.
In the above examples, all of the systems on the left are unlikely to receive a firmware update. The devices on the right do not have firmware because they do not have embedded systems.
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