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The CPU (central processing unit or processor) is the brains of your PC; everything that happens inside your PC has to interact with it. There's a bewildering number of processors out there for all budgets, with prices from around £25 for low-end CPUs up to £700 or more for the fastest models.
But with an equally bewildering amount of names and numbers, things aren't as simple as they once were. In this guide, we'll look at mainstream desktop and mobile processors. We won't be looking at specialised server or workstation models.
AMD vs Intel
Both Intel and AMD offer a wide range of CPUs. The two companies' products are in direct competition and are both capable of running any current PC software. The differences are in technical details that, although they may be used for marketing, don't necessarily mean much to the end user.
Pricing is also very competitive, with AMD's products being generally cheaper than comparable Intel models. At the budget end of the market, Intel's offering is the Celeron which is a cut-down version of the Pentium 4. AMD's budget processor, the Duron, is still available although it's getting harder to find.
In the mainstream desktop market it's Intel's Pentium 4 versus AMD's Athlon XP and Athlon 64. The chief difference between the two is that the Pentium 4 and Athlon XP are 32bit CPUs, while the Athlon 64 is a 64bit model (see later on for an explanation).
Just recently both companies have launched new ranges of processors, Intel with the Extreme Edition of the Pentium 4, an expensive model for the hardcore gaming market, and a new version of the Pentium 4 processor. You might see this referred to as Prescott to differentiate it from the previous Northwood version. The technical differences aren't huge, but the pricing hasn't changed for models at the same clock speed. So if you're given a choice, ask for the Prescott models.
AMD has also introduced the Athlon 64 FX range, a high-end gaming/workstation CPU that prompted Intel to launch the Pentium 4 Extreme Edition.
Other choices
If you're looking to build a very small system from scratch, then Via's Epia platform is worthy of your attention. This platform consists of tiny (17 x 17cm) motherboards with the Via C3 processor already fitted; you can't upgrade the CPU. At 1.2GHz, the fastest C3 may not be able to match its bigger desktop cousins in performance terms, but it allows you to build low cost, space saving systems.
What's in a number?
Processor naming can be highly confusing. Until recently Intel simply used the clock speed of the processor, for example 3.2GHz Pentium 4, while AMD uses names based on the processor's alleged capabilities. For example, an AMD XP3000+ may have a clock speed of 2.16GHz but in practice it performs more like a 3GHz processor.
Intel has now changed the way it names processors. The new names consist of the processor family (e.g. Pentium 4) and a three-digit number beginning with either 3, 5 or 7 as follows:
Pentium 4 desktop and Mobile - Pentium 4 will use 5xx
Celeron and the mobile - Celerons will use 3xx
Mobile Pentium Ms will use 7xx
The other two digits refer to the features and technical specifications of the processor. For example the 2GHz Pentium M processor is the 755 model. We think this will lead to even more confusion.
Sockets and pins
When you see the description of any processor you will see the socket type mentioned, either as a name or a number. This is simply the socket on the motherboard the processors were designed to fit. Socket 478 is the current Intel standard for Celerons and Pentium 4s. AMD's Socket A is for the Duron, Athlon and Athlon XP ranges. Socket 754 is for the Athlon 64 and Socket 940 for the Athlon 64-FX range.
Front-side what?
The front-side bus (FSB) is the interface between the CPU and the main system memory and determines the fastest type of memory your system can use. The faster the FSB the quicker data can be passed between the CPU and the memory.
Current Pentium 4s use a 200MHz FSB, but use 'quad-pumped' technology to enable data transfers to run at 800MHz. The latest Athlon XP 3200+ also uses a 200MHz FSB, but doubled to allow 400MHz data transfers.
Cache memory
Cache memory in a CPU helps eliminate data bottlenecks. The Level 2 (L2) cache is extra-fast memory built into the processor itself, and is used to store frequently accessed data so that the CPU isn't held up waiting for data to arrive. In general, the larger and faster the cache, the more efficient the processor.
There's also a Level 1 (L1) cache on most processors that is smaller and is used for caching the processor's internal instructions. You can't upgrade cache memory.
Hyper-threading
Most of Intel's current Pentium 4s and some of the Mobile Pentium 4s support Hyperthreading (HT). Put simply, HT gives you two virtual processors for the price of one. Using an HT-aware operating system such as Windows XP can give you a noticeable performance boost by allowing applications to use either or both of the two virtual CPUs, hence making the system much more responsive when running lots of applications at the same time. It's not quite as good as running a system with dual CPUs, but it's a lot cheaper.
64bit v 32bit
64bit processing for consumer PCs is still in its infancy, but AMD is nevertheless pushing its Athlon 64 range of 64bit CPUs. At the moment it's a bit of a red herring as you need a 64bit operating system to take advantage of this.
While several flavours of Linux support 64bit, the 64bit version of Windows XP won't appear until possibly late 2004. The Athlon 64 runs normal 32bit software as well as any other 32bit processor, so there's no disadvantage in buying it and you will get a little bit of extra future proofing.
Mobile processors
Intel and AMD both offer mobile versions of their processors, incorporating special power-saving features in order to maximise battery life and reduce the heat generated. Some manufacturers use the desktop versions in laptops, which is fine, but this often means that you don't get very good battery life.
These systems also usually need more cooling to get rid of the extra heat generated, so can be noisier than laptops using true mobile processors.
Intel Centrino systems use the Mobile Pentium-M processor, but the label 'Centrino' is only applied to systems that also use Intel's wireless networking chipset. Intel's battery-saving technology is known as Speedstep, AMD's as Powernow. Both try to save power by reducing the processor speed when there's not much processing work being done by the system.
What's best for you?
Before you rush out and buy the fastest CPU you can afford, take a moment to ask yourself what you need it for. If you only want to use basic applications such as email and web browsing, then there's no point buying the fastest one out there.
On the other hand, if you do a lot of gaming or use demanding applications such as audio or video editing, then you should look for things such as large amounts of L2 cache and fast FSB speeds. If you regularly have lots of applications running at the same time, you will see benefits from Intel's hyper-threading technology.
Codes and roadmaps
Intel and AMD give codenames to the various versions of their chips, and you'll often hear them being called by these. The first Pentium 4 was known as Willamette, which was later followed by the Northwood version. The latest Pentium 4s rejoice in the name of Prescott. Using a 0.13micron fabrication technology, Northwood chips crammed more transistors into the same volume, offering 512KB of L2 cache and faster clock speeds than the previous generation.
Prescotts use an even finer 0.09micron (90nanometre) manufacturing process with an FSB of 800MHz and have 1MB of L2 cache, giving a boost to performance over previous versions.
Intel's latest Pentium mobile processor is the recently released Dothan, which is now beginning to appear in laptops. It boasts 2MB of L2 cache, a 400MHz FSB, has a minimum speed of 1.7GHz and claims that, despite its increased performance, it continues to improve battery life.
AMD's Thoroughbred version of the Athlon XP, a 0.13micron processor with 256KB of L2 cache, superseded the original AthlonXP, Palomino. Next came Barton, still built on a 0.13micron process, but with improved L2 cache of 512KB. AMD continues to manufacture Barton Athlon XP processors, with Paris, the next generation, due to ship later this year.
It now also has the Athlon 64 (codenamed Hammer), mobile Athlon 64 and the Athlon 64 FX, aimed at high-end gaming and workstations. These continue to build on a 0.13micron process, but this will change in two generations for the Athlon 64, currently named Winchester and the next FX, called San Diego. All have 1MB of L2 cache and a bus speed of 1.6GHz, plus the mobile version has advanced power-saving capabilities.
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