What is CPU?

This is a beginners guide to CPU's. Please use the articles menu if you require more in depth articles. The CPU is often called the main processor of your PC. CPU stands for Central Processing Unit. When you are buying a PC from a high street store the main selling feature is always the speed of the CPU. You will notice it will say 2.4Ghz or 3Ghz PC. The truth is that the CPU is set at these speeds and has no relevance on what else you could have inside your PC.

What does the CPU look like?

The CPU will likely be the larger of the chips on the mainboard inside your PC. If you have bought the PC from new the CPU will be covered by a heatsink and fan. The heatsink and fan are extremely important. Without these the CPU would get too hot to work and possibly melt or burn out. Be very careful to replace the fan correctly if you remove it to have a look at your CPU.

What does the CPU do?

The CPU is the main processor of your PC. Everything that goes on in your PC at some point goes through your CPU. In reference to the human body the CPU is brain of the PC. It is artificial thinking but the CPU is where all the logic is applied. As a very basic example computer code is basically mathematics. if you wanted to calculate 2+7 you would need an input an output and a processor to add the logic. The logic in this case is simple addition. You would input 2+7 on a keyboard. This would be registered and sent to the CPU for analysis. The CPU would see that the addition logic is required and use this inbuilt logic to send the answer of 9. That of course is a simple instruction but the idea for all processors is the same. Input > Apply Logic > Output.

How do you measure the speed of a CPU?

The CPU's speed is a measure of MHz (megahertz) or more recently GHz (gigahertz). a chip with a megahertz rating of 900Mhz would be able to complete 900Million cycles every second. However don't be fully deceived by this figure alone. This figure only shows how many clock cycles the CPU can do in a second. How much being done in each cycle is another matter. I urge you to check out some benchmarks on CPU's before you you decide that the one with a faster rating has the best performance. Unfortunately the need for speed and higher clock rating have driven the CPU industry to work on this factor without really seeing what the performance of these chips are like.

Since CPU's have started to top out at fast speeds now, the major manufacturers are looking for ways to increase the processing power without increasing the actual speed of the CPU. The route they have taken with this problem is with multi-core CPU's. On a single chip these new processors have 2, 3, 4, or even more processing cores fitting. These cores chare the workload and process faster than traditional CPU's even though they are not rated at higher speeds. So remember don't just look at the rated speed as now more than ever this doesn't give you the full story.

Types of CPU

There are two main desktop CPU manufacturers, they are Intel and AMD. Both of these companies have a power CPU and a Budget CPU. The Power CPU's are the Core 2 Quad from Intel and the Phenom X3 and Phenom Quad from AMD. The budget CPU's are the Celeron from Intel and the Sempron from AMD. Price is a big factor between these CPU's. Check the latest prices of these CPU's below.

AMD X2 (Dual core CPU)
AMD Phenom X3
AMD Phenom Quad
AMD Sempron
Intel Core 2 Duo
Intel Core 2 Quad
Intel Celeron

Socket Types

Each range of CPU fits into a specific socket on your motherboard. motherboards are design with one socket type and cannot be made to take another. Socket types changes quite rapidly, a socket change usually happens when a new breed of CPU surfaces. A new CPU with a new layout requires to connect to the motherboard differently and so a new socket type is released on motherboards to handle the new CPU's correctly. Even if a CPU fits into a motherboards CPU socket Physically, there is no guarantee this will work. Always check the motherboards documentation for supported CPU's before making a purchase.

Socket Type	Compatible Processors
Socket 7	Original Pentiums, Cyrix 686, Cyrix MII, K6, K6-2 and K6-III
Socket 370	Intel Celeron, Intel PIII (not Cartridge), Cyrix III
Slot 1	Intel PII, Intel PIII (cartridge only)
Slot A	AMD Athlon (Cartridge only)
Socket A	AMD Athlon Thunderbird (not Cartridge), AMD Duron, AMD Athlon XP
Socket 423	Intel P4
Socket 478	Intel P4 (2nd Gen)
754-Pin Socket	Athlon 64
940-Pin Socket	Athlon 64-fx, Opteron
Socket AM2	Athlon 64 FX, Athlon X2
Socket 603 / 604	Intel Xeon
Socket T	Intel Pentium 4, Pentium Extreme, Pentium D, Core 2 Duo
LGA 771	Xeon
PAC418 / 611	Intel Itanium
Socket 775	Core 2 duo, Core 2 Extreme, Core 2 Quad
Socket AM2+	Athlon 64, Athlon 64 FX-62, Athlon 64 X2, Sempron, Sempron LE
Socket AM3	Athlon 64 X2, Phenom, Sempron, Sempron LE, Opteron
Socket F	Phenom, Opteron (Server range)

The CPU's Cache

The Cache on the CPU is a small amount of very fast memory which is situated on the CPU. the cache memory is very expensive which is why its available in very limited amounts. The CPU cache acts as the repeated memory storage for data that the CPU requires frequently. Having more on-die cache speeds up processing as the CPU doesn't have to wait to retrieve data from the main memory, which in computing terms is very slow compared to the cache. It ranges from about 64Kb to 4Mb and soon larger cache chips will be coming. For more information on CPU cache and how it works we have just the article.

Central Processing Unit Cache Memory

What is the CPU Cache?

The cache on your CPU has become a very important part of today's computing. The cache is a very high speed and very expensive piece of memory, which is used to speed up the memory retrieval process. Due to its expensive CPU's come with a relatively small amount of cache compared with the main system memory. Budget CPU's have even less cache, this is the main way that the top processor manufacturers take the cost out of their budget CPU's.

How does the CPU Cache work?

Without the cache memory every time the CPU requested data it would send a request to the main memory which would then be sent back across the memory bus to the CPU. This is a slow process in computing terms. The idea of the cache is that this extremely fast memory would store and data that is frequently accessed and also if possible the data that is around it. This is to achieve the quickest possible response time to the CPU. Its based on playing the percentages. If a certain piece of data has been requested 5 times before, its likely that this specific piece of data will be required again and so is stored in the cache memory.

Lets take a library as an example o how caching works. Imagine a large library but with only one librarian (the standard one CPU setup). The first person comes into the library and asks for Lord of the Rings. The librarian goes off follows the path to the bookshelves (Memory Bus) retrieves the book and gives it to the person. The book is returned to the library once its finished with. Now without cache the book would be returned to the shelf. When the next person arrives and asks for Lord of the Rings, the same process happens and takes the same amount of time.

If this library had a cache system then once the book was returned it would have been put on a shelf at the librarians desk. This way once the second person comes in and asks for Lord of the Rings, the librarian only has to reach down to the shelf and retrieve the book. This significantly reduces the time it takes to retrieve the book. Back to computing this is the same idea, the data in the cache is retrieved much quicker. The computer uses its logic to determine which data is the most frequently accessed and keeps them books on the shelf so to speak.

That is a one level cache system which is used in most hard drives and other components. CPU's however use a 2 level cache system. The principles are the same. The level 1 cache is the fastest and smallest memory, level 2 cache is larger and slightly slower but still smaller and faster than the main memory. Going back to the library, when Lord of the Rings is returned this time it will be stored on the shelf. This time the library gets busy and lots of other books are returned and the shelf soon fills up. Lord of the Rings hasn't been taken out for a while and so gets taken off the shelf and put into a bookcase behind the desk. The bookcase is still closer than the rest of the library and still quick to get to. Now when the next person come in asking for Lord of the Rings, the librarian will firstly look on the shelf and see that the book isn't there. They will then proceed to the bookcase to see if the book is in there. This is the same for CPU's. They check the L1 cache first and then check the L2 cache for the data they require.

Is more Cache always better?

The answer is mostly yes but certainly not always. The main problem with having too much cache memory is that the CPU will always check the cache memory before the main system memory. Looking at our library again as an example. If 20 different people come into the library all after different books that haven't been taken out in quite a while but the library has been busy before and so the shelf and the bookcase are both full we have a problem. Each time a person asks for a book the librarian will check the shelf and then check the bookcase before realising that the book has to be in the main library. The librarian each time then trots off to get the book from the library. If this library had a non cache system it would actually be quicker in this instance because the librarian would go straight to the book in the main library instead of checking the shelf and the bookcase.

As the fact that non cache systems only work in certain circumstances and so in certain applications CPU's are definitely better with a decent amount of cache. Applications such as MPEG encoders are not good cache users because they have a constant stream of completely different data.

Does cache only store frequently accessed data?

If the cache memory has space it will store data that is close to that of the frequently accessed data. Looking back again to our library. If the first person of the day comes into the library and takes out Lord of the Rings, the intelligent librarian may well place Lord of the Rings part II on the shelf. In this case when the person brings back the book, there is a good chance that they will ask for Lord of the Rings part II. As this will happen more times than not. It was well worth the Librarian going to fetch the second part of the book in case it was required.

Cache Hit and Cache Miss

Cache hit and cache miss are just simple terms for the accuracy of what goes into the CPU's cache. If the CPU accesses its cache looking for data it will either find it or it wont. If the CPU finds what's its after that's called a cache hit. If it has to go to main memory to find it then that is called a cache miss. The percentage of hits from the overall cache requests is called the hit rate. You will be wanting to get this as high as possible for best performance.

CPU Temperatures and Cooling

The cooling of the CPU is one of the most important things you have to do. Choosing the right cooling method for the CPU you have could be essential. The CPU itself will have an ideal working temperature. You may or may not be surprised to know that the warmer a CPU becomes the slower it actually performs until it stops altogether. Also bad cooling could lead to permanent damage to your CPU and Motherboard. So what are the ways of cooling your CPU and other components. Well we have the two easiest methods to start with, which are the most popular. These are the heatsink and standard fan.

The Heatsink

The Heatsink is a simple piece of either copper or Aluminium which sits onto of the Processor chip. The idea is that the heat sink transfers the heat from the CPU or GPU or even chipset and and disperse it into the air. Heatsinks are normally coupled with a fan to aid in removing the excess heat.

When using a heatsink always use a thermal paste (sometimes called goop). This thermal paste fills the small holes which appear in all solid materials. In this case the imperfections in the heatsink and the CPU are filled with the paste and this greatly improves the heat transfer. If you find that your Heatsink actually gets hotter when you use thermal paste, you are correct. The paste has increased the heat transfer and so the heat is now in the heatsink and not the CPU. This is much better for the Performance of the System as a whole.

CPU and Case Fans

CPU, GPU and chipset fans are almost always attached to heatsinks. The idea is the heatsink removes the heat from the CPU and the fan blasts the heatsink with the surrounding air cooling it down. The reason that you don't want to have the fan blowing air from the heatsink as you may expect is that the hot air would simply be blasted into the case heating up the other components. If you wish to have a set up where the air is removed from the heatsink you will want a duct attached to the fan which runs to the outside of the case, this is a very effective way of removing the hot air away from the case altogether. Case fans provide this function as well. The idea of the case fan (sometimes called exhaust fans) is to remove the warm air in the case which is generated by all components. Once the hot air has been expelled from the case, cooler air is drawn in. Again this is healthier for the system overall.

Find the best fans and other cooling equipment

Water Cooling

When we are talking of advanced cooling we are talking of water cooling. Now those of you that are into overclocking and high system performance may have already heard of water cooling. Those of you however that haven't may well be thinking what a bad idea it is to have water running round an electrical system. I am not going to say that water in your system is not dangerous because it obviously is. However when you using water cooling you should use de-ionized water. De-ionized water is a very bad conductor of electricity and so should give your system a chance of survival should a bit leak onto the circuitry.

Water is far better at absorbing heat than air. For this reason passing water over the heatsink will mean that the water will pick up a lot of the excess heat, a lot more than air being blasted onto it would. The water is then taken away from the system to cool and pumped back again to the CPU. This method of cooling is very efficient. If you are thinking of using a Peltier cooler (see below) then I would definitely recommend setting up a water cooling system.

Expense does play a part in water cooling. the parts are still quite expensive, although they are not as bad as they once were. If you are serious about having a performance system or are into overclocking your components then this is the way to go.

Peltier Cooling

Peltier cooling is definitely the odd one of the bunch. The Peltier cooling system actually creates heat. Slightly odd you may rightly presume. But the reason for this is that the Peltier cooler uses electricity. It looks like a standard heatsink, however there are two sides to the plate. Heat is electronically "pumped" from one side of the plate to the other away from the CPU. The hot side of the plate can get very hot indeed, a great deal hotter than a standard heatsink. This is why we suggest water cooling with a Peltier system. The upside of all this heat is the part you actually want to cool, the CPU, remains very cold. I think efficient is the wrong word for the Peltier system. It takes power and creates heat so its not really efficient. However it is excellent at the job its supposed to do, which is keep the CPU cool.

Time for the disclaimer and nasty information. As you are aware as with all computer modding and overclocking, things can go wrong if you are not careful. I cannot be held responsible for actions you may take when using these cooling systems. Please be aware that especially with Peltier systems, to have hardware monitoring available. Any component breakdown could fry your whole system. When done properly these systems can greatly increase performance. Always follow standard safety procedures.

Following on from the cooling article we look at the maximum temperatures of CPU's. Even though we use fans and heatsinks CPU temperatures can still raise if they are being used at full power for a long period of time. Using CPU's at high temperatures can lower cause system crashes in the short term and in the long term cause the life of your CPU to be greatly reduced. In extreme cases your CPU could burn out or melt onto the motherboard. This usually happens when a fan breaks down and goes unnoticed. Today's motherboards come with temperature monitoring software and hardware which actually shuts the computer off if the CPU temperature gets too hot. Even these however are not 100% fail proof. The only way to be sure is to check your fans and other cooling equipment regularly and also use CPU thermometers to check your CPU temperature is stable and not raising over time. CPU's have a rated maximum temperature sometimes called a critical temperature. What this boils down to (quick pun :} ) is what the manufacturer states is the maximum temperature the CPU will operate at. This is not to say you want your CPU to operate at this temperature as it will be borderline between working and burning out. Always try to keep at least 20C below this value if you can. Below is a table showing you the critical temperatures for most of the CPU's we use today.

Please be aware that as faster models are released even under the same name the thermal requirements may change. this table is meant for a guide only. Critical temperature is often referred as Critical Case temperature as CPU core temperatures are difficult to report accurately.

CPU	Critical Temperature
AMD Athlon Series
AMD Athlon (socket) up to 1Ghz	90°C
AMD Athlon (slot) all speeds	70°C
AMD Athlon Thunderbird 1.1Ghz+	95°C
AMD Athlon MP 1.33Ghz+	95°C
AMD Athlon XP 1.33Ghz+	90°C
AMD Athlon XP T-Bred upto 2100+	90°C
AMD Athlon XP T-Bred over 2100+	85°C
AMD Athlon XP Barton	85°C
AMD Athlon 64	70°C
AMD Athlon 64 (Socket 939, 1.4 volts)	65°C
AMD Athlon 64 FX (sledgehammer)	70°C
AMD Athlon FX (San Diego + Toledo + Windsor)	63°C
AMD Athlon X2 (Manchester + Toledo)	65°C
AMD Athlon X2 (Windsor)	70°C-72°C (1.35v - 1.25v)
AMD Athlon X2 (Brisbane)	78°C (1.25v)
AMD Phenom
AMD Phenom	70°C
AMD Phenom X3	70°C
AMD Phenom X4 (9100, 9750, 9850)	61°C
AMD Phenom X4 (9550, 9650)	70°C
AMD Sempron
AMD Sempron (T-bred/Barton core)	90°C
AMD Sempron (Paris core)	70°C
AMD Sempron (Manila)	69°C/78°C Model dependant
AMD Mobile Sempron	95°C
Intel Pentium D
Pentium D (Smithfield 805, 820)	63°C
Pentium D (Smithfield 830, 840)	69.8°C
Pentium D (Presler 915, 920, 930, 945, 960)	63.4°C
Pentium D (Presler 940, 950)	68.6°C
Intel Celeron Series
Celeron D (Prescott)	67°C
Celeron D (Cedar Mill)	69.2°C
Mobile Celeron	100°C
Intel Core 2 Duo
Intel core 2 Duo (Conroe E4300, E4400, E6300, E6400)	61.4°C
Intel Core 2 Duo (Conroe E4500, E4600, E4700)	73.3°C
Intel Core 2 Duo (Conroe E6320, E6420, E6540, E6550, E6600, E6700, E6750, E6850)	60.1°C
Intel Core 2 Duo (Wolfdale)	72.4°C
Mobile Core 2 Duo	100°C
Intel Core 2 Extreme
Intel Core 2 Extreme (Conroe)	60.4°C
Intel Core 2 Extreme (Kentsfield Q6700)	71°C
Intel Core 2 Extreme (Kentsfield Q6600)	62.2°C
Intel Core 2 Extreme (Kentsfield QX6700, QX6850)	64.5°C
Intel Core 2 Extreme (Kentsfield QX6800)	54.8°C
Intel Core 2 Extreme (Yorkfiled Q9300, Q9450, Q9550)	71.4°C
Intel Core 2 Extreme (Yorkfield QX9650)	64.5°C
Intel Core 2 Extreme (Yorkfield QX9770)	55.5°C
Intel Core 2 Extreme (Hypertown QX9775)	63°C
Intel Itanium 2
Intel Itanium 2 below 1Ghz	66°C
Intel Itanium 2 1Ghz - 1.6Ghz	83°C

To Illustrate what could happen to your CPU without adequate cooling Toms Hardware guide made a short video. This really is a must see. It shows you what happens to to CPU's should the heatsink/fan is removed. The results are very interesting.

Also for more a in depth look at processor specifications take a look at this page