Lab

Processors

Upgrading your processor can give your PC a new lease of life. Similarly, if you're building a new PC, choosing the right processor is of the utmost importance. Fortunately, whether you're planning an upgrade or a completely new computer, now is an excellent time to buy a processor. Intel has just launched its new Core 2 Duo range, and there have been big price cuts across AMD's range. You can now buy a huge amount of power for not a lot of money.

In this month's Labs test we've covered every current desktop range of processors from Intel and AMD. You may want a cutting-edge chip to go in a new PC, so we've included the latest Intel LGA775 and AMD Socket AM2 processors.

However, you may want to upgrade your current PC, so we've also reviewed the previous-generation Socket 939 processors and one Socket 754 chip. Either way, you'll find all the advice you need here.

Choosing a... Processor

Choosing the right processor is more complicated than just buying the one with the highest clock speed you can afford. You have to consider the tasks your PC will perform, as different processors run different types of application faster than others. The number of cores a processor has greatly influences where its strengths lie. However, other factors - such as how much cache is on the chip and what speed of memory it supports - are also important.

COMPATIBILITY CONUNDRUMS

Not all processors are compatible with all motherboards, so if you're upgrading a PC, you have the choice of keeping the motherboard and limiting your choice of processor, or replacing both board and processor to maximise your options. Either way, you must check that your eventual pairing is compatible.

You need a motherboard and processor that share the same socket type. AMD's current socket type is Socket AM2, and Intel's is LGA775. As Socket AM2 has only just replaced Socket 939, many people will still have 939 motherboards, so we have included 939 processors in this group test. Socket 754 came before Socket 939, and there are still some processors available for this socket. But the older the socket, the more limited the performance increase you'll be able to achieve with a straight chip swap.

Even if it has the right socket for a processor, a motherboard may not support it. For instance, dual-core Intel processors work only with certain chipsets. The dual-core Pentium D 800 and 900 series of processors require Intel's 945 chipset and above or an Nvidia nForce 4 motherboard, while Core 2 Duo processors need at least an Intel 965 or Nvidia nForce 5 board. The Core 2 Extreme X6800 will work only in an Intel 975 board, but the 975 chipset doesn't support Celeron processors.

Even if you have the right socket type and chipset, your motherboard may recognise the processor only if you update the BIOS. The only way to be sure a processor will definitely work in a board is to visit the board manufacturer's website and look up the processor compatibility of the board in question.

If you upgrade your motherboard to accommodate a processor upgrade, check whether you'll need to upgrade your RAM as well. The type of RAM you can fit depends on the motherboard and there are two types, DDR and DDR2, which are incompatible with each other. Most LGA775 motherboards require the faster DDR2 memory, though a few designed for single-core processors support DDR, which you're more likely to have in an existing PC. Socket 754 and 939 motherboards use DDR memory, and all the matching processors in the group require 400MHz DDR. Socket AM2 boards use DDR2 memory up to 800MHz.

CORE ARGUMENT

Processors used to be compared by their clock speeds, expressed in megahertz (MHz) and gigahertz (GHz). The clock speed shows how many processing cycles a processor does per second, so on a basic level the faster the clock speed the more work the processor can handle every second. The reality is more complex, and consequently both AMD and Intel now play down clock speeds, preferring to name processors using model numbers instead.

The real speed at which a processor runs depends on many factors and is also affected by the application you are running. Some of this is down to which instruction sets a processor supports. If a processor supports an extended instruction set, software designed to work with that set will run faster. Most of the processors in the group support the SSE, SSE2 and SSE3 instruction sets, while Intel's Core 2 Duo range supports SSE4. All the processors are ready for 64-bit operating systems thanks to EM64T or x86-64 instruction sets.

Processor manufacturers have recently started making dual-core processors. These are essentially two processors on one chip, and the idea is that both cores share the processing workload. However, this isn't always possible. When running more than one application, the work can be easily split between two cores. However, for a single application to run faster it needs to be divided into threads that can be shared between the cores. A single-threaded application will not run any faster on a dual-core processor, but a multithreaded application will. Our video-encoding and multitasking benchmarks take advantage of both cores.

A processor's onboard cache also affects its performance. The cache stores data from the most frequently accessed areas of the main memory. The more cache a processor has, the less often it has to fetch information from the system RAM, and as it is far quicker to access data in the cache than it is to access main memory, this improves performance.

Most dual-core processors have a separate allocation of cache for each core, but Intel's Core 2 Duo chips share a unified cache between the cores and dynamically allocate it to whichever core needs it at that time.

CLOCKING IN

For those who are more adventurous, there are some processors that are particularly suitable for overclocking, a technique that gets you more speed for nothing. Overclocking a processor causes it to run hotter and can cause damage, however, so you need to keep an eye on the processor's temperature if you try it.

A processor's clock speed is determined by its external bus clock speed, which is multiplied by circuitry inside the processor to derive the core clock speed. If you increase the external bus speed all processors will run faster, but this can speed up the memory, so both can suffer excess heat and instability. Intel's Extreme and AMD's FX processors let you change the processor multiplier, which allows you to speed up the chip without affecting the memory. Our reviews record the success we had overclocking various processors.